Biology Glossary

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  • Abiogenesis Early theory that held that some organisms originated from nonliving material.
  • Abnormal hemoglobin Hemoglobin molecule with a different shape due to an altered amino acid sequence (ultimately caused by an altered DNA base sequence), such as in the inherited disease sickle-cell anemia.
  • Abscisic acid A plant hormone that promotes dormancy in perennial plants and causes rapid closure of leaf stomata when a leaf begins to wilt.
  • Absolute time One of the two types of geologic time (relative time being the other), with a definite age date established mostly by the decay of radioactive elements, although ages may also be obtained by counting tree rings, decay of a specific type of atom, or annual sedimentary layers (such as varves in lakes or layers in a glacier). The term is in some disfavor because it suggests an exactness that may not be possible to obtain.
  • Absorption The process by which the products of digestion are transferred into the body's internal environment, enabling them to reach the cells.
  • Absorptive feeders Animals such as tapeworms that ingest food through the body wall.
  • Acetyl coa An intermediate compound formed during the breakdown of glucose by adding a two-carbon fragment to a carrier molecule (Coenzyme A or coa).
  • Acetylcholine A chemical released at neuromuscular junctions that binds to receptors on the surface of the plasma membrane of muscle cells, causing an electrical impulse to be transmitted. The impulse ultimately leads to muscle contraction.
  • Acid A substance that increases the number of hydrogen ions in a solution.
  • Acid rain The precipitation of sulfuric acid and other acids as rain. The acids form when sulfur dioxide and nitrogen oxides released during the combustion of fossil fuels combine with water and oxygen in the atmosphere.
  • Acoelomates Animals that do not have a coelom or body cavity; e.g., sponges and flatworms.
  • Acquired immunodeficiency syndrome (AIDS) A collection of disorders that develop as a result of infection by the human immunodeficiency virus (HIV), which attacks helper T cells, crippling the immune system and greatly reducing the body's ability to fight infection; results in premature death brought about by various diseases that overwhelm the compromised immune system.
  • Actin The protein from which microfilaments are composed; forms the contractile filaments of sarcomeres in muscle cells.
  • Action potential A reversal of the electrical potential in the plasma membrane of a neuron that occurs when a nerve cell is stimulated; caused by rapid changes in membrane permeability to sodium and potassium.
  • Active transport Transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentration) with the aid of proteins in the cell membrane and energy from ATP.
  • Adaptation Tendency of an organism to suit its environment; one of the major points of Charles Darwin's theory of evolution by natural selection: organisms adapt to their environment. Those organisms best adapted will have a greater chance of surviving and passing their genes on to the next generation.
  • Adaptive radiation The development of a variety of species from a single ancestral form; occurs when a new habitat becomes available to a population. Evolutionary pattern of divergence of a great many taxa from a common ancestral species as a result of novel adaptations or a recent mass extinction. Examples: mammals during the Cenozoic Era after the extinction of dinosaurs at the close of the Mesozoic Era flowering plants during the Cretaceous Period diversified because of their reproductive advantages over gymnosperm and non-seed plants that dominated the floras of the world at that time.
  • Adenine One of the four nitrogen-containing bases occurring in nucleotides, the building blocks of the organic macromolecule group known as nucleic acids (DNA and RNA). Adenine is also the base in the energy carrying molecule ATP (adenosine triphosphate) which is the energy coin of the cell.
  • Adenosine diphosphate (ADP) Lower energy form of ATP, having two (instead of the three in ATP) phosphhate groups attached to the adenine base and ribose sugar.
  • Adenosine triphosphate (ATP) A common form in which energy is stored in living systems; consists of a nucleotide (with ribose sugar) with three phosphate groups. The energy coin of the cell.
  • Adhesion The ability of molecules of one substance to adhere to a different substance.
  • Adrenocorticotropic hormone (ACTH) A hormone produced by the anterior pituitary that stimulates the adrenal cortex to release several hormones including cortisol.
  • Adventitious roots Roots that develop from the stem following the death of the primary root. Branches from the adventitious roots form a fibrous root system in which all roots are about the same size; occur in monocots.
  • Age structure The relative proportion of individuals in each age group in a population.
  • Aggregates Fairly random associations of animals with little or no internal organization; form in response to a single stimulus and disperse when the stimulus is removed; one of the three broad classes of social organization.
  • Albinism Genetic condition caused by the body's inability to manufacture pigments; an autosomal recessive trait.
  • Aldosterone A hormone secreted by the adrenal glands that controls the reabsorption of sodium in the renal tubule of the nephron.
  • Alleles Alternate forms of a gene.
  • Allergens Antigens that provoke an allergic reaction.
  • Alpha decay Type of radioactive decay in which a radioisotope emits a large but slow-moving particle consisting of two protons and two neutrons.
  • Alternation of generations A life cycle in which a multicellular diploid stage is followed by a haploid stage and so on; found in land plants and many algae and fungi. |
  • Altitudinal gradient As altitude increases, a gradient of cooler, drier conditions occurs.
  • Alveoli Tiny, thin-walled, inflatable sacs in the lungs where oxygen and carbon dioxide are exchanged.
  • Amensalism A symbiotic relationship in which members of one population inhibit the growth of another population without being affected.
  • Amino acid sequence Also known as the primary structure of a protein/polypeptide; the sequence of amino acids in a protein/polypeptide controlled by the sequence of DNA bases.
  • Amino acids The subunits (monomers) from which proteins (polymers) are assembled. Each amino acid consists of an amino functional group, and a carboxyl acid group, and differs from other amino acids by the composition of an R group.
  • Amniocentesis A method of prenatal testing in which amniotic fluid is withdrawn from the uterus through a needle. The fluid and the fetal cells it contains are analyzed to detect biochemical or chromosomal disorders.
  • Amniote egg An egg with compartmentalized sacs (a liquid-filled sac in which the embryo develops, a food sac, and a waste sac) that allowed vertebrates to reproduce on land.
  • Amoebocytes Amoeboid cells in sponges that occur in the matrix between the epidermal and collar cells. They transport nutrients.
  • Amphibians Class of terrestrial vertebrates which lay their eggs (and also mate) in water but live on land as adults following a juvenile stage where they live in water and breathe through gills. Amphibians were the first group of land vertebrates; today they are mostly restricted to moist habitats.
  • Anabolic reactions Reactions in cells in which new chemical bonds are formed and new molecules are made; generally require energy, involve reduction, and lead to an increase in atomic order.
  • Anaerobic Refers to organisms that are not dependent on oxygen for respiration.
  • Analogous structures Body parts that serve the same function in different organisms, but differ in structure and embryological development; e. G., the wings of insects and birds.
  • Anaphase Phase of mitosis in which the chromosomes begin to separate.
  • Anaphylactic shock See anaphylaxis.
  • Anaphylaxis A severe allergic reaction in which histamine is released into the circulatory system; occurs upon subsequent exposure to a particular antigen; also called anaphylactic shock.
  • Androecium Collective term applied to all of the male (stamen) parts of the flower.
  • Aneuploidy Variation in chromosome number involving one or a small number of chromosomes; commonly involves the gain or loss of a single chromosome.
  • Angina Chest pain, especially during physical exertion or emotional stress, that is caused by gradual blockage of the coronary arteries.
  • Angiosperms Flowering plants. First appearing at least 110 million years ago from an unknown gymnosperm ancestor, flowering planbts have risen to dominance in most of the world's floras. The male gametophyte is 2-3 cells contained within a pollen grain; the female gametophyte is usually eight cells contained within an ovule which is retaind on the sporophyte phase of the plant's life cycle.
  • Animalia Animal Kingdom. Multicellular eukaryotic group characterized by heterotrophic nutritional mode, usually organ and tissue development, and motility sometime during the organism's life history.
  • Annuals Plants that grow and reproduce sexually during one year.
  • Antagonistic muscles A pair of muscles that work to produce opposite effects&emdash;one contracts as the other relaxes: for example, the bicep and tricep muscles on opposite sides of your upper arm.
  • Anther The top of a stamen's filament; divided into pollen sacs in which the pollen grains form. |
  • Antibiotic resistance Tendency of certain bacteria to develop a resistance to commonly over-used antibiotics.
  • Antibiotics Substances produced by some microorganisms, plants, and vertebrates that kill or inhibit the growth of bacteria.
  • Antibodies Proteins produced by immune system cells that bind to foreign molecules and microorganisms and inactivate them.
  • Antibody-mediated immunity Immune reaction that protects primarily against invading viruses and bacteria through antibodies produced by plasma cells; also known as humoral immunity.
  • Anticodon A sequence of three nucleotides on the transfer RNA molecule that recognizes and pairs with a specific codon on a messenger RNA molecule; helps control the sequence of amino acids in a growing polypeptide chain.
  • Antidiuretic hormone (ADH) A hormone produced by the hypothalamus and released by the pituitary gland that increases the permeability of the renal tubule of the nephron and thereby increases water reabsorption; also known as vasopressin.
  • Antigenic determinant The site on an antigen to which an antibody binds, forming an antigen-antibody complex.
  • Antigens Molecules carried or produced by microorganisms that initiate antibody production; mostly proteins or proteins combined with polysaccharides.
  • Antinutrients Chemicals produced by plants as a defense mechanism; inhibit the action of digestive enzymes in insects that attack and attempt to eat the plants.
  • Anus The posterior opening of the digestive tract.
  • Aorta The artery that carries blood from the left ventricle for distribution throughout the tissues of the body. The largest diameter and thickest walled artery in the body.
  • Apical meristem A meristem (embryonic tissue) at the tip of a shoot or root that is responsible for increasing the plant's length.
  • Apnea A disorder in which breathing stops for periods longer than 10 seconds during sleep; can be caused by failure of the automatic respiratory center to respond to elevated blood levels of carbon dioxide.
  • Apocrine glands Sweat glands that are located primarily in the armpits and groin area; larger than the more widely distributed eccrine glands.
  • Appendicular skeleton The bones of the appendages (wings, legs, and arms or fins) and of the pelvic and pectoral girdles that join the appendages to the rest of the skeleton; one of the two components of the skeleton of vertebrates.
  • Appendix Blind sac at the end of the large intestine that usually ruptures during final exams; a vestigial organ in humans.
  • Archaea Proposed, but not widely accepted, sixth taxonomic kingdom that would include the archaebacteria.
  • Archaebacteria Ancient (over 3.5 billion years old) group of prokaryotes; some biologists want to place this group into a separate Kingdom, the Archaea. Most currently place it within the Kingdom Monera.
  • Archaeocyathids An extinct group of animals that were part of Cambrian-aged reef environments, but which were extinct by the close of the Cambrian Period.
  • Archean/Proterozoic Era The period of time beginning 4.6 billion years ago with the formation of the Earth and ending 570 million years ago.
  • Aridity The condition of receiving sparse rainfall; associated with cooler climates because cool air can hold less water vapor than warm air. Many deserts occur in relatively warm climates, however, because of local or global influences that block rainfall.
  • Arrector pili A muscle running from a hair follicle to the dermis. Contraction of the muscle causes the hair to rise perpendicular to the skin surface, forming "goose pimples."
  • Arteries Thick-walled vessels that carry blood away from the heart. Singular=artery.
  • Arterioles The smallest arteries; usually branch into a capillary bed.
  • Artificial selection The process in which breeders choose the variants to be used to produce succeeding generations.
  • Ascomycetes Division of fungi that contains the yeasts and morels; ascomycetes produce an ascus (or sac) in which ascospores are produced.
  • Ascus Structure produced by sac fungi in which sexual ascospores develop.
  • Asexual reproduction A method of reproduction in which genetically identical offspring are produced from a single parent; occurs by many mechanisms, including fission, budding, and fragmentation.
  • Assortment A way in which meiosis produces new combinations of genetic information. Paternal and maternal chromosomes line up randomly during synapsis, so each daughter cell is likely to receive an assortment of maternal and paternal chromosomes rather than a complete set from either.
  • Aster Short fibers produced by cells during mitosis and meiosis. These radiate from the centriole (if it is present).
  • Asteroid impacts Hypothesis that links certain mass extinction events with the impact of a comet or asteroid, most notably the mass extinction 65 million years that caused the disappearance of dinosaurs and many other reptilian groups. Asteroid impacts early in earth history also contributed to the formation of the atmosphere and oceans.
  • Asthma A respiratory disorder caused by allergies that constrict the bronchioles by inducing spasms in the muscles surrounding the lungs, by causing the bronchioles to swell, or by clogging the bronchioles with mucus.
  • Asymmetrical In animals, a term referring to organisms that lack a general body plan or axis of symmetry that divides the body into mirror-image halves.
  • Atmosphere The envelope of gases that surrounds the Earth; consists largely of nitrogen (78%) and oxygen (21%).
  • Atom The smallest indivisible particle of matter that can have an independent existence.
  • Atomic number The number of protons in the nucleus of an atom.
  • Atomic weight The sum of the weights of an atom's protons and neutrons, the atomic weight differs between isotopes of the same element.
  • Atrioventricular (AV) node Tissue in the right ventricle of the heart that receives the impulse from the atria and transmits it through the ventricles by way of the bundles of His and the Purkinje fibers.
  • Atrioventricular (AV) valve The valve between each auricle and ventricle of the heart.
  • Auricle The chamber of the heart that receives blood from the body returned to the heart by the veins. Also referred to as atrium.
  • Autonomic system The portion of the peripheral nervous system that stimulates smooth muscle, cardiac muscle, and glands; consists of the parasympathetic and sympathetic systems.
  • Autosomes The chromosomes other than the sex chromosomes. Each member of an autosome pair (in diploid organisms) is of similar length and in the genes it carries.
  • Autotrophic Refers to organisms that synthesize their nutrients and obtain their energy from inorganic raw materials.
  • Autotrophs Organisms that synthesize their own nutrients; include some bacteria that are able to synthesize organic molecules from simpler inorganic compounds.
  • Auxins A group of hormones involved in controlling plant growth and other functions; once thought responsible for phototropism by causing the cells on the shaded side of a plant to elongate, thereby causing the plant to bend toward the light.
  • Axial skeleton The skull, vertebral column, and rib cage; one of the two components of the skeleton in vertebrates.
  • Axillary buds Buds borne in the axil (where the leaf meets the stem) of a stem.
  • Axons Long fibers that carry signals away from the cell body of a neuron.
  • B
  • B cells Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circulate in the circulatory and lymph systems where they transform into antibody-producing plasma cells when exposed to antigens.
  • B memory cells Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if the antigen is encountered again.
  • Bacteriophages Viruses that attack and kill bacterial cells; composed only of DNA and protein.
  • Bark The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an inner layer of phloem.
  • Barr body Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated prior to cell division and thus is passed on to all descendant cells of the embryonic cell that had one of its X-chromosomes inactivated.
  • Barriers to gene flow Factors, such as geographic, mechanical, and behavioral isolating mechanisms that restrict gene flow between populations, leading to populations with differing allele frequencies.
  • Basal body A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.
  • Base A substance that lowers the hydrogen ion concentration in a solution.
  • Basidia Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spores form (sing.: basidium).
  • Basidiomycetes The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospores are produced. Includes mushrooms and toadstools.
  • Basidiospores The spores formed on the basidia of club fungi (Basidiomycetes).
  • Benthic zone One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling organisms.
  • Beta decay Type of radioactive decay in which a radioisotope emits a small, negatively-charged and fast-moving particle from its nucleus. The beta particle is similar in size, charge, and speed to an electron and is formed when a neutron in the radioisotope's nucleus converts to a proton.
  • Bicarbonate ions A weak base present in saliva that helps to neutralize acids in food.
  • Big bang theory A model for the evolution of the universe that holds that all matter and energy in the universe were concentrated in one point, which suddenly exploded. Subsequently, matter condensed to form atoms, elements, and eventually galaxies and stars.
  • Bilateral symmetry In animals, refers to those that have a single axis of symmetry.
  • Biliary system The bile-producing system consisting of the liver, gallbladder, and associated ducts.
  • Binary fission The method by which bacteria reproduce. The circular DNA molecule is replicated; then the cell splits into two identical cells, each containing an exact copy of the original cell's DNA.
  • Binding sites Areas on the ribosome within which trna-amino acid complexes fit during protein synthesis.
  • Binomial system of nomenclature A system of taxonomy developed by Linnaeus in the early eighteenth century. Each species of plant and animal receives a two-term name; the first term is the genus, and the second is the species.
  • Biochemical cycle The flow of an element through the living tissue and physical environment of an ecosystem; e. G., the carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus cycles.
  • Biochemical reactions Specific chemical processes that occur in living things.
  • Biochemistry Chemical processes associated with living things.
  • Biodiversity Biological diversity; can be measured in terms of genetic, species, or ecosystem diversity.
  • Biogeography The study of the distribution of plants and animals across the Earth.
  • Bioluminescent Refers to organisms that emit light under certain conditions.
  • Biomass The total weight of living tissue in a community.
  • Biome A large-scale grouping that includes many communities of a similar nature.
  • Biosphere All ecosystems on Earth as well as the Earth's crust, waters, and atmosphere on and in which organisms exist; also, the sum of all living matter on Earth.
  • Birds Taxonomic class of terrestrial vertebrates that are characterized by endothermy and feathers; descended from some group of reptiles (or possibly dinosaurs).
  • Birth rate The ratio between births and individuals in a specified population at a particular time.
  • Bladder A hollow, distensible organ with muscular walls that stores urine and expels it through the urethra.
  • Blastocoel The fluid-filled cavity at the center of a blastula.
  • Blastocyst The developmental stage of the fertilized ovum by the time it is ready to implant; formed from the morula and consists of an inner cell mass, an internal cavity, and an outer layer of cells (the trophoblast).
  • Blastula A ball of cells surrounding a fluid-filled cavity (the blastocoel) that is produced by the repeated cleavage of a zygote.
  • Blending Term applied to 19th century belief that parental traits "blended" in their offspring; disproven by Mendel's work.
  • Blood group or type One of the classes into which blood can be separated on the basis of the presence or absence of certain antigens; notably, the ABO types and the Rh blood group.
  • Body fossil The actual remains (however permineralized, compressed or otherwise post-mortem altered) of an organism; includes bones, shells, and teeth.
  • Bolus A mass of chewed food mixed with salivary secretions that is propelled into the espohagus during the swallowing phase of digestion.
  • Bony fish A term applied collectively to all groups of fish with bony (as opposed to cartilaginous) skeletons.
  • Bottlenecks Drastic short-term reductions in population size caused by natural disasters, disease, or predators; can lead to random changes in the population's gene pool.
  • Brachiopods A phylum of hinge-shelled animals that have left an excellent fossil record; brachiopods live on or in the ocean floor. |
  • Brachydactly Human genetic disorder that causes production of an extra digit; an autosomal dominant trait. Sometimes referred to as polydactly.
  • Brain The most anterior, most highly developed portion of the central nervous system.
  • Brain stem The portion of the brain that is continuous with the spinal cord and consists of the medulla oblongata and pons of the hindbrain and the midbrain.
  • Bronchi Tubes that carry air from the trachea to the lungs (sing.: bronchus).
  • Bronchioles Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli.
  • Bronchitis A respiratory disorder characterized by excess mucus production and swelling of the bronchioles; caused by long-term exposure to irritants such as cigarette smoke and air pollutants.
  • Brown algae Multicellular protistans placed in the Division Phaeophyta, includes kelp.
  • Brush border The collection of microvilli forming a border on the intestinal side of the epithelial cells of the small intestine.
  • Bryophytes The nonvascular plants, characterized by life cycles dominated by the gametophyte phase. This group includes the mosses, liverworts, and hornworts, which lack lignified conducting tissues.
  • Bud sports Buds that produce fruit that is different from the rest of the fruit on the tree; vegetatively propagated by grafting cuttings onto another plant.
  • Budding 1. Asexual production of new organisms; usually found in yeast; 2. The process by which HIV and similar viruses leave the cell (other than by lysing).
  • Buffers Chemicals that maintain ph values within narrow limits by absorbing or releasing hydrogen ions.
  • Bulbourethral glands Glands that secrete a mucus-like substance that is added to sperm and provides lubrication during intercourse.
  • Bursae Small sacs lined with synovial membrane and filled with synovial fluid; act as cushions to reduce friction between tendons and bones.
  • C
  • Calcitonin A hormone produced by the thyroid that plays a role in regulating calcium levels.
  • Calcium carbonate Chemical that also occurs in limestone and marble.
  • Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast.
  • Cambium A lateral meristem in plants. Types of cambiums include vascular, cork, and intercalary.
  • Cambrian Geologic period that begins the Paleozoic Era 570 million years ago. Marked in its beginning by a proliferation of animals with hard, preservable parts, such as brachiopods, trilobites, and archaeocyathids.
  • Campodactyly A dominant trait in which a muscle is improperly attached to bones in the little finger, causing the finger to be permanently bent.
  • Capillaries Small, thin-walled blood vessels that allow oxygen to diffuse from the blood into the cells and carbon dioxide to diffuse from the cells into the blood.
  • Capillary bed A branching network of capillaries supplied by arterioles and drained by venules.
  • Capsid The protein "shell" of a free virus particle. This definition is from the Glossary at the UCMP site at http://www.ucmp.berkeley.edu/glossary/gloss4cell.html
  • Capsule 1. Structure produced around certain bacteria; 2. Structure produced by the bryophyte sporophyte that contains spores produced by meiosis.
  • Carbohydrates Organic molecules composed of carbon, hydrogen, and oxygen that serve as energy sources and structural materials for cells of all organisms.
  • Cardiac cycle One heartbeat; consists of atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause.
  • Cardiac muscle The type of muscle that is found in the walls of the heart. Cardiac muscle is striated but branched, unlike the straight-shaped striated skeletal muscle cells.
  • Cardiovascular system The human circulatory system consisting of the heart and the vessels that transport blood to and from the heart.
  • Carnivores Term applied to a heterotroph, usually an animal, that eats other animals. Carnivores function as secondary, tertiary, or top consumers in food chains and food webs.
  • Carotenoids Major group of accessory pigments in plants; includes beta carotene.
  • Carpals The bones that make up the wrist joint.
  • Carpels The female reproductive structures of a flower; consisting of the ovary, style, and stigma.
  • Carrageenan Chemical extracted from red algae that is added to commercial ice creams as an emulsifying agent.
  • Carrying capacity The maximum population size that can be regularly sustained by an environment; the point where the population size levels off in the logistic growth model.
  • Casparian strip In plants, an impermeable waxy layer between the cells of the endodermis that stops water and solutes from entering the xylem, except by passing through the cytoplasm of adjacent cells.
  • Cast Type of fossil preservation where the original material of the fossil has decayed and been replaced later by another material, much the way a plaster cast is made in a mold.
  • Catabolic reactions Reactions in cells in which existing chemical bonds are broken and molecules are broken down; generally produce energy, involve oxidation, and lead to a decrease in atomic order.
  • Catastrophism Once-popular belief that events in earth history had occurred in the past a sudden events and by processes unlike those operating today. Periods of catastrophic change were followed by long periods of little change. A subgroup, the Diluvialists, contended that Noah's Flood was the last of many floods which had occurred throughout earth history.
  • Cell body In a neuron, the part that contains the nucleus and most of the cytoplasm and the organelles.
  • Cell cycle The sequence of events from one division of a cell to the next; consists of mitosis (or division) and interphase.
  • Cell plate In plants, a membrane-bound space produced during cytokinesis by the vesicles of the Golgi apparatus. The cell plate fuses with the plasma membrane, dividing the cell into two compartments.
  • Cell theory One of the four (or five) unifying concepts in biology. The cell theory states that all living things are composed of at least one cell and that the cell is the fundamental unit of function in all organisms. Corollaries: the chemical composition of all cells is fundamentally alike; all cells arise from preexisting cells through cell division.
  • Cell wall Structure produced by some cells outside their cell membrane; variously composed of chitin, peptidoglycan, or cellulose.
  • Cell-mediated immunity Immune reaction directed against body cells that have been infected by viruses and bacteria; controlled by T cells.
  • Cells The smallest structural units of living matter capable of functioning independently.
  • Cellular respiration The transfer of energy from various molecules to produce ATP; occurs in the mitochondria of eukaryotes, the cytoplasm of prokaryotes. In the process, oxygen is consumed and carbon dioxide is generated.
  • Cellulose A polysaccharide that is composed of unbranched chains of glucose; the major structural carbohydrate of plants, insoluble in water, and indigestible in the human intestine.
  • Cenozoic Era The period of geologic time beginning after the end of the Mesozoic Era 65 million years ago and encompassing the present. Commonly referred to as the age of mammals.
  • Central nervous system (CNS) The division of the nervous system that includes the brain and spinal cord.
  • Centriole Paired cellular organelle which functions in the organization of the mitotic spindle during cell division in eukaryotes.
  • Centromere A specialized region on each chromatid to which kinetochores and sister chromatids attach.
  • Cephalization The concentration of sensory tissues in the anterior part of the body (head).
  • Cerebellum That part of the brain concerned with fine motor coordination and body movement, posture, and balance; is part of the hindbrain and is attached to the rear portion of the brain stem.
  • Cerebral cortex The outer layer of gray matter in the cerebrum; consists mainly of neuronal cell bodies and dendrites in humans; associated with higher functions, including language and abstract thought.
  • Cerebrum The part of the forebrain that includes the cerebral cortex; the largest part of the human brain.
  • Cervix The lower neck of the uterus that opens into the vagina.
  • Channels Transport proteins that act as gates to control the movement of sodium and potassium ions across the plasma membrane of a nerve cell.
  • Chemical equilibrium The condition when the forward and reverse reaction rates are equal and the concentrations of the products remain constant.
  • Chemiosmosis The process by which ATP is produced in the inner membrane of a mitochondrion. The electron transport system transfers protons from the inner compartment to the outer; as the protons flow back to the inner compartment, the energy of their movement is used to add phosphate to ADP, forming ATP.
  • Chemotrophs Organisms (usually bacteria) that derive energy from inorganic reactions; also known as chemosynthetic.
  • Chiasma The site where the exchange of chromosome segments between homologous chromosomes takes place (crossing-over) (pl.: chiasmata).
  • Chitin A polysaccharide contained in fungi; also forms part of the hard outer covering of insects.
  • Chlamydia A sexually transmitted disease caused by a parasitic bacterium that lives inside cells of the reproductive tract.
  • Chlorofluorocarbons (cfcs) Chemical substances used in refrigerators, air conditioners, and solvents that drift to the upper stratosphere and dissociate. Chlorine released by cfcs reacts with ozone, eroding the ozone layer.
  • Chlorophyll The pigment in green plants that absorbs solar energy.
  • Chlorophyll a The green photosynthetic pigment common to all photosynthetic organisms.
  • Chlorophyll b An accessory chlorophyll found in green algae and plants.
  • Chlorophyll c An accessory chlorophyll found in some protistans.
  • Chlorophyta The taxonomic division that contains what are commonly called the green algae.
  • Chloroplasts Disk-like organelles with a double membrane found in eukaryotic plant cells; contain thylakoids and are the site of photosynthesis. ATP is generated during photosynthesis by chemiosmosis.
  • Cholecystokinin A hormone secreted in the duodenum that causes the gallbladder to release bile and the pancreas to secrete lipase.
  • Chorion The two-layered structure formed from the trophoblast after implantation; secretes human chorionic gonadotropin.
  • Chorionic villi sampling (CVS) A method of prenatal testing in which fetal cells from the fetal side of the placenta (chorionic villi) are extracted and analyzed for chromosomal and biochemical defects.
  • Chromatid Generally refers to a strand of a replicated chromosome; consists of DNA and protein.
  • Chromatin A complex of DNA and protein in eukaryotic cells that is dispersed throughout the nucleus during interphase and condensed into chromosomes during meiosis and mitosis.
  • Chromosome theory of inheritance Holds that chromosomes are the cellular components that physically contain genes; proposed in 1903 by Walter Sutton and Theodore Boveri.
  • Chromosomes Structures in the nucleus of a eukaryotic cell that consist of DNA molecules that contain the genes.
  • Chrysophytes Protistan division that is referred to as the golden brown algae; includes the diatoms.
  • Cilia Hair-like organelles extending from the membrane of many eukaryotic cells; often function in locomotion (sing.: cilium).
  • Circadian rhythms Biorhythms that occur on a daily cycle.
  • Circulatory system One of eleven major body organ systems in animals; transports oxygen, carbon dioxide, nutrients, and waste products between cells and the respiratory system and carries chemical signals from the endocrine system; consists of the blood, heart, and blood vessels.
  • Circulatory system, closed A system that uses a continuous series of vessels of different sizes to deliver blood to body cells and return it to the heart; found in echinoderms and vertebrates.
  • Circulatory system, open A system in which the circulating fluid is not enclosed in vessels at all times; found in insects, crayfish, some mollusks, and other invertebrates.
  • Classes Taxonomic subcategories of phyla.
  • Clavicle The collar bone.
  • Cleavage furrow A constriction of the cell membrane at the equator of the cell that marks the beginning of cytokinesis in animal cells. The cell divides as the furrow deepens.
  • Climax community The stage in community succession where the community has become relatively stable through successful adjustment to its environment.
  • Clitoris A short shaft with a sensitive tip located where the labia minora meet; consists of erectile tissue and is important in female sexual arousal.
  • Clone An exact copy of a DNA segment; produced by recombinant DNA technology.
  • Closed community A community in which populations have similar range boundaries and density peaks; forms a discrete unit with sharp boundaries.
  • Codominance A type of inheritance in which heterozygotes fully express both alleles.
  • Codon A sequence of three nucleotides in messenger RNA that codes for a single amino acid.
  • Coelom In animals, a body cavity between the body wall and the digestive system that forms during preadult development.
  • Coelomates Animals that have a coelom or body cavity lined with mesoderm.
  • Coenzymes Chemicals required by a number of enzymes for proper functioning; also known as enzyme cofactors.
  • Cohesion The force that holds molecules of the same substance together.
  • Cohesion-adhesion theory Describes the properties of water that help move it through a plant. Cohesion is the ability of water molecules to stick together (held by hydrogen bonds), forming a column of water extending from the roots to the leaves; adhesion is the ability of water molecules to stick to the cellulose in plant cell walls, counteracting the force of gravity and helping to lift the column of water.
  • Collenchyma One of the three major cell types in plants; are elongated and have thicker walls than parenchyma cells and are usually arranged in strands; provide support and are generally in a region that is growing.
  • Colonial 1. Level of organization intermediate between unicellular and multicellular - organisms are composed of multiple cells but fail to exhibit specialization of those cells. Examples: Volvox, a colonial alga. Click HERE to view a series of images of Volvox. 2. Term applied to organisms that occur in a fixed location, with one generation growing atop previous generations, as in coral reefs.
  • Commensalism A symbiotic relationship in which one species benefits and the other is not affected.
  • Community All species or populations living in the same area.
  • Community age One of the factors that helps cause the latitudinal diversity gradient. Tropical communities have had more time to evolve because they have been less disrupted by advancing ice sheets and other relatively recent climatic changes.
  • Community simplification The reduction of overall species diversity in a community; generally caused by human activity.
  • Community succession The sequential replacement of species in a community by immigration of new species and by local extinction of old ones.
  • Compact bone The outer dense layer that forms the shaft of the long bones; made up of concentric layers of mineral deposits surrounding a central opening.
  • Companion cells Specialized cells in the phloem that load sugars into the sieve elements and help maintain a functional plasma membrane in the sieve elements.
  • Competition One of the biological interactions that can limit population growth; occurs when two species vie with each other for the same resource.
  • Competitive exclusion Competition between species that is so intense that one species completely eliminates the second species from the area.
  • Competitive release Occurs when one of two competing species is removed from an area, thereby releasing the remaining species from one of the factors that limited its population size.
  • Complement system A chemical defense system that kills microorganisms directly, supplements the inflammatory response, and works with, or complements, the immune system.
  • Complementary nucleotides The bonding preferences of nucleotides, Adenine with Thymine, and Cytosine with Guanine. Also referred to as complementary base pairing.
  • Complete dominance The type of inheritance in which both heterozygotes and dominant homozygotes have the same phenotype.
  • Complete flower Condition in which all flower parts are present. Example: lily.
  • Compound A substance formed by two or more elements combined in a fixed ratio.
  • Compound leaf A leaf in which the blade forms small leaflets. Compound leaves that have several small leaflets originating from a central axis are termed pinnately compound; example: rose. Compound leaves that have their leaflets originating from a common point are termed palmately compound; example: palm.
  • Compression Type of fossilization in which the fossil is flattened (compressed)m by the weight of overlying sediment.
  • Conditioned response The response to a stimulus that occurs when an animal has learned to associate the stimulus with a certain positive or negative effect.
  • Cones Light receptors in primates' eyes that operate in bright light; provide color vision and visual acuity.
  • Conifers Group of gymnosperms that reproduce by cones and have needle-like leaves (in general); includes the pines.
  • Connective tissue Animal tissue composed of cells embedded in a matrix (gel, elastic fibers, liquid, or inorganic minerals). Includes loose, dense, and fibrous connective tissues that provide strength (bone, cartilage), storage (bone, adipose), and flexibility (tendons, ligaments).
  • Consumers The higher levels in a food pyramid; consist of primary consumers, which feed on the producers, and secondary consumers, which feed on the primary consumers.
  • Continuous variation Occurs when the phenotypes of traits controlled by a single gene cannot be sorted into two distinct phenotypic classes, but rather fall into a series of overlapping classes.
  • Contractile vacuole Organelle in many eukaryotes that acts as a bilge pump in the active transport of excess water from the cell.
  • Contrast In relation to microscopes, the ability to distinguish different densities of structures.
  • Convergent evolution The development of similar structures in distantly related organisms as a result of adapting to similar environments and/or strategies of life. Example: wings of birds and insects, the body shape of dolphins, sharks, and the extinct marine reptiles known as ichthyosaurs.
  • Convergent plate boundary The boundary between two plates that are moving toward one another.
  • Coprolites Fossilized feces.
  • Cork The outer layer of the bark in woody plants; composed of dead cells.
  • Cork cambium A layer of lateral meristematic tissue between the cork and the phloem in the bark of woody plants.
  • Coronary arteries Arteries that supply the heart's muscle fibers with nutrients and oxygen.
  • Corpus callosum Tightly bundled nerve fibers that connect the right and left hemispheres of the cerebrum.
  • Corpus luteum A structure formed from the ovulated follicle in the ovary; secretes progesterone and estrogen.
  • Cortex 1) The outer part of an organ, e.g., the adrenal cortex, which produces several steroid hormones; 2) in plants, the region of the stem or root between the epidermis and the vascular bundle(s).
  • Cortisol The primary glucocorticoid hormone; released by the adrenal cortex.
  • Cotyledon A leaf-like structure that is present in the seeds of flowering plants; appears during seed germination and sometimes is referred to as a seed leaf.
  • Countercurrent flow An arrangement by which fish obtain oxygen from the water that flows through their gills. Water flows across the respiratory surface of the gill in one direction while blood flows in the other direction through the blood vessels on the other side of the surface.
  • Courtship behavior Behavioral sequences that precede mating.
  • Covalent bond A chemical bond created by the sharing of electrons between atoms.
  • Cranium The braincase; composed of several bones fitted together at immovable joints.
  • Cretaceous Period The geologic period between the Jurassic Period (140 milliojn years ago) and the Tertiary Period (beginning 65 million years ago). The Cretaceous was marked by a mass extinction that closed the period along with the reign of the nonavian dinosaurs.
  • Cristae Structures formed by the folding of the inner membrane of a mitochondrion (sing.: crista).
  • Crossing-over During the first meiotic prophase, the process in which part of a chromatid is physically exchanged with another chromatid to form chromosomes with new allele combinations.
  • Crossopterygians A type of lobe-finned fish with lungs that were ancestral to amphibians.
  • Crustaceans A large taxonomic class of arthropods that includes lobsters, shrimps, and crabs.
  • Cuticle A film composed of wax and cutin that occurs on the external surface of plant stems and leaves and helps to prevent water loss.
  • Cyanobacteria Blue-green bacteria; unicellular or filamentous chains of cells that carry out photosynthesis.
  • Cycadeoids A group of gymnosperm seed plants not closely rated to, but superficially similar to, the cycads. Cycads and cycadeoids were dominant floristic elements of early and middle Mesozoic landscapes. This groupo is also known as the Bennettitaleans.
  • Cycads Group of gymnosperm seed plants that have large fern-like leaves and reproduce by cones but not flowers.
  • Cycle A recurring sequence of events; e. G., the secretion of certain hormones at regular intervals.
  • Cyclin A protein found in the dividing cells of many organisms that acts as a control during cell division.
  • Cystic fibrosis An autosomal recessive genetic disorder that causes the production of mucus that clogs the airways of the lungs and the ducts of the pancreas and other secretory glands.
  • Cytokinesis The division of the cytoplasm during cell division.
  • Cytokinins A group of hormones that promote cell division and inhibit aging of green tissues in plants.
  • Cytology The branch of biology dealing with cell structure.
  • Cytoplasm The viscous semiliquid inside the plasma membrane of a cell; contains various macromolecules and organelles in solution and suspension.
  • Cytosine One of the pyrimidine nitrogenous bases occurring in both DNA and RNA.
  • Cytoskeleton A three-dimensional network of microtubules and filaments that provides internal support for the cells, anchors internal cell structures, and functions in cell movement and division.
  • Cytoxic T cells T cells that destroy body cells infected by viruses or bacteria; also attack bacteria, fungi, parasites, and cancer cells and will kill cells of transplanted organs if they are recognized as foreign; also known as killer T cells.
  • D
  • Dark reactions The photosynthetic process in which food (sugar) molecules are formed from carbon dioxide from the atmosphere with the use of ATP; can occur in the dark as long as ATP is present.
  • Death rate The ratio between deaths and individuals in a specified population at a particular time.
  • Decay series Most radioisotopes do not decay into a stable daughter element in one single decay, but rather through a series of radioactive intermediaries.
  • Deciduous Term applied to trees that lose the leaves and have a dormancy period at least once per year.
  • Deletion The loss of a chromosome segment without altering the number of chromosomes.
  • Dendrites Short, highly branched fibers that carry signals toward the cell body of a neuron.
  • Dendrochronology The process of determining the age of a tree or wood used in structures by counting the number of annual growth rings.
  • Deoxyribonucleic acid (DNA) A nucleic acid composed of two polynucleotide strands wound around a central axis to form a double helix; the repository of genetic information. Nucleic acid that functions as the physical carrier of inheritance for 99% of all species. The molecule is double-stranded and composed of two strands in an antiparallel and complementary arrangement. The basic unit, the nucleotide, consists of a molecule of deoxyribose sugar, a phosphate group, and one of four nitrogenous bases.
  • Deoxyribose Five-carbon sugar found in nucleotides of DNA.
  • Depth diversity gradient The increase in species richness with increasing water depth until about 2000 meters below the surface, where species richness begins to decline.
  • Dermal system Plant organ system that provides the covering for the plant.
  • Dermis One of the two layers of skin; a connective tissue layer under the epidermis containing elastic and collagen fibers, capillary networks, and nerve endings.
  • Desert biome Characterized by dry conditions and plants and animals that have adapted to those conditions; found in areas where local or global influences block rainfall.
  • Desmosome A circular region of membrane cemented to an adjacent membrane by a molecular glue made of polysaccharides; found in tissues that undergo stretching.
  • Deuterostomes Animals in which the first opening that appears in the embryo becomes the anus while the mouth appears at the other end of the digestive system. Main groups include chordates and echinoderms.
  • Devonian Period of geologic time from 410 - 360 million years before the present. Life on land diversified, with the amphibians appearing late in this period. Plants underwent major changes, including the development of forests and seeds. In the water, fish diversified into all modern groups as well as numerous now-extinct forms.
  • Diabetes mellitus, Types I and II A disorder associated with defects in insulin action. Type I diabetes is characterized by inadequate insulin secretion; Type II diabetes is characterized by impaired insulin secretion in response to elevated blood glucose levels or by loss of sensitivity to insulin by target cells.
  • Diaphragm A dome-shaped muscle that separates the thoracic and abdominal cavities.
  • Diastole The filling of the ventricle of the heart with blood.
  • Diatomaceous earth Fossilized deposits of diatoms; used for abrasives, polishes and as a filtering agent.
  • Dicots One of the two main types of flowering plants; characterized by having two cotyledons, floral organs arranged in cycles of four or five, and leaves with reticulate veins; include trees (except conifers) and most ornamental and crop plants.
  • Dictyosomes Organelles in plant cells composed of a series of flattened membrane sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum. Also known as the Golgi Apparatus.
  • Diencephalon Part of the forebrain; consists of the thalamus and hypothalamus.
  • Diffusion The spontaneous movement of particles from an area of higher concentration to an area of lower concentration.
  • Digestion The process of breaking down food into its molecular and chemical components so that these nutrient molecules can cross plasma membranes.
  • Digestive system One of eleven major body organ systems in animals; converts food from the external environment into nutrient molecules that can be used and stored by the body and eliminates solid wastes; involves five functions: movement, secretion, digestion, absorption, and elimination.
  • Dihybrid cross In genetics, a cross that involves two sets of characteristics.
  • Dinoflagellates Single-celled to colonial protistans characterized by two flagella, one girdling the cell and the other trailing the cell. Some dinoflagellates exist in coral, in a symbiotic relationship. These dinoflagellates are termed the zooxanthellae. Other dinoflagellates occur in such high numbers that the water is colored red, a phenomenon known as a red tide.
  • Dinosaurs Any of the Mesozoic diapsids (once considered to be reptiles) belonging to the groups designated as ornithischians and saurischians.
  • Dioecious Term applied to plants having separate male and female plants.
  • Diploid Cells that contain homologous chromosomes. The number of chromosomes in the cells is the diploid number and is equal to 2n (n is the number of homologous pairs).
  • Directional selection A process of natural selection that tends to favor phenotypes at one extreme of the phenotypic range.
  • Disaccharides 1. Sugars made up of two monosaccharides held together by a covalent bond; e.g., sucrose and lactose. 2. Type of sugar (saccharide) composed of two sugar molecules bonded together with an ester (covalent) bond examples include sucrose, maltose, and lactose.
  • Discontinuous variation Occurs when the phenotypes of traits controlled by a single gene can be sorted into two distinct phenotypic classes.
  • Disruptive selection A process of natural selection that favors individuals at both extremes of a phenotypic range.
  • Distal tubule The section of the renal tubule where tubular secretion occurs.
  • Divergent evolution The divergence of a single interbreeding population or species into two or more descendant species.
  • Divergent plate boundary The boundary between two tectonic plates that are moving apart.
  • Diversity The different types of organisms that occur in a community.
  • DNA hybridization The formation of hybrid DNA molecules that contain a strand of DNA from two different species. The number of complementary sequences in common in the two strands is an indication of the degree of relatedness of the species.
  • DNA ligase In recombinant DNA technology, an enzyme that seals together two DNA fragments from different sources to form a recombinant DNA molecule.
  • DNA polymerase In DNA replication, the enzyme that links the complementary nucleotides together to form the newly synthesized strand.
  • Dominance The property of one of a pair of alleles that suppresses the expression of the other member of the pair in heterozygotes.
  • Dominance hierarchy A social structure among a group of animals in which one is dominant and the others have subordinate nonbreeding positions.
  • Dominant Refers to an allele of a gene that is always expressed in heterozygotes.
  • Double fertilization A characteristic of angiosperms in which a pollen tube carries two sperm cells to the female gametophyte in the ovule. One sperm cell fuses with the egg cell and gives rise to a diploid embryo The other sperm cell fuses with the two polar cells to form a triploid cell that develops into the endosperm.
  • Duodenum The upper part of the small intestine.
  • Duplication An extra copy of a chromosome segment without altering the number of chromosomes.
  • Dystrophin Protein making up only 0.002% of all protein in skeletal muscle but which appears vital for proper functioning of the muscle. Sufferers of muscular dystrophy appear to lack dystrophin.
  • E
  • Eccrine glands Sweat glands that are linked to the sympathetic nervous system and are widely distributed over the body surface.
  • Ecological niche The role an organism occupies and the function it performs in an ecosystem; closely associated with feeding.
  • Ecological time A timescale that focuses on community events that occur on the order of tens to hundreds of years.
  • Ecology The study of how organisms interact with each other and their physical environment.
  • Ecosystem The community living in an area and its physical environment.
  • Ecotones Well-deþned boundaries typical of closed communities.
  • Ecotype A subdivision of a species; a stage in the formation of a species such that reproductive isolation has occurred.
  • Ectoderm The outer layer of cells in embryonic development; gives rise to the skin, brain, and nervous system. Also, the outermost tissue layer in þatworms.
  • Ectotherms Animals with a variable body temperature that is determined by the environment. Examples: fish, frogs, and reptiles.
  • Effector In a closed system, the element that initiates an action in response to a signal from a sensor. In human systems, a muscle or gland often serves as an effector.
  • Ejaculatory duct In males, a short duct that connects the vas deferens from each testis to the urethra.
  • Electron A subatomic particle with a negative charge. Electrons circle the atom's nucleus in regions of space known as orbitals.
  • Electron acceptor A molecule that forms part of the electron transport system that transfers electrons ejected by chlorophyll during photosynthesis. Part of the energy carried by the electrons is transferred to ATP, part is transferred to NADPH, and part is lost in the transfer system.
  • Electron transport 1) A series of coupled oxidation/reduction reactions where electrons are passed like hot potatoes from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH). ATP is formed by this process. 2) coupled series of oxidation/reduction reactions during which ATP is generated by energy transfer as electrons move from high reducing state to lower reducing state. |
  • Electrostatic attraction The attraction between atoms of opposite charge that holds the atoms together in ionic bonds.
  • Element A substance composed of atoms with the same atomic number; cannot be broken down in ordinary chemical reactions.
  • Elongation During protein synthesis, the growth of the polypeptide chain through the addition of amino acids; the second step in translation. |
  • Embryo sac Alternate term applied to the angiosperm female gametophyte contained within a megaspore.
  • Embryo Term applied to the zygote after the beginning of mitosis that produces a multicellular structure.
  • Emphysema Lung disease characterized by shortness of breath, often associated with smoking.
  • Endergonic Chemical reactions that require energy input to begin.
  • Endochondral ossification The process by which human bones form from cartilage.
  • Endocrine system One of eleven major body organ systems in animals; a system of glands that works with the nervous system in controlling the activity of internal organs, especially the kidneys, and in coordinating the long-range response to external stimuli.
  • Endocytosis The incorporation of materials from outside the cell by the formation of vesicles in the plasma membrane. The vesicles surround the material so the cell can engulf it.
  • Endoderm The inner layer of cells in embryonic development that gives rise to organs and tissues associated with digestion and respiration. Also, the inner tissue layer in þatworms.
  • Endodermis A layer of cells surrounding the vascular cylinder of plants.
  • Endometrium The inner lining of the uterus.
  • Endoplasmic reticulum (ER) A network of membranous tubules in the cytoplasm of a cell; involved in the production of phospholipids, proteins, and other functions. Rough ER is studded with ribosomes; smooth ER is not.
  • Endoskeleton An internal supporting skeleton with muscles on the outside; in vertebrates, consists of the skull, spinal column, ribs, and appendages.
  • Endosperm A food storage tissue that provides nutrients to the developing embryo in angiosperms; formed from the triploid cell produced when a sperm cell fertilizes the central cell. Some endosperm is solid (as in corn), some is liquid (as in coconut).
  • Endosymbiosis Theory that attempts to explain the origin of the DNA-containing mitochondria and chloroplasts in early eukaryotes by the engulfing of various types of bacteria that were not digested but became permanent additions to the ancestral "eukaryote".
  • Endothermic A reaction that gives off energy. The product is in a lower energy state than the reactants.
  • Endotherms Animals that have the ability to maintain a constant body temperature over a wide range of environmental conditions.
  • Endothermy The internal control of body temperature; the ability to generate and maintain internal body heat.
  • Energy The ability to bring about changes or to do work.
  • Energy flow The movement of energy through a community via feeding relationships.
  • Energy of activation The minimum amount of energy required for a given reaction to occur; varies from reaction to reaction.
  • Entropy The degree of disorder in a system. As energy is transferred from one form to another, some is lost as heat; as the energy decreases, the disorder in the system&emdash;and thus the entropy&emdash;increases.
  • Enzymes Protein molecules that act as catalysts in biochemical reactions.
  • Eon The longest umit of geological time.
  • Epidermis 1. The outermost layer of skin consisting of several layers of epithelial cells&emdash;notably, keratinocytes&emdash;and, in the inner layer of the epidermis, basal cells and melanocytes. 2. The outer layer of cells in the plant body, often covered by a waxy cuticle.
  • Epididymis A long, convoluted duct on the testis where sperm are stored.
  • Epiglottis A þap of tissue that closes off the trachea during swallowing.
  • Epinephrine A hormone produced by the adrenal medulla and secreted under stress; contributes to the "Þght or þight" response.
  • Epistasis The masking of the effects of one gene by the action of another, example: widow's peak masked by the baldness gene.
  • Epithelial tissue Cells in animals that are closely packed in either single or multiple layers, and which cover both internal and external surfaces of the animal body. Also referred to as epithelium.
  • Epoch Subdivision of a geological period.
  • Eras One of the major divisions of the geologic time scale.
  • Erythrocytes Red blood cells; doubly concave, enucleated cells that transport oxygen in the blood.
  • Esophagus The muscular tube extending between and connecting the pharynx to the stomach.
  • Estrogen A female sex hormone that performs many important functions in reproduction.
  • Ethylene A gaseous plant hormone that stimulates fruit ripening and the dropping of leaves.
  • Eubacteria The subunit of the Monera that includes the true bacteria such as E. Coli. One of the three major groups of prokaryotes in the Kingdom Monera. The eubacteria have cell walls containing peptidoglycan.
  • Euglenoids Term applied to a division of protozoans that have one long flagellum, no cell wall, and which may have chloroplasts.
  • Eukaryote A type of cell found in many organisms including single-celled protists and multicellular fungi, plants, and animals; characterized by a membrane-bounded nucleus and other membraneous organelles; an organism composed of such cells. The first eukaryotes are encountered in rocks approximately 1.2-1.5 billion years old.
  • Euphotic zone The upper part of the marine biome where light penetrates and photosynthesis occurs; usually extends to about 200 meters below the water surface.
  • Eutrophication "Runaway" growth of aquatic plants that occurs when agricultural fertilizers containing phosphorus and nitrogen run off into lakes and ponds; also ultimately increases the plant death rate with the result that the bacterial decomposition of the dead plants uses up oxygen, causing Þsh and other organisms to suffocate.
  • Evaporation The part of the hydrologic cycle in which liquid water is converted to vapor and enters the atmosphere.
  • Evolution 1) The change in life over time by adaptation, variation, over-reproduction, and differential survival/reproduction, a process referred to by Charles Darwin and Alfred Wallace as natural selection. 2) Descent with modification.
  • Evolutionary tree A diagram showing the evolutionary history of organisms based on differences in amino acid sequences. Organisms with fewer differences are placed closer together while those with more differences are further apart.
  • Excretion The process of removing the waste products of cellular metabolism from the body.
  • Excretory system One of eleven major body systems in animals; regulates the volume and molecular and ionic constitution of internal body þuids and eliminates metabolic waste products from the internal environment.
  • Exine Outer covering of pollen grains, often containing sporopollenin, an acid-resistant polysaccharide that allows pollen grains to become fossils.
  • Exocytosis The process in which a membrane-enclosed vesicle Þrst fuses with the plasma membrane and then opens and releases its contents to the outside.
  • Exon The DNA bases that code for an amino acid sequence. Exons are separated by introns that code for no amino acid sequences.
  • Exoskeleton A hard, jointed, external covering that encloses the muscles and organs of an organism; typical of many arthropods including insects.
  • Exothermic A reaction where the product is at a higher energy level than the reactants.
  • Exponential rate An extremely rapid increase, e.g., in the rate of population growth.
  • Expression In relation to genes, the phenotypic manifestation of a trait. Expression may be age-dependent (e.g., Huntington disease) or affected by environmental factors (e.g., dark fur on Siamese cats).
  • Extinction The elimination of all individuals in a group, both by natural (dinosaurs, trilobites) and human-induced (dodo, passenger pigeon, liberals [:)]) means.
  • Extracellular digestion A form of digestion found in annelids, crustaceans, and chordates including vertebrates; takes place within the lumen of the digestive system, and the resulting nutrient molecules are transferred into the blood or body þuid.
  • Extracellular route Path taken by water through the root in which water moves through the spaces between cell walls of the cortex parenchyma.
  • Eyespot 1. A pigmented photoreceptor in euglenoids. The eyespot senses light and orients the cell for maximum rates of photosynthesis. . Term applied to a photosenstive area in starfish.
  • F
  • Families 1. In taxonomy, term applied to subcategories within orders. 2. Term applied to a group of similar things, such as languages, chromosomes, etc.
  • Fats 1. Triglycerides that are solid at room temperature. 2. A legendary pool player from Minnesota?
  • Fauna Term referring collectively to all animals in an area. The zoological counterpart of flora.
  • Feces Semisolid material containing undigested foods, bacteria, bilirubin, and water that is produced in the large intestine and eliminated from the body. Frequently noted as "hitting the fan".
  • Femur The upper leg bone.
  • Fermentation The synthesis of ATP in the absence of oxygen through glycolysis. |
  • Fertilization The fusion of two gametes (sperm and ovum) to produce a zygote that develops into a new individual with a genetic heritage derived from both parents. Strictly speaking, fertilization can be divided into the fusion of the cells (plasmogamy) and the fusion of nuclei (karyogamy).
  • Fibroblast A term applied to a cell of connective tissue that is separated from similar cells by some degree of matrix material; fibroblasts secrete elastin and collagen protein fibers.
  • Fibrous root A root system found in monocots in which branches develop from the adventitious roots, forming a system in which all roots are about the same size and length.
  • Filaments Slender, thread-like stalks that make up the stamens of a þower; topped by the anthers.
  • Filter feeders Organisms such as sponges that feed by removing food from water that Þlters through their body.
  • Filtration The removal of water and solutes from the blood; occurs in the glomerulus of the nephron.
  • First law of thermodynamics (conservation) Energy is neither created nor destroyed, it changes from one form to another.
  • Fitness A measure of an individual's ability to survive and reproduce; the chance that an individual will leave more offspring in the next generation than other individuals.
  • Flagella long, whip-like locomotion organelles found in both prokaryotic and eukaryotic cells; sing.: flagellum. Eukaryotic flagella have an internal arrangement of microtubules in a 9 + 2 array.
  • Flame cell A specialized cell at the blind end of a nephridium that Þlters body þuids.
  • Flora Term collectively applied to all of the plants in an area. The botanical counterpart of fauna.
  • Flowers The reproductive structures in angiosperm sporophytes where gametophytes are generated.
  • Fluid feeders Animals such as aphids, ticks, and mosquitoes that pierce the body of a host plant or animal and obtain food from ingesting its þuids.
  • Fluid-mosaic Widely accepted model of the plasma membrane in which proteins (the mosaic) are embedded in lipids (the þuid).
  • Follicles (ovary) Structures in the ovary consisting of a developing egg surrounded by a layer of follicle cells.
  • Follicles (thyroid) Spherical structures that make up the thyroid gland; contain a gel-like colloid surrounded by a single layer of cells, which secrete thyroglobulin into the colloid.
  • Follicle-stimulating hormone (FSH) A hormone secreted by the anterior pituitary that promotes gamete formation in both males and females.
  • Fontanels Membranous areas in the human cranial bones that do not form bony structures until the child is 14 to 18 months old; know as "soft spots."
  • Food chain The simplest representation of energy þow in a community. At the base is energy stored in plants, which are eaten by small organisms, which in turn are eaten by progressively larger organisms; the food chain is an oversimpliþcation in that most animals do not eat only one type of organism.
  • Food pyramid A way of depicting energy þow in an ecosystem; shows producers (mostly plants or other phototrophs) on the Þrst level and consumers on the higher levels.
  • Food web A complex network of feeding interrelations among species in a natural ecosystem; more accurate and more complex depiction of energy þow than a food chain.
  • Foraminifera Single-celled protists that secrete a shell or test. Accumulations of the shells of dead foraminifera and other microscopic sea creatures form chalk deposits. |
  • Forebrain The part of the brain that consists of the diencephalon and cerebrum.
  • Fossil 1. The remains or traces of prehistoric life preserved in rocks of the Earth's crust. 2. Any evidence of past life.
  • Fossil fuels Fuels that are formed in the Earth from plant or animal remains; e.g., coal, petroleum, and natural gas.
  • Fossil record 1. The observed remains of once-living organisms taken as a whole. 2. The album Meet the Beatles.
  • Founder effect The difference in gene pools between an original population and a new population founded by one or a few individuals randomly separated from the original population, as when an island population is founded by one or a few individuals; often accentuates genetic drift.
  • Fovea The area of the eye in which the cones are concentrated.
  • Freshwater biome The aquatic biome consisting of water containing fewer salts than the waters in the marine biome; divided into two zones: running waters (rivers, streams) and standing waters (lakes, ponds).
  • Frontal lobe The lobe of the cerebral cortex that is responsible for motor activity, speech, and thought processes.
  • Fruit A ripened ovary wall produced from a flower.
  • Fucoxanthin Brown accessory pigment found in and characteristic of the brown algae.
  • Fungi Nonmobile, heterotrophic, mostly multicellular eukaryotes, including yeasts and mushrooms.
  • G
  • Gaia A hypothetical superorganism composed of the Earth's four spheres: the biosphere, hydrosphere, lithosphere, and atmosphere.
  • Gametes Haploid reproductive cells (ovum and sperm).
  • Gametophyte The haploid stage of a plant exhibiting alternation of generations, generates gametes by the process of mitosis.
  • Ganglia Clusters of neurons that receive and process signals; found in þatworms and earthworms.
  • Gap junctions Junctions between the plasma membranes of animal cells that allow communication between the cytoplasm of adjacent cells.
  • Gastric pits The folds and grooves into which the stomach lining is arranged.
  • Gastrin A hormone produced by the pyloric gland area of the stomach that stimulates the secretion of gastric acids.
  • Gastroesophageal sphincter A ring of muscle at the junction of the esophagus and the stomach that remains closed except during swallowing to prevent the stomach contents from entering the esophagus.
  • Gene pool The sum of all the genetic information carried by members of a population. Note: there is no diving in the deep end of the gene pool!
  • Gene therapy The insertion of normal or genetically altered genes into cells through the use of recombinant DNA technology; usually done to replace defective genes as part of the treatment of genetic disorders.
  • Genera Taxonomic subcategories within families (sing.: genus), composed of one or more species.
  • Genes speciþc segments of DNA that control cell structure and function; the functional units of inheritance. Sequence of DNA bases usually code for a polypeptide sequence of amino acids.
  • Genetic code The linear series of nucleotides, read as triplets, that speciþes the sequence of amino acids in proteins. Each triplet speciþes an amino acid, and the same codons are used for the same amino acids in almost all life-forms, an indication of the universal nature of the code.
  • Genetic divergence The separation of a population's gene pool from the gene pools of other populations due to mutation, genetic drift, and selection. Continued divergence can lead to speciation.
  • Genetic drift Random changes in the frequency of alleles from generation to generation; especially in small populations, can lead to the elimination of a particular allele by chance alone.
  • Genetic maps Diagrams showing the order of and distance between genes; constructed using crossover information.
  • Genetics The study of the structure and function of genes and the transmission of genes from parents to offspring.
  • Genital herpes A sexually transmitted disease caused by the herpes virus; results in sores on the mucus membranes of the mouth or genitals.
  • Genome 1. The set of genes carried by an individual. 2. The set of genes shared by members of a reproductive unit such as a population or species.
  • Genotype The genetic (alleleic) makeup of an organism with regard to an observed trait.
  • Geographic isolation Separation of populations of a species by geographic means (distance, mountains, rivers, oceans, etc.) That lead to reproductive isolation of those populations.
  • Geographic range The total area occupied by a population.
  • Geological time The span of time that has passed since the formation of the Earth and its physical structures; also, a timescale that focuses on events on the order of thousands of years or more.
  • Geotropism Plants' response to gravity: roots grow downward, showing positive geotropism, while shoots grow upward in a negative response.
  • Germ cells Collective term for cells in the reproductive organs of multicellular organisms that divide by meiosis to produce gametes.
  • Gestation Period of time between fertilization and birth of an animal. Commonly called pregnancy.
  • Gibberellins A group of hormones that stimulate cell division and elongation in plants. Gibberellic acid (GA), the first of this class to be discovered, causes bolting (extreme elongation) of stems. GA is also applied to certain plants to promote larger fruits.
  • Gill slits Opening or clefts between the gill arches in Þsh. Water taken in by the mouth passes through the gill slits and bathes the gills. Also, rudimentary grooves in the neck region of embryos of air-breathing vertebrates such as humans; a characteristic of chordates.
  • Ginkgos Group of seed plants today restricted to a single genus (Ginkgo biloba); ginkgos were more diverse during the Mesozoic Era.
  • Glial cells Nonconducting cells that serve as support cells in the nervous system and help to protect neurons.
  • Glomerulus A tangle of capillaries that makes up part of the nephron; the site of Þltration.
  • Glucagon A hormone released by the pancreas that stimulates the breakdown of glycogen and the release of glucose, thereby increasing blood levels of glucose. Glucagon and insulin work together to maintain blood sugar levels.
  • Glucocorticoids A group of steroid hormones produced by the adrenal cortex that are important in regulating the metabolism of carbohydrates, fats, and proteins.
  • Glucose A six-carbon single sugar; the most common energy source.
  • Glycogen Polysaccharide consisting of numerous monosaccharide glucoses linked together. The animal equivalent of starch.
  • Glycolipids Polysaccharides formed of sugars linked to lipids, a part of the cell membrane.
  • Glycolysis The universal cellular metabolic process in the cell's cytoplasm where 6-carbon glucose is split into two 3-carbon pyruvate molecules, and some ATP and NADH are produced. Click here to view the On-Line Biology Book chapter on glycolysis.
  • Glycoproteins Polysaccharides formed of sugars linked to proteins. On the outer surface of a membrane, they act as receptors for molecular signals originating outside the cell.
  • Gnetales Group of seed plants restricted to three genera today (Gnetum, Ephedra, and Welwitschia); the possible outgroup for flowering plants.
  • Golden brown algae Common name applied to the protistan division Chrysophyta.
  • Golgi complex Organelles in animal cells composed of a series of þattened sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum.
  • Gonadotropin-releasing hormone (gnrh) A hormone produced by the hypothalamus that controls the secretion of luteinizing hormone.
  • Gonadotropins Hormones produced by the anterior pituitary that affect the testis and ovary; include follicle-stimulating hormone and luteinizing hormone.
  • Gonads The male and female sex organs.
  • Gondwana Name applied to the ancient (Paleozoic-early Mesozoic) southern hemisphere supercontinent that rifted apart to form present-day Antarctica, India, Africa, Australia, and South America. The southern part of Pangaea.
  • Gonorrhea A sexually transmitted disease that is caused by a bacterium that inþames and damages epithelial cells of the reproductive system.
  • Grana A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloroplasts.
  • Grasslands biome Occurs in temperate and tropical regions with reduced rainfall or prolonged dry seasons; characterized by deep, rich soil, an absence of trees, and large herds of grazing animals.
  • Green algae Common name for algae placed in the division Chlorophyta.
  • Greenhouse effect The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radiate it back to the surface; so-called because the gases are transparent to sunlight but not to heat and thus act like the glass in a greenhouse.
  • Ground system Plant tissue system, composed mainly of parenchyma cells with some collenchyma and sclerenchyma cells, that occupies the space between the epidermis and the vascular system; is involved in photosynthesis, water and food storage, and support; one of the four main tissue systems in plants.
  • Growth hormone (GH) A peptide hormone produced by the anterior pituitary that is essential for growth.
  • Growth rings Features of woody stems produced by plants growing in areas with seasonal (as opposed to year-long) growth. The growth ring marks the position of the vascular cambium at the cessation of the previous year's growth.
  • Guanine One of the nitrogenous bases in nucleic acids, guanine is one of the two purine bases.
  • Guard cells Specialized epidermal cells that flank stomates and whose opening and closing regulates gas exchange and water loss. |
  • Gymnosperms Flowerless, seed-bearing land plants; the Þrst seed plants; living groups include the pines, ginkgos, and cycads. Naked seeds.
  • Gynoecium Collective term for all of the carpels (or pistils) in a flower. Some flowers have many pistils that are partially or wholly fused.
  • H
  • Habitat disruption A disturbance of the physical environment in which a population lives.
  • Hair bulb The base of a hair; contains cells that divide mitotically to produce columns of hair cells.
  • Hair root The portion of a hair that extends from the skin's surface to the hair bulb.
  • Hair shaft The portion of a hair that extends above the skin's surface.
  • Half-life The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element.
  • Halophiles A group of archaebacteria that are able to tolerate high salt concentrations.
  • Haploid Cells that contain only one member of each homologous pair of chromosomes (haploid number = n). At fertilization, two haploid gametes fuse to form a single cell with a diploid number of chromosomes.
  • Hardwoods Term applied to dicot trees, as opposed to softwoods, a term applied to gymnosperms.
  • Haversian canal The central opening of compact bone; contains nerves and blood vessels.
  • Heart The multicellular, chambered, muscular structure that pumps blood through the circulatory system by alternately contracting and relaxing.
  • Heartwood Inner rings of xylem that have become clogged with metabolic by-products and no longer transport water; visible as the inner darker areas in the cross section of a tree trunk.
  • Helper T cells A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present.
  • Hemizygous Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans).
  • Hemoglobin A red pigment in red blood cells that can bind with oxygen and is largely responsible for the blood's oxygen-carrying capacity. Hemoglobin is composed of four polypeptide chains, two alpha (a) and two beta (b) chains.
  • Hemophilia A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting factors, usually Factor VII. Hemophiliacs suffer from an inability to clot their blood.
  • Hepatitis B A potentially serious viral disease that affects the liver; can be transmitted through sexual contact or through contact with infected blood.
  • Herbaceous Term applied to a nonwoody stem/plant with minimal secondary growth.
  • Herbivores Term pertaining to a heterotroph, usually an animal, that eats plants or algae. Herbivores function in food chains and food webs as primary consumers.
  • Heterogametic sex The sex with two different chromosomes, such as males in humans and Drosophila.
  • Heterotrophic Refers to organisms, such as animals, that depend on preformed organic molecules from the environment (or another organism) as a source of nutrients/energy.
  • Heterotrophs Organisms that obtain their nutrition by breaking down organic molecules in foods; include animals and fungi.
  • Heterozygous Having two different alleles (one dominant, one recessive) of a gene pair.
  • Histamine A chemical released during the inþammatory response that increases capillary blood þow in the affected area, causing heat and redness.
  • Histone proteins Proteins associated with DNA in eukaryote chromosomes.
  • Homeobox genes Pattern genes that establish the body plan and position of organs in response to gradients of regulatory molecules.
  • Homeostasis The ability to maintain a relatively constant internal environment.
  • Hominid Primate group that includes humans and all fossil forms leading to man only.
  • Hominoid Primate group that includes common ancestors of humans and apes.
  • Homologous structures Body parts in different organisms that have similar bones and similar arrangements of muscles, blood vessels, and nerves and undergo similar embryological development, but do not necessarily serve the same function; e.g., the þipper of a whale and the forelimb of a horse.
  • Homologues A pair of chromosomes in which one member of the pair is obtained from the organism's maternal parent and the other from the paternal parent; found in diploid cells. Also commonly referred to as homologous chromosomes.
  • Homozygous Having identical alleles for a given gene.
  • Hormones Chemical substances that are produced in the endocrine glands and travel in the blood to target organs where they elicit a response.
  • Human chorionic gonadotropin (hcg) A peptide hormone secreted by the chorion that prolongs the life of the corpus luteum and prevents the breakdown of the uterine lining.
  • Human Genome Project Federally funded project to determine the DNA base sequence of every gene in the human genome.
  • Human immunodeficiency virus (HIV) The retrovirus that attacks T-cells in the human immune system, destroying the body's defenses and allowing the development of AIDS.
  • Huntington disease A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 years; caused by an expansion of a trinucleotide repeat and inherited as a dominant trait.
  • Hydrogen bond A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charges.
  • Hydrophilic Water-loving. Term applied to polar molecules that can form a hydrogen bond with water.
  • Hydrophobic Water-fearing.Term applied to nonpolar molecules that cannot bond with water.
  • Hydrophytic leaves The leaves of plants that grow in water or under conditions of abundant moisture.
  • Hydrosphere The part of the physical environment that consists of all the liquid and solid water at or near the Earth's surface.
  • Hydrostatic skeleton Fluid-Þlled closed chambers that give support and shape to the body in organisms such as jellyþsh and earthworms. No to be confused with the water-vascular system of echinoderms.
  • Hypertension High blood pressure; blood pressure consistently above 140/90.
  • Hypertonic A solution having a high concentration of solute.
  • Hyphae The multinucleate or multicellular Þlaments that make up the mycelium (body) of a fungus (sing.: hypha).
  • Hypothalamus A region in the brain beneath the thalamus; consists of many aggregations of nerve cells and controls a variety of autonomic functions aimed at maintaining homeostasis.
  • Hypothesis An idea that can be experimentally tested; an idea with the lowest level of confidence.
  • Hypotonic A solution having a low concentration of solute.
  • I
  • Ice age Interval of geologic time between 2 million and 10,000 years ago during which the northern hemisphere experienced several episodes of continental glacial advance and retreat along with a climatic cooling. The icing over of Antarctica was also completed during this time.
  • Ileum The third and last section of the small intestine.
  • Immovable joint A joint in which the bones interlock and are held together by Þbers or bony processes that prevent the joint from moving; e.g., the bones of the cranium.
  • Immune system One of the eleven major body organ systems in vertebrates; defends the internal environment against invading microorganisms and viruses and provides defense against the growth of cancer cells.
  • Immunoglobulins The Þve classes of protein to which antibodies belong (igd, igm, igg, iga, ige).
  • Implantation The process in which the blastocyst embeds in the endometrium.
  • Incomplete dominance A type of inheritance in which the heterozygote has a phenotype intermediate to those of the homozygous parents.
  • Incomplete flower Condition in which one or more "typical" flower parts are absent. Example: grass flowers such as corn tassels which are male.
  • Incus One of the three bones comprising the middle ear of mammals.
  • Inflammation A reaction to the invasion of microorganisms through the skin or through the epithelial layers of the respiratory, digestive, or urinary system; characterized by four signs: redness, swelling, heat, and pain.
  • Inflammatory response The body's reaction to invading infectious microorganisms; includes an increase in blood þow to the affected area, the release of chemicals that draw white blood cells, an increased þow of plasma, and the arrival of monocytes to clean up the debris.
  • Ingestive feeders Animals that ingest food through a mouth.
  • Inheritance of acquired characteristics Lamarck's view that features acquired during an organism's lifetime would be passed on to succeeding generations, leading to inheritable change in species over time.
  • Initiation The Þrst step in translation; occurs when a messenger RNA molecule, a ribosomal subunit, and a transfer RNA molecule carrying the Þrst amino acid bind together to form a complex; begins at the start codon on mrna.
  • Initiation codon (AUG) Three-base sequence on the messenger RNA that codes for the amino acid methionine; the start command for protein synthesis.
  • Insertion A type of mutation in which a new DNA base is inserted into an existing sequence of DNA bases. This shifts the reference frame in protein synthesis, resulting (sometimes) in altered amino acid sequences.
  • Insulin A hormone secreted by the pancreas that stimulates the uptake of glucose by body cells. Insulin works antagonistically with glucagon to control blood sugar levels.
  • Integration The process of combining incoming information; one of the functions of the nervous system.
  • Integument Something that covers or encloses, e.g., the skin.
  • Integumentary system The skin and its derivatives (hair, nails, feathers, horns, antlers, and glands), which in multicellular animals protect against invading foreign microorganisms and prevent the loss or exchange of internal þuids.
  • Interferons Proteins released by cells in response to viral infection; activate the synthesis and secretion of antiviral proteins.
  • Internal environment In multicellular organisms, the aqueous environment that is outside the cells but inside the body.
  • Interneurons Neurons that process signals from one or more sensory neurons and relay signals to motor neurons. Aka connector neurons.
  • Internodes The stem regions between nodes in plants.
  • Interphase The period between cell divisions when growth and replacement occur in preparation for the next division; consists of gap 1 (G1), synthesis (S), and gap 2 (G2).
  • Interstitial Being situated within a particular organ or tissue.
  • Interstitial fluid Fluid surrounding the cells in body tissues; provides a path through which nutrients, gases, and wastes can travel between the capillaries and the cells.
  • Intracellular digestion A form of digestion in which food is taken into cells by phagocytosis; found in sponges and most protozoa and coelenterates.
  • Intracellular parasites Viruses that enter a host cell and take over the host's cellular machinery to produce new viral particles.
  • Intracellular route Path taken by water through the cells of the root between the epidermis and the xylem, moving through plasmodesmata.
  • Intron In eukaryotes, bases of a gene transcribed but later excised from the mrna prior to exporting from the nucleus and subsequent translation of the message into a polypeptide.
  • Inversion A reversal in the order of genes on a chromosome segment.
  • Ion An atom that has lost or gained electrons from its outer shell and therefore has a positive or negative charge, respectively; symbolized by a superscript plus or minus sign and sometimes a number, e.g., H+, Na+1, Cl-2.
  • Ionic bond A chemical bond in which atoms of opposite charge are held together by electrostatic attraction.
  • Isotonic Term applied to two solutions with equal solute concentrations.
  • Isotopes Atoms with the same atomic number but different numbers of neutrons; indicated by adding the mass number to the element's name, e.g., carbon 12 or 12C.
  • J
  • Jejunum The second portion of the small intestine. Also, a popular month for weddings!
  • Jurassic Period Middle period of the Mesozoic Era, between 185-135 million years ago. Characterized by the (possible) origin of angiosperms and the continued split of the worldwide supercontinent of Pangaea.
  • K
  • Karyotype The chromosomal characteristics of a cell; also, a representation of the chromosomes aligned in pairs.
  • Keratin A Þbrous protein that Þlls mature keratinocytes near the skin's surface.
  • Keratinocytes The basic cell type of the epidermis; produced by basal cells in the inner layer of the epidermis.
  • Kidney stones Crystallized deposits of excess wastes such as uric acid, calcium, and magnesium that may form in the kidney.
  • Killer T cells See cytoxic T cells.
  • Kilocalorie The energy needed to heat 1000 grams of water from 14.5 to 15.5 degrees C.
  • Kinetochores Structures at the centromeres of the chromosomes to which the Þbers of the mitotic spindle connect.
  • Kingdoms Five broad taxonomic categories (Monera, Protista, Plantae, Fungi, Animalia) into which organisms are grouped, based on common characteristics.
  • Klinefelter syndrome In humans, a genetically determined condition in which the individual has two X and one Y chromosome. Affected individuals are male and typically tall and infertile.
  • Kreb's cycle Biochemical cycle in cellular aerobic metabolism where acetyl coa is combined with oxaloacetate to form citric acid; the resulting citric acid is converted into a number of other chemicals, eventually reforming oxaloacetate; NADH, some ATP, and FADH2 are produced and carbon dioxide is released.
  • L
  • Labia majora The outer folds of skin that cover and protect the genital region in women.
  • Labia minora Thin membranous folds of skin outside the vaginal opening.
  • Lactose intolerance A genetic trait characterized by the absence of the enzyme lactase, which breaks down lactose, the main sugar in milk and other dairy products.
  • Langerhans' cells Epidermal cells that participate in the inþammatory response by engulfing microorganisms and releasing chemicals that mobilize immune system cells.
  • Large intestine Consists of the cecum, appendix, colon, and rectum; absorbs some nutrients, but mainly prepares feces for elimination.
  • Larva A stage in the development of many insects and other organisms including sea urchins and sponges. In sponges, sexual reproduction results in the production of motile ciliated larvae.
  • Larynx A hollow structure at the beginning of the trachea. The vocal cords extend across the opening of the larynx.
  • Lateral roots Roots extending away from the main (or taproot) root.
  • Latitudinal diversity gradient The decrease in species richness that occurs as one moves away from the equator.
  • Latitudinal gradient As latitude increases, a gradient of cooler, drier conditions occurs.
  • Laurasia The northern part of the supercontinent of Pangaea, composed of the present-day North America, Europe, and Asia.
  • Laurentia Name applied to the "core" of North America in the times from the breakup of the precambrian supercontinent Rodinia to the formation of Pangaea.
  • Law of the minimum Holds that population growth is limited by the resource in shortest supply.
  • L-dopa A chemical related to dopamine that is used in the treatment of Parkinson's disease.
  • Leaf primordia Young leaves, recently formed by the shoot apical meristem, located at the tip of a shoot.
  • Leaf veins Vascular tissue in leaves, arranged in a net-like network (reticulate vennation) in dicots, and running parallel (parallel vennation) to each other in monocots.
  • Leaves The site of photosynthesis; one of the three major organs in plants.
  • Leukocytes White blood cells; primarily engaged in fighting infection.
  • Lichens Autotrophic organisms composed of a fungus (sac or club fungus) and a photosynthetic unicellular organism (e.g., a cyanobacterium or alga) in a symbiotic relationship; are resistant to extremes of cold and drought and can grow in marginal areas such as Arctic tundra.
  • Life history The age at sexual maturity, age at death, and age at other events in an individual's lifetime that inþuence reproductive traits.
  • Ligaments Dense parallel bundles of connective tissue that strengthen joints and hold the bones in place.
  • Light reactions The photosynthetic process in which solar energy is harvested and transferred into the chemical bonds of ATP; can occur only in light.
  • Lignin A polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the wall; related term lignified.
  • Linkage The condition in which the inheritance of a specific chromosome is coupled with that of a given gene. The genes stay together during meiosis and end up in the same gamete.
  • Lipases Enzymes secreted by the pancreas that are active in the digestion of fats.
  • Lipids One of the four classes of organic macromolecules. Lipids function in the long-term storage of biochemical energy, insulation, structure and control. Examples of lipids include the fats, waxes, oils and steroids (e.g. Testosterone, cholesterol).
  • Lithosphere The solid outer layer of the Earth; includes both the land area and the land beneath the oceans and other water bodies.
  • Lobe-finned Fish with muscular fins containing large jointed bones that attach to the body; one of the two main types of bony fish.
  • Logistic growth model A model of population growth in which the population initially grows at an exponential rate until it is limited by some factor; then, the population enters a slower growth phase and eventually stabilizes.
  • Long-day plants Plants that þower in the summer when nights are short and days are long; e.g., spinach and wheat.
  • Loop of Henle A U-shaped loop between the proximal and distal tubules in the kidney.
  • Lungfish A type of lobe-finned fish that breathe by a modified swim bladder (or lung) as well as by gills.
  • Lungs Sac-like structures of varying complexity where blood and air exchange oxygen and carbon dioxide; connected to the outside by a series of tubes and a small opening. In humans, the lungs are situated in the thoracic cavity and consist of the internal airways, the alveoli, the pulmonary circulatory vessels, and elastic connective tissues.
  • Luteal phase The second half of the ovarian cycle when the corpus luteum is formed; occurs after ovulation.
  • Luteinizing hormone (LH) A hormone secreted by the anterior pituitary gland that stimulates the secretion of testosterone in men and estrogen in women.
  • Lymph Interstitial þuid in the lymphatic system.
  • Lymph hearts Contractile enlargements of vessels that pump lymph back into the veins; found in fish, amphibians, and reptiles.
  • Lymphatic circulation A secondary circulatory system that collects þuids from between the cells and returns it to the main circulatory system; the circulation of the lymphatic system, which is part of the immune system.
  • Lymphatic system A network of glands and vessels that drain interstitial þuid from body tissues and return it to the circulatory system.
  • Lymphocytes White blood cells that arise in the bone marrow and mediate the immune response; include T cells and B cells.
  • Lyon hypothesis Idea proposed by Mary Lyon that mammalian females inactivate one or the other X-chromosome during early embryogenesis. This deactivated chromosome forms the Barr body. |
  • Lysosomes Membrane-enclosed organelles containing digestive enzymes. The lysosomes fuse with food vacuoles and enzymes contained within the lysosome chemically breakdown and/or digest the food vacuole's contents.
  • M
  • Macroevolution The combination of events associated with the origin, diversification, extinction, and interactions of organisms which produced the species that currently inhabit the Earth. Large scale evolutionary change such as the evolution of new species (or even higher taxa) and extinction of species.
  • Macromolecules Large molecules made up of many small organic molecules that are often referred to as monomers; e.g., carbohydrates, lipids, proteins, and nucleic acids. Macromolecules are polymers of monomers.
  • Macronucleus In ciliates, the large nucleus that carries up to several hundred copies of the genome and controls metabolism and asexual reproduction.
  • Macronutrients 1. Elements needed by plants in relatively large (primary) or smaller (secondary) quantities. 2. Foods needed by animals daily or on a fairly regular basis.
  • Macrophages A type of white blood cell derived from monocytes that engulf invading antigenic molecules, viruses, and microorganisms and then display fragments of the antigen to activate helper T cells; ultimately stimulating the production of antibodies against the antigen.
  • Malleus One of the bones comprising the middle ear of mammals.
  • Malpighian tubules The excretory organs of insects; a set of long tubules that open into the gut.
  • Mammal-like reptiles Group of Permian-Triassic reptiles having some possible mammalian features, notably a more prominent dentary (tooth-bearing) bone and reduction of the incus and malleus (which are part of the reptilian jaw along with the dentary). The mammal-like reptiles are thought to have been the reptile group from which the mammals later evolved.
  • Mantle In mollusks, a membranous or muscular structure that surrounds the visceral mass and secretes a shell if one is present.
  • Marine biome The aquatic biome consisting of waters containing 3.5% salt on average; includes the oceans and covers more than 70% of the Earth's surface; divided into benthic and pelagic zones.
  • Marsupials Pouched mammals. The young develop internally, but are born while in an embryonic state and remain in a pouch on the mother's abdomen until development is complete; this group includes kangaroos, koalas, and opossums. One of the three reproductive "strategies" of living mammals g-laying and placental being the other two), marsupials finish development in a pouch or under hairy coverings attached to the mother.
  • Mass extinction A time during which extinction rates are generally accelerated so that more than 50% of all species then living become extinct; results in a marked decrease in the diversity of organisms. Mass extinctions are thought to have occurred numerous times in Earth history, often from a variety of reasons: impacts, tectonism, changes in primary productivity of the seas, etc.
  • Mast cells Cells that synthesize and release histamine, as during an allergic response; found most often in connective tissue surrounding blood vessels.
  • Matter Anything that has mass and occupies space.
  • Matter cycling The þow of matter through various organisms and the physical environment of an ecosystem.
  • Maximum sustainable yield (MSY) The maximum number of a food or game population that can be harvested without harming the population's ability to grow back.
  • Medulla 1. A term referring to the central portion of certain organs; e.g., the medulla oblongata of the brain and the adrenal medulla, which synthesizes epinephrine and norepinephrine. 2. In more common usage, the area in the brain that regulates breathing, heartbeat, blood pressure and similar activities.
  • Medulla oblongata The region of the brain that, with the pons, makes up the hindbrain; controls heart rate, constriction and dilation of blood vessels, respiration, and digestion.
  • Medusa The motile bell-shaped form of body plan in cnidarians; e.g., jellyfish.
  • Megakarocytes Cells found in the bone marrow that produce platelets.
  • Megaspore mother cell Cells that undergo meiosis to produce megaspores.
  • Megaspores Four haploid cells produced by meiosis in the ovule of a þower. Usually, three of these cells degenerate, with the remaining cell becoming the female gametophyte phase of the plant's life cycle. Large (palynologists consider the megaspores to generally be above 200 micrometers in diameter) spores that develop into the megagametophyte, which in turn produces eggs.
  • Meiosis Cell division in which the chromosomes replicate, followed by two nuclear divisions. Each of the resulting gametes (in animals, spores in plants) receives a haploid set of chromosomes. Reduction/division by which ploidy, the number of sets of homologous chromosomes, is reduced in the formation of haploid cells that become gametes (or gametophytes in plants).
  • Meissner's corpuscles Sensory receptors concentrated in the epidermis of the fingers and lips that make these areas very sensitive to touch.
  • Melanin A pigment that gives the skin color and protects the underlying layers against damage by ultraviolet light; produced by melanocytes in the inner layer of the epidermis.
  • Melanocytes The cells in the inner layer of the epidermis that produce melanin.
  • Membrane-attack complex (MAC) A large cylindrical multiprotein complex formed by the complement system; kills invading microorganisms by embedding in their plasma membrane, creating a pore through which þuid þows, ultimately causing the cell to burst.
  • Menstrual cycle The recurring secretion of hormones and associated uterine tissue changes; typically 28 days in length.
  • Menstruation The process in which the uterine endometrium breaks down and sheds cells, resulting in bleeding; occurs approximately once a month. The first day marks the beginning of the menstrual and ovarian cycles.
  • Meristematic tissue Embryonic tissue located at the tips of stems and roots and occasionally along their entire length; can divide to produce new cells; one of the four main tissue systems in plants.
  • Mesentary Epithelial cells supporting the digestive organs.
  • Mesoderm The middle layer of cells in embryonic development; gives rise to muscles, bones, and structures associated with reproduction. The middle embryonic tissue layer. Cells and structures arising from the mesoderm include the bone, blood, muscle, skin, and reproductive organs.
  • Mesoglea A gel-like matrix that occurs between the outer and inner epithelial layers in cnidarians.
  • Mesophyll Layer of leaf tissue between the epidermis layers; literally meaning "middle of the leaf".
  • Mesophytic leaves The leaves of plants that grow under moderately humid conditions with abundant soil and water.
  • Mesozoic Era The period of geologic time beginning 245 million years ago and ending 65 million years ago; the age of the dinosaurs and cycads, the Mesozoic falls between the Paleozoic and Cenozoic Eras and includes the Triassic, Jurassic, and Cretaceous Periods.
  • Messenger RNA (mrna) "Blueprint" for protein synthesis that is transcribed from one strand of the DNA (gene) and which is translated at the ribosome into a polypeptide sequence.
  • Metabolic pathway A series of individual chemical reactions in a living system that combine to perform one or more important functions. The product of one reaction in a pathway serves as the substrate for the following reaction. Examples include glycolysis and Kreb's cycle.
  • Metabolism The sum of all chemical reactions (energy exchanges) in cells.
  • Metamorphosis The process of changing from one form to another; e.g., in insects, from the larval stage to the pupal stage to the reproductive adult stage.
  • Metaphase The stage of eukaryotic cell division (mitosis or meiosis) in which the chromosomes line up at the equator of the cell.
  • Metastasis The process in which cancer cells break away from the original tumor mass and establish new tumor sites elsewhere in the body.
  • Methanogens A group of archaebacteria that produce methane as a by product of their metabolism.
  • Methionine The amino acid coded for by the initiation codon; all polypeptides begin with methionine, although post-translational reactions may remove it.
  • Micelles Structures formed when bile salts surround digested fats in order to enable the water-insoluble fats to be absorbed by the epithelial cells lining the small intestine.
  • Microevolution A small-scale evolutionary event such as the formation of a species from a preexisting one or the divergence of reproductively isolated populations into new species.
  • Microfilaments Rods composed of actin that are found in the cytoskeleton and are involved in cell division and movement. |
  • Microgametophyte Stage of the plant life cycle that develops from or within a microspore. The microgametophyte produces sperm in specialized structures known as antheridia.
  • Micronucleus In the protistan group known as the ciliates, the small nucleus containing a single copy of the genome that is used for sexual reproduction.
  • Micronutrients Elements that are required by plants in very small quantities, but are toxic in large quantities: iron, manganese, molybdenum, copper, boron, zinc, and chloride.
  • Micropyle The end of the embryo sac where the egg cell and synergids are located.
  • Microsporangia Structures of the sporophyte in which microspores are produced by meiosis. In flowering plants the microsporangia are known as anther sacs.
  • Microspore mother cell Cells in the microsporangium that undergo meiosis to produce microspores. In flowering plants the microspore is known as the pollen grain, and contains a three-celled male.
  • Microspores Four haploid cells produced by the meiotic division in the pollen sacs of þowers or microsporangia of gymnosperms. Microspores undergo mitotic division and become encased in a thick protective wall to form pollen grains. Small, size usually less than 200 micrometers, spores produced by meiosis. Microspores either germinate into the male gametophyte or have the male gametophyte develop inside the microspore wall.
  • Microtubules Filaments about 25 nanometers in diameter found in cilia, þagella, and the cytoskeleton.
  • Microvilli Hair-like projections on the surface of the epithelial cells of the villi in the small intestine; increase the surface area of the intestine to improve absorption of digested nutrients.
  • Midbrain A network of neurons that connects with the forebrain and relays sensory signals to other integrating centers.
  • Middle lamella A layer composed of pectin that cements two adjoining plant cells together.
  • Migration Movement of organisms either permanently (as in the migration of humans to the Americas) or temporarily (migratory birds such as Canadian geese).
  • Mineralocorticoids A group of steroid hormones produced by the adrenal cortex that are important in maintaining electrolyte balance.
  • Minerals Trace elements required for normal metabolism, as components of cells and tissues, and in nerve conduction and muscle contraction.
  • Minimum viable population (MVP) The smallest population size that can avoid extinction due to breeding problems or random environmental þuctuations.
  • Mitochondria Self-replicating membrane-bound cytoplasmic organelles in most eukaryotic cells that complete the breakdown of glucose, producing NADH and ATP (singular term: mitochondrion). The powerhouse of the cell. Organelles within eukaryotes that generate (by chemiosmosis) most of the ATP the cell needs to function and stay alive.
  • Mitosis The division of the cell's nucleus and nuclear material of a cell; consists of four stages: prophase, metaphase, anaphase, and telophase. Cell xeroxing. Mitosis occurs only in eukaryotes. The DNA of the cell is replicated during interphase of the cell cycle and then segregated during the four phases of mitosis.
  • Mitotic spindle A network of microtubules formed during prophase. Some microtubules attach to the centromeres of the chromosomes and help draw the chromosomes apart during anaphase.
  • Mold Type of fossil preservation where the original material of the fossil has decayed but has left an impression in the surrounding sediments. Molds are often filled with a different material, producing strikingly beautiful fossils.
  • Mole Avogadro's number (6.02 X 1023 atoms) of a substance.
  • Molecular biology Field of biology that studies the molecular level of organization.
  • Molecules Units of two or more atoms held together by chemical bonds. The combination of atoms by chemical bonds with the component atoms in definite porportions, such as water (two H to one O).
  • Monera Prokaryotic kingdom that includes (in the most widely accepted classification system) archaebacteria, eubacteria, and cyanobacteria. Members of this kingdom were among the first forms of life over 3.5 billion years ago.
  • Monocots One of the two major types of þowering plants; characterized by having a single cotyledon, þoral organs arranged in threesd or multiples of three, and parallel-veined leaves; include grasses, cattails, lilies, and palm trees. One of the two major groups in the Angiosperms, monocots are characterized by having a single seed leaf (cotyledon), flower parts in 3's or multiples of 3, monoaperturate pollen (although some dicots also have this feature), parallel veins in their leaves, and scattered vascular bundles in their stems.
  • Monoculture The growth of only one species in a given area; such as a cornfield or other agricultural field.
  • Monocytes White blood cells that clean up dead viruses, bacteria, and fungi and dispose of dead cells and debris at the end of the inþammatory response.
  • Monohybrid cross In genetics, a cross that involves only one characteristic.
  • Monomer An organic chemical unit linked to other units (usually by a covalent bond formed by the removal of water) to produce a larger molecule (macromolecule) known as a polymer.
  • Monophyletic group A group of organisms descended from a common ancestor. For example: your immediate family may be considered such a group, being descended from a common ancestral group (grandparents, etc.).
  • Monosaccharides Simple carbohydrates, usually with a five- or six-carbon skeleton; e.g., glucose and fructose. A carbohydrate composed of a single sugar unit, such as glucose, ribose, deoxyribose, etc.
  • Monotremes Egg-laying mammals; e.g., the spiny anteater and the duck-billed platypus.
  • Morph A distinct phenotypic variant within a population.
  • Morphological convergence The evolution of basically dissimilar structures to serve a common function. For example: the wings of birds and insects.
  • Morula The solid-ball stage of the pre-emplantation embryo.
  • Mosaic evolution A pattern of evolution where all features of an organism do not evolve at the same rate. Some characteristics are retained from the ancestral condition while others are more recently evolved.
  • Motor (efferent) pathways The portion of the peripheral nervous system that carries signals from the central nervous system to the muscles and glands.
  • Motor neurons Neurons that receive signals from interneurons and transfer the signals to effector cells that produce a response. Nerve cells connected to a muscle or gland. Sometimes also known as effector neurons.
  • Motor output A response to the stimuli received by the nervous system. A signal is transmitted to organs that can convert the signals into action, such as movement or a change in heart rate.
  • Motor units Consist of a motor neuron with a group of muscle fibers; form the units into which skeletal muscles are organized; enable muscles to contract on a graded basis.
  • Mouth The oral cavity; the entrance to the digestive system where food is broken into pieces by the teeth and saliva begins the digestion process.
  • Mucus A thick, lubricating fluid produced by the mucous membranes that line the respiratory, digestive, urinary, and reproductive tracts; serves as a barrier against infection and, in the digestive tract, moistens food, making it easier to swallow.
  • Multicellular Organisms composed of multiple cells and exhibiting some division of labor and specialization of cell structure and function.
  • Multinucleate Cells having more than one nucleus per cell.
  • Muscle fibers Long, multinucleated cells found in skeletal muscles; made up of myofibrils. One of the four major groups of vertebrate cell/tissue types. Muscle cells contract/relax, allowing movement of and/or within the animal. | |
  • Muscular system One of eleven major body organ systems in animals; allows movement and locomotion, powers the circulatory, digestive, and respiratory systems, and plays a role in regulating temperature.
  • Mutation Any heritable change in the nucleotide sequence of DNA; can involve substitutions, insertions, or deletions of one or more nucleotides.
  • Mutation rate The average occurrence of mutations in a species per a given unit of time.
  • Mutualism A form of symbiosis in which both species benefit. A type of symbiosis where both organisms benefit. The classic example is lichens, which is a symbiosis between an alga and a fungus. The alga provides food and the fungus provides water and nutrients.
  • Mycelium The mass of interwoven filaments of hyphae in a fungus.
  • Mycorrhiza Occurs when a fungus (basidiomycete or zygomycete) weaves around or into a plant's roots and forms a symbiotic relationship. Fungal hyphae absorb minerals from the soil and pass them on to the plant roots while the fungus obtains carbohydrates from the plant (pl.: mycorrhizae).
  • Myelin sheath Layers of specialized glial cells, called Schwann cells, that coat the axons of many neurons.
  • Myofibrils Striated contractile microfilaments in skeletal muscle cells.
  • Myosin Thick protein filaments in the center sections of sarcomeres. |
  • N
  • Nares Nostrils; the openings in the nose through which air enters.
  • Nastic movement A plant's response to a stimulus in which the direction of the response is independent of the direction of the stimulus. Non-directional plant movements.
  • Natural selection The process of differential survival and reproduction of Þtter genotypes; can be stabilizing, directional, or disruptive. Better adapted individuals are more likely to survive to reproductive age and thus leave more offspring and make a larger contribution to the gene pool than do less Þt individuals. The differential survival and reproductive successes of individuals in a variable population that powers the evolutionary process. When all individuals survive and reproduce (except for chance occurrences) natural selection works at a lower rate, if at all.
  • Nectaries Nectar-secreting organs in þowering plants that serve as insect feeding stations and thus attract insects, which then assist in the transfer of pollen.
  • Negative feedback control Occurs when information produced by the feedback reverses the direction of the response; regulates the secretion of most hormones.
  • Negative feedback loop A biochemical pathway where the products of the reaction inhibit production of the enzyme that controlled their formation.
  • Negative feedback The stopping of the synthesis of an enzyme by the accumulation of the products of the enzyme-mediated reaction.
  • Nektonic organisms "Swimmers"; one of the two main types of organisms in the pelagic zone of the marine biome.
  • Nephridium The excretory organ in þatworms and other invertebrates; a blind-ended tubule that expels waste through an excretory pore.
  • Nephron A tubular structure that is the Þltering unit of the kidney; consists of a glomerulus and renal tubule.
  • Nerve cord A dorsal tubular cord of nervous tissue above the notochord of a chordate.
  • Nerve net An interconnected mesh of neurons that sends signals in all directions; found in radially symmetrical marine invertebrates, such as jellyþsh and sea anemones, that have no head region or brain.
  • Nerves Bundles of neuronal processes enclosed in connective tissue that carry signals to and from the central nervous system.
  • Nervous system One of eleven major body organ systems in animals; coordinates and controls actions of internal organs and body systems, receives and processes sensory information from the external environment, and coordinates short-term reactions to these stimuli. | |
  • Net primary productivity (NPP) The rate at which producer (usually plants) biomass is created in a community.
  • Net secondary productivity (NSP) The rate at which consumer and decomposer biomass is produced in a community.
  • Neural tube A tube of ectoderm in the embryo that will form the spinal cord.
  • Neuromuscular junction The point where a motor neuron attaches to a muscle cell.
  • Neurons Highly specialized cells that generate and transmit bioelectric impulses from one part of the body to another; the functional unit of the nervous system. A cell of the nerve tissue having a cell body input zone of dendrites and an output zone of an axon (of varying length). The electrochemical nerve impulse/message is transmitted by neurons. |
  • Neurotoxin Chemical that paralyzes nerves. Neurotoxins are produced by a variety of organisms, most notably some of the heterotrophic dinoflagellates.
  • Neurotransmitters Chemicals released from the tip of an axon into the synaptic cleft when a nerve impulse arrives; may stimulate or inhibit the next neuron. The chemical that crosses the synaptic cleft and causes the transmission of the nerve message in an adjacent neuron or the stimulation of an effector cell (muscle or gland).
  • Neutron An uncharged subatomic particle in the nucleus of an atom. The large (mass approximately equal to 1 atomic mass unit), electrically neutral particle that may occur in the atomic nucleus.
  • Niche The biological role played by a species.
  • Niche overlap The extent to which two species require similar resources; speciþes the strength of the competition between the two species.
  • Nicotine adenine dinucleotide phosphate (NADP+) A substance to which electrons are transferred from photosystem I during photosynthesis; the addition of the electrons reduces NADP, which acquires a hydrogen ion to form NADPH, which is a storage form of energy that can be transferred to the Calvin Cycle for the production of carbohydrate.
  • Node The stem region of a plant where one or more leaves attach. Where leaves are attached to stems.
  • Node of Ranvier A gap between two of the Schwann cells that make up an axon's myelin sheath; serves as a point for generating a nerve impulse.
  • Nondisjunction The failure of chromosomes to separate properly during cell division. The unequal segregation of chromosomes during meiosis. This forms cells with either too many (possibly one or more single or sets of chromosomes too many) or too few chromosomes. Thought to be a common cause for Down Syndrome, where sufferers often have an extra copy of chromosome 21.
  • Nonvascular plants Plants lacking lignified vascular tissue (xylem), vascularized leaves, and having a free-living, photosynthetic gametophyte stage that dominates the life cycle. Common examples are the mosses and liverworts.
  • Norepinephrine A hormone produced in the adrenal medulla and secreted under stress; contributes to the "Þght or þight" response.
  • Notochord In chordates, a cellular rod that runs the length of the body and provides dorsal support. Also, a structure of mesoderm in the embryo that will become the vertebrae of the spinal column. The stiff rod-like structure that all members of the Phylum Chordata develop at some stage during their life.
  • Nuclear area In prokaryotic cells, a region containing the cell's genetic information. Unlike the nucleus in eukaryotic cells, it is not surrounded by a membrane.
  • Nuclear pores Openings in the membrane of a cell's nuclear envelope that allow the exchange of materials between the nucleus and the cytoplasm.
  • Nucleic acids Polymers composed of nucleotides; e.g., DNA and RNA.
  • Nucleoid The area of the prokaryotic cytoplasm where the chromatin is localized.
  • Nucleolus A round or oval body in the nucleus of a eukaryotic cell; consists of DNA and RNA and produces ribosomal RNA (pl.: nucleoli).
  • Nucleosomes Spherical bodies formed by coils of chromatin. The nucleosomes in turn are coiled to form the Þbers that make up the chromosomes.
  • Nucleotide sequences The genetic code encrypted in the sequence of bases along a nucleic acid.
  • Nucleotides The subunits of nucleic acids; composed of a phosphate, a sugar, and a nitrogen-containing base. The fundamental structural unit of the nucleic acid group of organic macromolecules. Some nucleotides are involved in information storage (as nucleotides in DNA), protein synthesis (as nucleotides in RNA), and energy transfers (as single nucleotide ATP, GTP, and double nucleotide NADH and NADPH).
  • Nucleus (atom) An atom's core; contains protons and one or more neutrons (except hydrogen, which has no neutrons).
  • Nucleus (cell) The largest, most prominent organelle in eukaryotic cells; a round or oval body that is surrounded by the nuclear envelope and contains the genetic information necessary for control of cell structure and function.
  • Nyctinasty A nastic movement in a plant that is caused by light and dark.
  • O
  • Occipital lobe The lobe of the cerebral cortex located at the rear of the head; is responsible for receiving and processing visual information.
  • Oils Triglycerides that are liquid at room temperature.
  • Oncogenes Genes that can activate cell division in cells that normally do not divide or do so only slowly. A gene that when over-expressed leads to cancer, but which normally functions in cell division.
  • One gene, one enzyme hypothesis Holds that a single gene controls the production, specificity, and activity of each enzyme in a metabolic pathway. Thus, mutation of such a gene changes the ability of the cell to carry out a particular reaction and disrupts the entire pathway.
  • One gene one polypeptide hypothesis" A revision of the one gene, one enzyme hypothesis. Some proteins are composed of different polypeptide chains encoded by separate genes, so the hypothesis now holds that mutation in a gene encoding a specifc polypeptide can alter the ability of the encoded protein to function and thus produce an altered phenotype.
  • Oocyte A cell that will/is undergo/ing development into a female gamete.
  • Oogenesis The production of ova. The development of a diploid cell into a haploid ovum or egg cell.
  • Open community A community in which the populations have different density peaks and range boundaries and are distributed more or less randomly.
  • Opposable The capability of being placed against the remaining digits of a hand or foot; e.g., the ability of the thumb to touch the tips of the fingers on that hand.
  • Opsins Molecules in cone cells that bind to pigments, creating a complex that is sensitive to light of a given wavelength.
  • Orders Taxonomic subcategories of classes.
  • Ordovician extinction Paleozoic-aged mass extinction possibly related to glaciation in the southern-hemisphere supercontinent Gondwana.
  • Ordovician Period Geologic period of the Paleozoic Era after the Cambrian Period between 500 and 435 million years ago. Major advances during this period include the bony fish and possibly land plants (during the late Ordovician).
  • Organ systems Groups of organs that perform related functions.
  • Organelles Cell components that carry out individual functions; e.g., the cell nucleus and the endoplasmic reticulum. Subcellular structures (usually membrane-bound and unique to eukaryotes) that perform some function, e.g. Chloroplast, mitochondrion, nucleus. |
  • Organism An individual, composed of organ systems (if multicellular). Multiple organisms make up a population.
  • Organs Differentiated structures consisting of tissues and performing some specific function in an organism. Structures made of two or more tissues which function as an integrated unit. E.g. The heart, kidneys, liver, stomach.
  • Orgasm Rhythmic muscular contractions of the genitals (sex organs) combined with waves of intense pleasurable sensations; in males, results in the ejaculation of semen.
  • Osmoconformers Marine organisms that have no system of osmoregulation and must change the composition of their body þuids as the composition of the water changes; include invertebrates such as jellyþsh, scallops, and crabs.
  • Osmoregulation The regulation of the movement of water by osmosis into and out of cells; the maintenance of water balance within the body.
  • Osmoregulators Marine vertebrates whose body þuids have about one-third the solute concentration of seawater; must therefore undergo osmoregulation.
  • Osmosis Diffusion of water molecules across a membrane in response to differences in solute concentration. Water moves from areas of high-water/low-solute concentration to areas of low-water/high-solute concentration. Diffusion of water across a semi-permeable barrier such as a cell membrane, from high water potential (concentration) to lower water potential (concentration).
  • Osmotic pressure Pressure generated by water moving by osmosis into or out of a cell.
  • Ossification The process by which embryonic cartilage is replaced with bone.
  • Osteoarthritis A degenerative condition associated with the wearing away of the protective cap of cartilage at the ends of bones. Bone growths or spurs develop, restricting movement and causing pain.
  • Osteoblasts Bone-forming cells.
  • Osteoclasts Cells that remove material to form the central cavity in a long bone.
  • Osteocytes Bone cells that lay down new bone; found in the concentric layers of compact bone. Bone cell, a type of connective tissue.
  • Osteoporosis A disorder in which the mineral portion of bone is lost, making the bone weak and brittle; occurs most commonly in postmenopausal women.
  • Out of Africa hypothesis Holds that modern human populations (Homo sapiens) are all derived from a single speciation event that took place in a restricted region in Africa.
  • Ovaries 1) In animals, the female gonads, which produce eggs (ova) and female sex hormones. ) In þowers, part of the female reproductive structure in the carpel; contain the ovules, where egg development occurs. The lower part of the carpel that contains the ovules within which the female gametophyte develops.
  • Overkill The shooting, trapping, or poisoning of certain populations, usually for sport or economic reasons.
  • Oviducts Tubes that connect the ovaries and the uterus; transport sperm to the ova, transport the fertilized ova to the uterus, and serve as the site of fertilization; also called the fallopian tubes or uterine tubes.
  • Ovulation The release of the oocyte onto the surface of the ovary; occurs at the midpoint of the ovarian cycle. The release of the ovum (egg) from the ovary after the peaking of luteinizing hormone concentration in the blood during the menstrual cycle. |
  • Ovule In seed plants, a protective structure in which the female gametophyte develops, fertilization occurs, and seeds develop; contained within the ovary. Structures inside the ovary of the flower within which the female gametophyte develops after megasporogenesis has produced a megaspore inside each ovule.
  • Ovum The female gamete, egg.
  • Oxidation The loss of electrons from the outer shell of an atom; often accompanied by the transfer of a proton and thus involves the loss of a hydrogen ion. The loss of electrons or hydrogens in a chemical reaction.
  • Oxytocin A peptide hormone secreted by the posterior pituitary that stimulates the contraction of the uterus during childbirth.
  • Ozone A triatomic (O3) form of oxygen that is formed in the stratosphere when sunlight strikes oxygen atoms. This atmospheric ozone helps Þlter radiation from the sun.
  • P
  • Pacemaker. See sinoatrial node.
  • Pacinian corpuscles Sensory receptors located deep in the epidermis that detect pressure and vibration.
  • Paleontology The study of ancient life by collection and analysis of fossils.
  • Paleozoic Era The period of time beginning 570 million years ago ending 245 million years ago; falls between the Proterozoic and Mesozoic Eras and is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.
  • Palindrome A sequence that reads the same in either direction; in genetics, refers to an enzyme recognition sequence that reads the same on both strands of DNA.
  • Palisade Layer of mesophyll cells in leaves that are closely placed together under the epidermal layer of the leaf. Palisade parenchyma: Columnar cells located just below the upper epidermis in leaves the cells where most of the light absorbtion in photosynthesis occurs. |
  • Palynology The study of palynomorphs and other acid-resistant microfossils usually produced by plants, protists, and fungi.
  • Palynomorph Generic term for any object a palynologist studies.
  • Pancreas A gland in the abdominal cavity that secretes digestive enzymes into the small intestine and also secretes the hormones insulin and glucagon into the blood, where they regulate blood glucose levels. A digestive organ that produces trypsin, chymotrypsin and other enzymes as a pancreatic juice, but which also has endocrine functions in the production of the hormones somatostatin, insulin, and glucagon.
  • Pancreatic islets Clusters of endocrine cells in the pancreas that secrete insulin and glucagon; also known as islets of Langerhans.
  • Pangaea The name proposed by German meteorologist Alfred Wegener for a supercontinent that existed at the end of the Paleozoic Era and consisted of all the Earth's landmasses.
  • Parallel evolution The development of similar characteristics in organisms that are not closely related (not part of a monophyletic group) due to adaptation to similar environments and/or strategies of life.
  • Parasites Organisms that live in, with, or on another organism. The parasites beneþt from the association without contributing to the host, usually they cause some harm to the host.
  • Parasitism A form of symbiosis in which the population of one species beneþts at the expense of the population of another species; similar to predation, but differs in that parasites act more slowly than predators and do not always kill the host. A type of symbiosis in which one organism benefits at the expense of the other, for example the influenza virus is a parasite on its human host. Viruses, are obligate intracellular parasites.
  • Parasympathetic system The subdivision of the autonomic nervous system that reverses the effects of the sympathetic nervous system. Part of the autonomic nervous system that controls heartbeat, respiration and other vital functions.
  • Parenchyma One of the three major cell types in plants. Parenchyma cells have thin, usually multisided walls, are unspecialized but carry on photosynthesis and cellular respiration and can store food; form the bulk of the plant body; found in the þeshy tissue of fruits and seeds, photosynthetic cells of leaves, and the vascular system. Generalized plant cells whose numerous functions include photosynthesis, storage, bulk of herbaceous stem tissues, lateral transport in woody stems. Parenchyma are variously shaped but are characterized by thin walls and remain alive at functional maturity.
  • Parietal lobe The lobe of the cerebral cortex that lies at the top of the brain; processes information about touch, taste, pressure, pain, and heat and cold.
  • Passive transport Diffusion across a plasma membrane in which the cell expends no energy.
  • Pectin A substance in the middle lamella that cements adjoining plant cells together.
  • Pectoral girdle In humans, the bony arch by which the arms are attached to the rest of the skeleton; composed of the clavicle and scapula.
  • Pedigree analysis A type of genetic analysis in which a trait is traced through several generations of a family to determine how the trait is inherited. The information is displayed in a pedigree chart using standard symbols.
  • Pelagic zone One of the two basic subdivisions of the marine biome; consists of the water above the sea þoor and its organisms.
  • Pelvic girdle In humans, the bony arch by which the legs are attached to the rest of the skeleton; composed of the two hipbones.
  • Pelvis The hollow cavity formed by the two hipbones.
  • Penicillin The first of the so-called wonder drugs; discovered by Sir Alexander Fleming.
  • Pepsin An enzyme produced from pepsinogen that initiates protein digestion by breaking down protein into large peptide fragments. An enzyme, produced by the stomach, that chemically breaks down peptide bonds in polypeptides and proteins.
  • Pepsinogen An inactive form of pepsin; synthesized and stored in cells lining the gastric pits of the stomach.
  • Peptic ulcer Damage to the epithelial layer of the stomach lining; generally caused by bacterial infection.
  • Peptide bond A covalent bond that links two amino acids together to form a polypeptide chain. A covalent bond between the amine end of one amino acid and the acid end of another amino acid.
  • Peptides Short chains of amino acids.
  • Perennials Plants that persist in the environment for more than one year (as in the case of annuals).
  • Perichondrium A layer of connective tissue that forms around the cartilage during bone formation. Cells in the perichondrium lay down a peripheral layer that develops into compact bone.
  • Period The fundamental unit in the hierarchy of time units; a part of geologic time during which a particular sequence of rocks designated as a system was deposited. Units of geological time that are the major subdivisions of Eras.
  • Periosteum A Þbrous membrane that covers bones and serves as the site of attachment for skeletal muscles; contains nerves, blood vessels, and lymphatic vessels.
  • Peripheral nervous system The division of the nervous system that connects the central nervous system to other parts of the body. Components of the nervous system that transmit messages to the central nervous system.
  • Peristalsis Involuntary contractions of the smooth muscles in the walls of the esophagus, stomach, and intestines that propel food along the digestive tract. Waves of muscle contraction in the esophagus that propel food from the oral cavity to the stomach.
  • Permian Period The last geologic time period of the Paleozoic Era, noted for the greatest mass extinction in earth history, when nearly 96% of species died out.
  • Peroxisomes Membrane-bound vesicles in eukaryotic cells that contain oxidative enzymes.
  • Pesticides Chemicals that are applied to agricultural crops or domesticated plants and which kill or inhibit growth of insects.
  • Petals Usually brightly colored elements of a þower that may produce fragrant oils; nonreproductive structures that attract pollinators. Sterile leaf-like (white, colorless, but usually colored) structures in flowers that serve to attract pollinators.
  • Petiole The generally non-leafy part of the leaf that attaches the leaf blade to the stem; celery and rhubarb are examples of a leaf petiole that we use as food. The stalk connecting the leaf blade to the stem.
  • Petrifaction Mode of fossilization where f organic matter is replaced with silica.
  • PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (rubp) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH.
  • PGAL (phosphoglyceraldehyde) A substance formed from PGA during the dark reaction of photosynthesis. Some PGAL leaves the cycle and can be converted to glucose, while other PGAL molecules are used to reform ribulose biphosphate (rubp) to continue the dark reaction.
  • Ph The negative logarithm of the H+ ion concentration. The ph is a measure of the acidity or basic character of a solution. Since it measures a fraction, the larger the ph number, the less H ions are present in a solution.
  • Phagocytes White blood cells that can engulf (by phagocytosis) and destroy microorganisms including viruses and bacteria; cells in this category include neutrophils and monocytes.
  • Phagocytosis A form of endocytosis in which white blood cells surround and engulf invading bacteria or viruses.
  • Pharynx The passageway between the mouth and the esophagus and trachea. Food passes from the pharynx to the esophagus, and air passes from the pharynx to the trachea.
  • Phenotype The observed properties or outward appearance of a trait. The physical expression of the alleles posessed by an organism.
  • Pheromones Chemical signals that travel between organisms rather than between cells within an organism; serve as a form of communication between animals.
  • Phloem Tissue in the vascular system of plants that moves dissolved sugars and other products of photosynthesis from the leaves to other regions of the plant. Phloem tissue consists of cells called sieve tubes and companion cells. Cells of the vascular system in plants that transport food from leaves to other areas of the plant. | |
  • Phosphate group A chemical group composed of a central phosphorous bonded to three or four oxygens. The net charge on the group is negative.
  • Phospholipids Asymmetrical lipid molecules with a hydrophilic head and a hydrophobic tail. Lipids with a phosphate group in place of one of the three fatty acid chains. Phospholipids are the building blocks of cellular membranes. Phospholipids have hydrophilic heads (glycerol and phosphate) and hydrophobic tails (the non-polar fatty acids).
  • Phosphorylation The chemical attachment of phosphorous to a molecule, usually associated with the storage of energy in the covalent bond that is also formed. Example: attachment of the third phosphate group to ADP in the formation of the higher energy form, ATP. Photophosphorylation is a type of phosphorylation associated with the formation of ATP in the photosynthesis process.
  • Photic zone The layer of the ocean that is penetrated by sunlight; extends to a depth of about 200 meters.
  • Photoperiodism The ability of certain plants to sense the relative amounts of light and dark in a 24-hour period; controls the onset of þowering in many plants.
  • Photosynthesis The process by which plant cells use solar energy to produce ATP. The conversion of unusable sunlight energy into usable chemical energy, associated with the actions of chlorophyll.
  • Photosystems Clusters of several hundred molecules of chlorophyll in a thylakoid in which photosynthesis takes place. Eukaryotes have two types of photosystems: I and II. The series of green photoreceptive pigments involved in the light reactions, which occur in the thylakoids of the chloroplast (in eukaryotes). Energy from light is passed to the electrons as they move through the photosystem pigments. | | |
  • Phototrophs Organisms that use sunlight to synthesize organic nutrients as their energy source; e.g., cyanobacteria, algae, and plants.
  • Phototropism The reaction of plants to light in which the plants bend toward the light. Plant response to light by unequal growth caused by concentration of the plant hormone Indole Acetic Acid (IAA, an auxin) on the darker side of the plant shoot.
  • Phycocyanin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.
  • Phycoerythrin An accessory pigment found in cyanobacteria and the chloroplasts of red algae.
  • Phylogenetic Pertaining to a phylogeny.
  • Phylogeny 1) the study of evolutionary relationships within a monophyletic group. 2) evolutionary hypotheses represented as a dendrogram or branching diagram. |
  • Phylum The broadest taxonomic category within kingdoms (pl.: phyla).
  • Phytochrome A pigment in plant leaves that detects day length and generates a response; partly responsible for photoperiodism.
  • Phytoplankton A þoating layer of photosynthetic organisms, including algae, that are an important source of atmospheric oxygen and form the base of the aquatic food chain.
  • Pilus Projection from surface of a bacterial cell (F+) that can donate genetic material to another (F-).
  • Pineal gland A small gland located between the cerebral hemispheres of the brain that secretes melatonin.
  • Pioneer community The initial community of colonizing species.
  • Pistil Female reproductive structures in flowers, consisting of the stigma, style, and ovary. Also known as a carpel in some books. |
  • Pith Central area in plant stems, largely composed of parenchyma tissue modified for storage. |
  • Pituitary gland A small gland located at the base of the brain; consists of an anterior and a posterior lobe and produces numerous hormones. The master gland of the endocrine system, the pituitary releases hormones that have specific targets as well as those that stimulate other glands to secrete hormones. Part of the pituitary is nerve tissue, the rest is glandular epithelium. |
  • Placenta An organ produced from interlocking maternal and embryonic tissue in placental mammals; supplies nutrients to the embryo and fetus and removes wastes.
  • Placental mammals One of three groups of mammals that carry their young in the mother's body for long periods during which the fetus is nourished by the placenta. Humans are placental mammals.
  • Planaria Small free-living þatworms (Phylum Platyhelminthes) with bilateral symmetry and cephalization. The freshwater type is often used as an experimental organism.
  • Planktonic organisms "Floaters"; one of the two main types of organisms in the pelagic zone of the marine biome.
  • Plantae The plant kingdom; nonmobile, autotrophic, multicellular eukaryotes. Kingdom of the plants, autotrophic eukaryotes with cellulose in their cell walls and starch as a carbohydrate storage product, with chlorophylls a and b as photosynthesis pigments.
  • Plasma The liquid portion of the blood. Along with the extracellular þuid, it makes up the internal environment of multicellular organisms.
  • Plasma cells Cells produced from B cells that synthesize and release antibodies.
  • Plasmids Self-replicating, circular DNA molecules found in bacterial cells; often used as vectors in recombinant DNA technology. Small circles of double-stranded DNA found in some bacteria. Plasmids can carry from four to 20 genes. Plasmids are a commonly used vector in recombinant DNA studies.
  • Plasmodesmata Junctions in plants that penetrate cell walls and plasma membranes, allowing direct communication between the cytoplasm of adjacent cells (sing.: plasmodesma).
  • Plasmolysis Osmotic condition in which a cell loses water to its outside environment.
  • Plastids Membrane-bound organelles in plant cells that function in storage (of food or pigments) or food production. Term for any double membrane-bound organelle. Chloroplasts contain the chemicals for photosynthesis, amyloplasts (also known as leukoplasts) store starch, chromoplasts contain colorful pigments such as in the petals of a flower or epidermis of a fruit.
  • Plate tectonics The movement of the plates that make up the surface of the Earth. The revolutionary paradigm in geology that the earth's crust is composed of rigid segments (plates) in constant (although considered slow in a human-scale time frame) motion (tectonics) relative to each other.
  • Platelets In vertebrates, cell fragments that bud off from the megakaryocytes in the bone marrow; carry chemicals needed for blood clotting. Cell fragment functioning in blood clotting.
  • Pleiotropic A term describing a genotype with multiple phenotypic effects. For example: sickle-cell anemia produces a multitude of consequences in those it affects, such as heart disease, jidney problem, etc.
  • Pleistocene The first geologic epoch of the Quaternary Period of the Cenozoic Era that ended 10,000 years ago with the retreat of the last glaciers.
  • Pleura A thin sheet of epithelium that covers the inside of the thoracic cavity and the outer surface of the lungs.
  • Pleural cavity The space between the sheets of pleura (one covering the inside of the thoracic cavity, the other covering the outside of the lungs).
  • Polar covalent bond A covalent bond in which atoms share electrons in an unequal fashion. The resulting molecule has regions with positive and negative charges. The presence of polar covalent bonds allows other polar molecules to surround molecule: example: glucose sugar in water.
  • Pollen grains The containers for male gametophytes of seed plants produced in a microsporangium by meiosis. Microspores produced by seed plants that contain the male gametophyte.
  • Pollen tube Structure produced by the tube nucleus in the pollen grain through which the sperm nucleus (or nuclei in angiosperms) proceed to travel through to reach the egg.
  • Pollination The transfer of pollen from the anthers to the stigma by a pollinating agent such as wind, insects, birds, bats, or in a few cases the opening of the flower itself.
  • Polygenic inheritance Occurs when a trait is controlled by several gene pairs; usually results in continuous variation.
  • Polymer Organic molecule composed of smaller units known as monomers. A large molecule composed of smaller subunits, for example starch is a polymer of glucose, proteins are polymers of amino acids.
  • Polymerase chain reaction (PCR) A method of amplifying or copying DNA fragments that is faster than cloning. The fragments are combined with DNA polymerase, nucleotides, and other components to form a mixture in which the DNA is cyclically amplified.
  • Polynucleotides Long chains of nucleotides formed by chemical links between the sugar and phosphate groups.
  • Polyp The sessile form of life history in cnidarians; e.g., the freshwater hydra.
  • Polyploidy Abnormal variation in the number of chromosome sets. The condition when a cell or organism has more than the customary two sets of chromosomes. This is an especially effective speciation mechanism in plants since the extra chromosomes will establish reproductive isolation with the parental population(s), an essential for speciation. |
  • Polysaccharides Long chains of monosaccharide units bonded together; e.g., glycogen, starch, and cellulose.
  • Pons The region that, with the medulla oblongata, makes up the hindbrain, which controls heart rate, constriction and dilation of blood vessels, respiration, and digestion.
  • Population A group of individuals of the same species living in the same area at the same time and sharing a common gene pool. A group of potentially interbreeding organisms in a geographic area.
  • Population dynamics The study of the factors that affect the growth, stability, and decline of populations, as well as the interactions of those factors.
  • Portal system An arrangement in which capillaries drain into a vein that opens into another capillary network.
  • Positive feedback Biochemical control where the accumulation of the product stimulates production of an enzyme responsible for that product's production.
  • Positive feedback control Occurs when information produced by the feedback increases and accelerates the response.
  • Precambrian Informal term describing 7/8 of geologic time from the beginning of the earth to the beginning of the Cambrian Period of the Paleozoic Era. During this time the atmosphere and oceans formed, life originated (or possibly "colonized" Earth), eukaryotes and simple animals evolved and by the end of the precambrian they began to accumulate hard preservable parts, the common occurrence of which marks the beginning of the Cambrian.
  • Precipitation The part of the hydrologic cycle in which the water vapor in the atmosphere falls to Earth as rain or snow.
  • Predation One of the biological interactions that can limit population growth; occurs when organisms kill and consume other living organisms.
  • Predatory release Occurs when a predator species is removed from a prey species such as by great reduction in the predator's population size or by the migration of the prey species to an area without major predators. The removal of the predator releases the prey from one of the factors limiting its population size.
  • Prehensile movement The ability to seize or grasp.
  • Prenatal testing Testing to detect the presence of a genetic disorder in an embryo or fetus; commonly done by amniocentesis or chorionic villi sampling.
  • Presymptomatic screening Testing to detect genetic disorders that only become apparent later in life. The tests are done before the condition actually appears, such as with Huntington disease.
  • Prey switching The tendency of predators to switch to a more readily available prey when one prey species becomes rare; allows the Þrst prey population to rebound and helps prevent its extinction.
  • Primary body Those parts of a plant produced by the shoot and root apical meristems.
  • Primary cell wall The cell wall outside the plasma membrane that surrounds plant cells; composed of the polysaccharide cellulose.
  • Primary compounds Chemicals made by plants and needed for the plant's own metabolism.
  • Primary growth Cells produced by an apical meristem. The growth a plant by the actions of apical meristems on the shoot and root apices in producing plant primary tisues.
  • Primary macronutrients Elements that plants require in relatively large quantities: nitrogen, phosphorus, and potassium.
  • Primary meristems The apical meristems on the shoot and root apices in plants that produce plant primary tissues.
  • Primary root The Þrst root formed by a plant.
  • Primary structure The sequence of amino acids in a protein.
  • Primates The taxonomic order of mammals that includes prosimians (lemurs and tarsiers), monkeys, apes, and humans; characteristics include large brain, stereoscopic vision, and grasping hand.
  • Principle of independent assortment Mendel's second law; holds that during gamete formation, alleles in one gene pair segregate into gametes independently of the alleles of other gene pairs. As a result, if enough gametes are produced, the collective group of gametes will contain all combinations of alleles possible for that organism.
  • Principle of segregation Mendel's Þrst law; holds that each pair of factors of heredity separate during gamete formation so that each gamete receives one member of a pair.
  • Prions Infectious agents composed only of one or more protein molecules without any accompanying genetic information.
  • Producers The Þrst level in a food pyramid; consist of organisms that generate the food used by all other organisms in the ecosystem; usually consist of plants making food by photosynthesis.
  • Progesterone One of the two female reproductive hormones secreted by the ovaries.
  • Prokaryote Type of cell that lacks a membrane-bound nucleus and has no membrane organelles; a bacterium. Prokaryotes are more primitive than eukaryotes. Cells lacking membrane-bound organelles and having a single circular chromosome, and ribosomes surrounded by a cell membrane. Prokaryotes were the first forms of life on earth, evolving over 3.5 billion years ago.
  • Prolactin A hormone produced by the anterior pituitary; secreted at the end of pregnancy when it activates milk production by the mammary glands.
  • Promoter The speciþc nucleotide sequence in DNA that marks the beginning of a gene.
  • Prophase 1) The Þrst stage of mitosis during which chromosomes condense, the nuclear envelope disappears, and the centrioles divide and migrate to opposite ends of the cell. 2) The first stage of mitosis and meiosis (although in meiosis this phase is denoted with either a roman numeral I or II) where the chromatin condenses to form chromosomes, nucleolus dissolves, nuclear envelope dissolves, and the spindle begins to form.
  • Prostaglandins A class of fatty acids that has many of the properties of hormones; synthesized and secreted by many body tissues and have a variety of effects on nearby cells.
  • Prostate gland A gland that is located near and empties into the urethra; produces a secretion that enhances sperm viability. Gland involved in the reproductive system in males, the prostate secretes a sperm activating chemical into the semen during the arousal/ejaculation response.
  • Proteinoids Polymers of amino acids formed spontaneously from inorganic molecules; have enzyme-like properties and can catalyze chemical reactions.
  • Proteins Polymers made up of amino acids that perform a wide variety of cellular functions. One of the classes of organic macromolecules that function as structural and control elements in living systems. Proteins are polymers of amino acids linked together by peptide bonds.
  • Prothallus In ferns, a small heart-shaped bisexual gametophyte.
  • Protista The taxonomic Kingdom from which the other three eukaryotic kingdoms (Fungi, Animalia and Plantae) are thought to have evolved. The earliest eukaryotes were single-celled organisms that would today be placed in this admittedly not monophyletic group. The endosymbiosis theory suggests that eukaryotes may have evolved independently several times.
  • Protists Single-celled organisms; a type of eukaryote. Protista
  • Proton A subatomic particle in the nucleus of an atom that carries a positive charge. The positively charged (+1) subatomic particle located in the atomic nucleus and having a mass slightly less than that of a neutron. Elements differ by the number of protons in their atoms.
  • Protostomes Animals in which the Þrst opening that appears in the embryo becomes the mouth; e.g., mollusks, annelids, and arthropods.
  • Protozoa Single-celled protists grouped by their method of locomotion. This group includes Paramecium, Amoeba, and many other commonly observed protists. |
  • Proximal tubule The winding section of the renal tubule where most reabsorption of water, sodium, amino acids, and sugar takes place.
  • Pseudocoelom In nematodes, a closed þuid-containing cavity that acts as a hydrostatic skeleton to maintain body shape, circulate nutrients, and hold the major body organs.
  • Pseudocoelomates Animals that have a body cavity that is in direct contact with the outer muscular layer of the body and does not arise by splitting of the mesoderm; e.g., roundworms.
  • Pseudopodia Temporary cytoplasmic extensions from a cell that enables it to move (sing.: pseudopodium).
  • Pulmonary artery The artery that carries blood from the right ventricle of the vertebrate heart to the lungs. Artery carrying oxygen-poor blood from the heart to the lungs.
  • Pulmonary circuit The loop of the circulatory system that carries blood to and from the lungs.
  • Pulmonary vein The vein that carries oxygenated blood from the lungs to the left atrium of the heart. Veins carrying oxygenated blood from the lungs to the heart.
  • Punctuated equilibrium A model that holds that the evolutionary process is characterized by long periods with little or no change interspersed with short periods of rapid speciation.
  • Purine One of the groups of nitrogenous bases that are part of a nucleotide. Purines are adenine and guanine, and are double-ring structures.
  • Pyloric sphincter The ring of muscle at the junction of the stomach and small intestine that regulates the movement of food into the small intestine.
  • Pyrimidine One of the groups of nitrogenous bases that are part of a nucleotide. Pyrimidines are single ringed, and consist of the bases thymine (in DNA), uracil (replacing thymine in RNA), and cytosine.
  • Q
  • Quantum models of speciation Models of evolution that hold that speciation sometimes occurs rapidly as well as over long periods, as the classical theory proposed.
  • Quaternary Period The most recent geologic period of the Cenozoic Era, the Quaternary began 2 million years ago with the growth of northern hemisphere continental glaciers and the ice age.
  • Quaternary structure In some proteins, a fourth structural level created by interactions with other proteins. Aspect of protein structure determined by the number and arrangement of polypeptides in a large protein such as hemoglobin.
  • R
  • Race A subdivision of a species that is capable of interbreeding with other members of the species.
  • Radially symmetrical In animals, refers to organisms with their body parts arranged around a central axis. Such animals tend to be circular or cylindrical in shape.
  • Radiation Energy emitted from the unstable nuclei of radioactive isotopes.
  • Radioactive decay The spontaneous decay of an atom to an atom of a different element by emission of a particle from its nucleus (alpha and beta decay) or by electron capture.
  • Radioisotope Term applied to a radioactive isotope, such as carbon-14 or uranium 238. Radioisotope nuclei are unstable and spontaneously breakdown and emit one of a number of types of radiation.
  • Radiometric time Type of absolute time determined by the relative porportions of radioisotopes to stable daughter isotopes.
  • Ray-finned Taxonomic group of fish, such as trout, tuna, salmon, and bass, that have thin, bony supports holding the Þns away from the body and an internal swim bladder that changes the buoyancy of the body; one of the two main types of bony Þshes.
  • Reabsorption The return to the blood of most of the water, sodium, amino acids, and sugar that were removed during Þltration; occurs mainly in the proximal tubule of the nephron.
  • Receptacle The base that attaches a þower to the stem.
  • Receptor Protein on or protruding from the cell surface to which select chemicals can bind. The opiate receptor in brain cells allows both the natural chemical as well as foreign (opiate) chemicals to bind.
  • Recessive Refers to an allele of a gene that is expressed when the dominant allele is not present. An allele expressed only in homozygous form, when the dominant allele is absent.
  • Recombinant DNA molecules New combinations of DNA fragments formed by cutting DNA segments from two sources with restriction enzyme and then joining the fragments together with DNA ligase. Interspecies transfer of genes usually through a vector such as a virus or plasmid.
  • Recombinant DNA technology A series of techniques in which DNA fragments are linked to self-replicating forms of DNA to create recombinant DNA molecules. These molecules in turn are replicated in a host cell to create clones of the inserted segments.
  • Recombination A way in which meiosis produces new combinations of genetic information. During synapsis, chromatids may exchange parts with other chromatids, leading to a physical exchange of chromosome parts; thus, genes from both parents may be combined on the same chromosome, creating a new combination.
  • Red algae Common name for the algae placed in the division Rhodophyta.
  • Red blood cell Component of the blood that transports oxygen with the hemoglobin molecule. See also erythrocyte
  • Red tides Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red structures inside the dinoflagellates cause the water to have a reddish color.
  • Reduction The gain of an electron or a hydrogen atom. The gain of electrons or hydrogens in a chemical reaction.
  • Reductional division The Þrst division in meiosis; results in each daughter cell receiving one member of each pair of chromosomes.
  • Reflex A response to a stimulus that occurs without conscious effort; one of the simplest forms of behavior.
  • Reflex arc Pathway of neurons, effector(s) and sensory receptors that participate in a reflex.
  • Region of division The area of cell division in the tip of a plant root.
  • Region of elongation The area in the tip of a plant root where cells grow by elongating, thereby increasing the length of the root.
  • Region of maturation (differentiation) The area where primary tissues and root hairs develop in the tip of a plant root.
  • Relative time Type of geologic time (absolute time being the other) that places events in a sequence relative to each other.
  • Renal tubule The portion of the nephron where urine is produced.
  • Renin An enzyme secreted by the kidneys that converts angiotensinogen into angiotensin II.
  • Replication Process by which DNA is duplicated prior to cell division.
  • Reproductive isolating mechanism Biological or behavioral characteristics that reduce or prevent interbreeding with other populations; e.g., the production of sterile hybrids. Establishment of reproductive isolation is considered essential for development of a new species.
  • Reproductive system One of eleven major body organ systems in animals; is responsible for reproduction and thus the survival of the species.
  • Reptiles Taxonomic class of vertebrates characterized by scales and amniotic eggs; the first truly terrestrial vertebrate group.
  • Resolution In relation to microscopes, the ability to view adjacent objects as distinct structures.
  • Resource partitioning The division of resources such that a few dominant species exploit most of the available resources while other species divide the remainder; helps explain why a few species are abundant in a community while others are represented by only a few individuals.
  • Respiration 1) breathing as part of gas exchange; or 2) cellular metabolism.
  • Respiratory surface A thin, moist, epithelial surface that oxygen can cross to move into the body and carbon dioxide can cross to move out of the body.
  • Respiratory system One of eleven major body organ systems in animals; moves oxygen from the external environment into the internal environment and removes carbon dioxide from the body.
  • Resting potential The difference in electrical charge across the plasma membrane of a neuron.
  • Restriction enzymes A series of enzymes that attach to DNA molecules at speciþc nucleotide sequences and cut both strands of DNA at those sites. A bacterial enzyme that cuts DNA at a specific recognition sequence. This is a bacterial defense against viral DNA and plasmid DNA and is now used as an important tool in biotechnology.
  • Restriction fragment length polymorphism (RFLP) A heritable difference in DNA fragment length and fragment number; passed from generation to generation in a codominant way.
  • Retina The inner, light-sensitive layer of the eye; includes the rods and cones.
  • Retroviruses Viruses that contain a single strand of RNA as their genetic material and reproduce by copying the RNA into a complementary DNA strand using the enzyme reverse transcriptase. The single-stranded DNA is then copied, and the resulting double-stranded DNA is inserted into a chromosome of the host cell. | | |
  • Reverse transcriptase An enzyme used in the replication of retroviruses; aids in copying the retrovirus's RNA into a complementary strand of DNA once inside the host cell. | |
  • Reverse transcription Process of transcribing a single-stranded DNA from a single-stranded RNA (the reverse of transcription); used by retroviruses as well as in biotechnology.
  • Rheumatoid arthritis A crippling form of arthritis that begins with inþammation and thickening of the synovial membrane, followed by bone degeneration and disþgurement.
  • Rhizoids Filamentous structures in the plants group known as bryophytes that attach to a substrate and absorb moisture. The term is also applied to similar structures found outside the bryophytes.
  • Rhizome In ferns, a horizontal stem with upright leaves containing vascular tissue.
  • Rhodopsin A visual pigment contained in the rods of the retina in the eye..
  • Ribonucleic acid (RNA) Nucleic acid containing ribose sugar and the base Uracil; RNA functions in protein synthesis. The single starnded molecule transcribed from one strand of the DNA. There are three types of RNA, each is involved in protein synthesis. RNA is made up nucleotides containing the sugar ribose, a phosphate group, and one of four nitrogenous bases (adenine, uracil, cytosine or guanine).
  • Ribose Sugar found in nucleotides of RNA and in ATP.
  • Ribosomal RNA One of the three types of RNA; rrna is a structural component in ribosomes.
  • Ribosomal subunits Two units that combine with mrna to form the ribosomal-mrna complex at which protein synthesis occurs.
  • Ribosomes Small organelles made of rrna and protein in the cytoplasm of prokaryotic and eukaryotic cells; aid in the production of proteins on the rough endoplasmic reticulum and ribosome complexes. The site of protein synthesis. The ribosome is composed of two subunits that attach to the mrna at the beginning of protein synthesis and detach when the polypeptide has been translated.
  • RNA polymerase During transcription, an enzyme that attaches to the promoter region of the DNA template, joins nucleotides to form the synthesized strand of RNA and detaches from the template when it reaches the terminator region.
  • RNA transcript Term applied to RNA transcribed in the nucleus.
  • Rodinia Name applied to the precambrian supercontinent.
  • Rods Light receptors in primates' eyes that provide vision in dim light.
  • Root cap Structure that covers and protects the apical meristem in plant roots. Cells forming a protective series of layers over the root meristem.
  • Root hairs Extensions of the root epidermis that increase the root's ability to absorb water.
  • Root system Plant organ systems that anchors the plant in place, stores excess sugars, and absorbs water and mineral nutrients. That part of the plant below ground level.
  • Root-leaf-vascular system axis Refers to the arrangement in vascular plants in which the roots anchor the plant and absorb water and nutrients, the leaves carry out photosynthesis, and the vascular system connects the roots and leaves, carrying water and nutrients to the leaves and carrying sugars and other products of photosynthesis from the leaves to other regions of the plant.
  • Roots Organs, usually occurring underground, that absorb nutrients and water and anchor the plant; one of the three major plant organ systems.
  • Rubp Ribulose biphosphate; the 5-carbon chemical that combines with carbon dioxide at the beginning of the Calvin Cycle.
  • S
  • S phase That period of interphase when new DNA is synthesized as part of replication of the chromatin.
  • Salivary amylase An enzyme secreted by the salivary glands that begins the breakdown of complex sugars and starches.
  • Salivary glands Glands that secrete salvia into the mouth.
  • Saprophytes Organisms that obtain their nutrients from decaying plants and animals. Saprophytes are important in recycling organic material.
  • Sapwood Layers of secondary xylem that are still functional in older woody plants; visible as the outer lighter areas in the cross section of a tree trunk.
  • Sarcomeres The functional units of skeletal muscle; consist of Þlaments of myosin and actin.
  • Saturated fat A fat with single covalent bonds between the carbons of its fatty acids.
  • Schwann cells Specialized glial cells that form the myelin sheath that coats many axons. Cells surrounding the axons of some neurons, thus forming the myelin sheath.
  • Scientific method Systematic apporach of observation, hypothesis formation, hypothesis testing and hypothesis evaluation that forms the basis for modern science.
  • Sclereids Plant cells with thick secondary walls that provide the gritty textures in pears.
  • Sclerenchyma One of the three major cell types in plants; have thickened, rigid, secondary walls that are hardened with lignin; provide support for the plant. Sclerenchyma cells include Þbers and sclereids. Plant tissue type consisting of elongated cells with thickened secondary walls for support of the plant. |
  • Scrotum In mammals, a pouch of skin located outside the body cavity into which the testes descend; provides proper temperature for the testes.
  • Second law of thermodynamics (entropy) The energy available after a chemical reaction is less than that at the beginning of a reaction; energy conversions are not 100% efficient.
  • Second messenger The mechanism by which nonsteroid hormones work on target cells. A hormone binds to receptors on the cell's plasma membrane activating a molecule&emdash;the second messenger&emdash;that activates other intracellular molecules that elicit a response. The second messenger can be cyclic AMP, cyclic GMP, inositol triphosphate, diacrylglycerol, or calcium. | |
  • Secondary (lateral) meristems Plant meristems that produce secondary growth from a cambium.
  • Secondary cell wall In woody plants, a second wall inside the primary cell wall; contains alternating layers of cellulose and lignin.
  • Secondary compounds Plant products that are not important in metabolism but serve other purposes, such as attracting animals for pollination or killing parasites.
  • Secondary extinction The death of one population due to the extinction of another, often a food species.
  • Secondary growth Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth is secondary xylem, better known as wood. | |
  • Secondary immunity Resistance to an antigen the second time it appears. Because of the presence of B and T memory cells produced during the Þrst exposure to the antigen, the second response is faster and more massive and lasts longer than the primary immune response.
  • Secondary macronutrients Elements that plants require in relatively small quantities: calcium, magnesium, and sulfur.
  • Secondary phloem Phloem produced by the vascular cambium in a woody plant stem or root.
  • Secondary structure The structure of a protein created by the formation of hydrogen bonds between different amino acids; can be a pleated sheet, alpha helix, or random coil. Shape of a protein caused by attraction between R-groups of amino acids.
  • Secondary xylem Xylem produced by the vascular cambium in a woody plant stem or root; wood.
  • Secretin A hormone produced in the duodenum that stimulates alkaline secretions by the pancreas and inhibits gastric emptying.
  • Secretion The release of a substance in response to the presence of food or speciþc neural or hormonal stimulation.
  • Sediment Loose aggregate of solids derived from preexisting rocks, or solids precipitated from solution by inorganic chemical processes or extracted from solution by organisms.
  • Sedimentary rock Any rock composed of sediment, i.e., solid particles and dissolved minerals. Examples include rocks that form from sand or mud in riverbeds or on the sea bottom.
  • Seed coat The tough outer layer of the seed, derived from the outer layers of the ovule.
  • Seed Structure produced by some plants in which the next generation sporophyte is surrounded by gametophyte nutritive tissues. An immature sporophyte in an arrested state of development, surrounded by a protective seed coat.
  • Segments Repeating units in the body parts of some animals.
  • Segregation Separation of replicated chromosomes to opposite sides of the cell. Distribution of alleles on chromosomes into gametes during meiosis.
  • Selective breeding The selection of individuals with desirable traits for use in breeding. Over many generations, the practice leads to the development of strains with the desired characteristics.
  • Selectively permeable Term describing a barrier that allows some chemicals to pass but not others. The cell membrane is such a barrier.
  • Semen A mixture of sperm and various glandular secretions.
  • Semiconservative replication Process of DNA replication in which the DNA helix is unwound and each strand serves as a template for the synthesis of a new complementary strand, which is linked to the old strand. Thus, one old strand is retained in each new molecule.
  • Semilunar valve A valve between each ventricle of the heart and the artery connected to that ventricle.
  • Seminal vesicles Glands that contribute fructose to sperm. The fructose serves as an energy source. The structures that add fructose and hormones to semen.
  • Seminiferous tubules Tubules on the interior of the testes where sperm are produced.
  • Sensor In a closed system, the element that detects change and signals the effector to initiate a response.
  • Sensory (afferent) pathways The portion of the peripheral nervous system that carries information from the organs and tissues of the body to the central nervous system.
  • Sensory cortex A region of the brain associated with the parietal lobe.
  • Sensory input Stimuli that the nervous system receives from the external or internal environment; includes pressure, taste, sound, light, and blood ph.
  • Sensory neurons Neurons that carry signals from receptors and transmit information about the environment to processing centers in the brain and spinal cord. Neurons carrying messages from sensory receptors to the spinal cord. Sometimes referred to as an afferent neuron.
  • Sepals Modified leaves that protect a flower's inner petals and reproductive structures. Small, leaf-like structures in flowers that enclose and protect the developing flower. These are often green, but in many monocots they are the same color as the petals (in which case the term tepal is applied since sepals and petals look so much alike).
  • Separation Splitting of the cytoplasm by cytokinesis (= cytokinesis).
  • Severe combined immunodeficiency (SCID) A genetic disorder in which afþicted individuals have no functional immune system and are prone to infections. Both the cell-mediated immune response and the antibody-mediated response are absent.
  • Sex chromosomes The chromosomes that determine the sex of an organism. In humans, females have two X chromosomes, and males have one X chromosome and one Y chromosome. Chromosome that determines the gender (sex) of the individual. Human males have a large X and a smaller Y sex chromosomes, while human females have two X sex chromosomes.
  • Sex hormones A group of steroid hormones produced by the adrenal cortex. Hormones that are produced in the gonads and promoted development and maintainence of the secondary sex characteristics and structures, prepare the female for pregnancy, and aid in development of gametes. Males produce testosterone, while females produce estrogen and progesterone.
  • Sex linkage The condition in which the inheritance of a sex chromosome is coupled with that of a given gene; e.g., red-green color blindness and hemophilia in humans. Traits located on the X-chromosome.
  • Sexual reproduction A system of reproduction in which two haploid sex cells (gametes) fuse to produce a diploid zygote.
  • Shoot The plant stem; provides support for the leaves and þowers; one of the three major plant organs; also referred to as the shoot system.
  • Short-day plants Plants that þower during early spring or fall when nights are relatively long and days are short; e.g., poinsettia and dandelions.
  • Sickle cell anemia Human autosomal recessive disease that causes production of abnormal red blood cells that collapse (or sickle) and cause circulatory problems.
  • Sieve cells Conducting cells in the phloem of vascular plants. See sieve elements
  • Sieve elements Tubular, thin-walled cells that form a system of tubes extending from the roots to the leaves in the phloem of plants; lose their nuclei and organelles at maturity, but retain a functional plasma membrane.
  • Sieve plates Pores in the end walls of sieve elements that connect the sieve elements together. The end walls of sieve tube cells that are perforated (sieves).
  • Sieve tube members Phloem cells that form long sieve tubes. See sieve elements.
  • Silica Silicon dioxide.
  • Silurian Period The geological time period of the Paleozoic Era following the Ordovician, between 435 and 395 million years ago, when plants colonized the land.
  • Simple leaf A leaf in which the blade does not form leaflets.
  • Sink A body or process that acts as a storage device or disposal mechanism; e.g., plants and the oceans act as sinks absorbing atmospheric carbon dioxide. Also, a location in a plant where sugar is being consumed, either in metabolism or by conversion to starch.
  • Sinoatrial (SA) node A region of modiþed muscle cells in the right atrium that sends timed impulses to the heart's other muscle cells, causing them to contract; the heart's pacemaker.
  • Sister chromatids Chromatids joined by a common centromere and carrying identical genetic information (unless crossing-over has occurred).
  • Skeletal muscle Muscle that is generally attached to the skeleton and causes body parts to move; consists of muscle Þbers. Voluntary muscle cells that have a striated appearance. These muscles control skeletal movements and are normally under conscious control.
  • Skeletal system One of eleven major body organ systems in animals; supports the body, protects internal organs, and, with the muscular system, allows movement and locomotion.
  • Skin One of eleven major body organ systems in animals; the outermost layer protecting multicellular animals from the loss or exchange of internal þuids and from invasion by foreign microorganisms; composed of two layers: the epidermis and dermis.
  • Sleep movement In legumes, the movement of the leaves in response to daily rhythms of dark and light. The leaves are horizontal in daylight and folded vertically at night.
  • Sliding filament model Model of muscular contraction in which the actin Þlaments in the sarcomere slide past the myosin Þlaments, shortening the sarcomere and therefore the muscle. | | |
  • Slime molds Protistans that may represent a transition between protistans and fungi.
  • Small intestine A coiled tube in the abdominal cavity that is the major site of chemical digestion and absorption of nutrients; composed of the duodenum, jejunum, and ileum.
  • Smog A local alteration in the atmosphere caused by human activity; mainly an urban problem that is often due to pollutants produced by fuel combustion.
  • Smooth muscle Muscle that lacks striations; found around circulatory system vessels and in the walls of such organs as the stomach, intestines, and bladder. Involuntary, not striated cells that control autonomic functions such as digestion and artery contraction.
  • Social behavior Behavior that takes place in a social context and results from the interaction between and among individuals.
  • Societies The most highly organized type of social organization; consist of individuals that show varying degrees of cooperation and communication with one another; often have a rigid division of labor.
  • Sodium-potassium pump The mechanism that uses ATP energy to reset the sodium and potassium ions after transmission of a nerve impulse.
  • Soil Weathered rocks and minerals combined with air, water and organic matter that can support plants.
  • Somatic Relating to the non-gonadal tissues and organs of an organism's body.
  • Somatic cell A cell that is not or will not become a gamete; the cells of the body.
  • Somatic nervous system The portion of the peripheral nervous system consisting of the motor neuron pathways that innervate skeletal muscles.
  • Somatic senses All senses except vision, hearing, taste, and smell; include pain, temperature, and pressure.
  • Somatostatin Pancreatic hormone that controls the rate of nutrient absorption into the bloodstream.
  • Somites Mesodermal structures formed during embryonic development that give rise to segmented body parts such as the muscles of the body wall.
  • Special senses Vision, hearing, taste, and smell.
  • Species One or more populations of interbreeding or potentially interbreeding organisms that are reproductively isolated in nature from all other organisms. Populations of individuals capable of interbreeding and producing viable, fertile offspring. The least inclusive taxonomic category commonly used.
  • Species diversity The number of living species on Earth.
  • Species packing The phenomenon in which present-day communities generally contain more species than earlier communities because organisms have evolved more adaptations over time.
  • Species richness The number of species present in a community.
  • Sperm The male gamete.
  • Spermatogenesis The development of sperm cells from spermatocytes to mature sperm, including meiosis.
  • Spicules Needle-shaped skeletal elements in sponges that occur in the matrix between the epidermal and collar cells.
  • Spinal cord A cylinder of nerve tissue extending from the brain stem; receives sensory information and sends output motor signals; with the brain, forms the central nervous system. Nerve cell collections extending from the base of the brain to just below the last rib vertebrae.
  • Spindle apparatus Microtubule construction that aligns and segregates chromosomes during eukaryotic cell division.
  • Spleen An organ that produces lymphocytes and stores erythrocytes.
  • Spongy bone The inner layer of bone; found at the ends of long bones and is less dense than compact bone. Some spongy bone contains red marrow.
  • Spongy mesophyll Parenchyma cells found in plant leaves that are irregularly shaped and have large intracellular spaces. |
  • Sporangia The structures in which spores are produced (sing.: sporangium).
  • Spores Impervious structures formed by some cells that encapsulate the cells and protect them from the environment; haploid cells that can survive unfavorable conditions and germinate into new haploid individuals or act as gametes in fertilization.
  • Sporophyte The diploid stage of a plant exhibiting alternation of generations. The diploid, spore producing phase of the plant life cycle.
  • Sporozoans Members of the protists that are referred to as slime molds; may include organisms resembling the ancestors of fungi.
  • Stability One of the phases of a population's life cycle. The population's size remains roughly constant, þuctuating around some average density. Also, the ability of a community to persist unchanged.
  • Stabilizing selection A process of natural selection that tends to favor genotypic combinations that produce an intermediate phenotype; selection against the extremes in variation.
  • Stalk A leaf's petiole; the slender stem that supports the blade of a leaf and attaches it to a larger stem of the plant.
  • Stamens The male reproductive structures of a þower; usually consist of slender, thread-like filaments topped by anthers. The male reproductive structures in the flower, composed of a filament and anther.
  • Stapes One of the three bones that function in hearing.
  • Start codon The codon (AUG) on a messenger RNA molecule where protein synthesis begins.
  • Steinkerns Internal casts of a fossil. Steinkerns may reveal internal anatomy of an organism, such as muscle attachment, and other details of soft tissue structure.
  • Stem cells Cells in bone marrow that produce lymphocytes by mitotic division.
  • Sternum The breastbone.
  • Steroids Compounds with a skeleton of four rings of carbon to which various side groups are attached; one of the three main classes of hormones.
  • Sticky ends Term applied to DNA sequences cut with restriction enzymes where the cuts will bond with each other or with another sequence cut with the same enzyme.
  • Stigma Part of the female reproductive structure of the carpel of a þower; the sticky surface at the tip of the style to which pollen grains attach. The receptive surface of the pistil (of the flower) on which pollen is placed by a pollinator. |
  • Stimulus A physical or chemical change in the environment that leads to a response controlled by the nervous system.
  • Stolons Stems that grow along the surface of the ground; a method of plant vegetaive propagation.
  • Stomach The muscular organ between the esophagus and small intestine that stores, mixes, and digests food and controls the passage of food into the small intestine.
  • Stomata Pores on the underside of leaves that can be opened or closed to control gas exchange and water loss. Openings in the epidermis (usually of the leaf) that allow gas exchange. |
  • Stomatal apparatus The stomata and guard cells that control the size of the stoma. |
  • Stop codon The codon on a messenger RNA molecule where protein synthesis stops.
  • Stratification The division of water in lakes and ponds into layers with different temperatures and oxygen content. Oxygen content declines with depth, while the uppermost layer is warmest in summer and coolest in winter.
  • Stressed community A community that is disturbed by human activity, such as road building or pollution, and is inadvertently simpliþed. Some species become superabundant while others disappear.
  • Stroma The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thylakoids, grana) inside the chloroplast.
  • Stromatolite A sedimentological and biological "fossil" representinmg colonies of bacteria altenating with layers of sediments. Becoming more common during the Proterozoic, stromatolites persist today in marine environments where grazing by herbivorous organisms is limited. |
  • Style Part of the female reproductive structure in the carpel of a þower; formed from the ovary wall. The tip of the style carries the stigma to which pollen grains attach. Part of the pistil that separates the stigma from the ovary. |
  • Subatomic particles The three kinds of particles that make up atoms: protons, neutrons, and electrons.
  • Suberin Waxy, waterproof chemical in some plant cells, notably cork (in stems) and endodermis cells (in roots).
  • Subspecies A taxonomic subdivision of a species; a population of a particular region genetically distinguishable from other such populations and capable of interbreeding with them.
  • Substitution A type of mutation in which one base is substituted for another.
  • Substrate feeders Animals such as earthworms or termites that eat the soil or wood through which they burrow.
  • Sudden infant death syndrome (SIDS) A disorder resulting in the unexpected death during sleep of infants, usually between the ages of two weeks and one year. The causes are not fully understood, but are believed to involve failure of automatic respiratory control.
  • Superior vena cava Blood from the head returns to the heart through this main vein.
  • Suppressor T cells T cells that slow down and stop the immune response of B cells and other T cells. Immune system cells that shut off the antibody production when an infection is under control.
  • Suprachiasmic nucleus (SCN) A region of the hypothalamus that controls internal cycles of endocrine secretion.
  • Symbiosis An interactive association between two or more species living together; may be parasitic, commensal, or mutualistic. The relationship between two organisms.
  • Sympathetic system The subdivision of the autonomic nervous system that dominates in stressful or emergency situations and prepares the body for strenuous physical activity, e.g., causing the heart to beat faster.
  • Synapse The junction between an axon and an adjacent neuron.
  • Synapsis The alignment of chromosomes during meiosis I so that each chromosome is beside its homologue.
  • Synaptic cleft The space between the end of a neuron and an adjacent cell.
  • Synaptic vesicles Vesicles at the synapse end of an axon that contain the neurotransmitters.
  • Synergid Cells in the embryo sac of angiosperms that flank the egg cell. The pollen tube grows through one (usually the smaller) of the synergids.
  • Synovial joint The most movable type of joint. The bones are covered by connective tissue, the interior of which is Þlled with synovial þuid, and the ends of the bones are covered with cartilage.
  • Syphilis A sexually transmitted disease caused by a bacterial infection that produces an ulcer on the genitals and can have potentially serious effects if untreated.
  • Systematics The classiþcation of organisms based on information from observations and experiments; includes the reconstruction of evolutionary relatedness among living organisms. Currently, a system that divides organisms into Þve kingdoms (Monera, Protista, Plantae, Fungi, Animalia) is widely used.
  • Systemic circuit The loop of the circulatory system that carries blood through the body and back to the heart.
  • Systole The contraction of the ventricles that opens the semilunar valve and forces blood into the arteries.
  • Systolic pressure The peak blood pressure when ventricles contract.
  • T
  • T cells The type of lymphocyte responsible for cell-mediated immunity; also protects against infection by parasites, fungi, and protozoans and can kill cancerous cells; circulate in the blood and become associated with lymph nodes and the spleen.
  • Taiga biome The region of coniferous forest extending across much of northern Europe, Asia, and North America; characterized by long, cold winters and short, cool summers and by acidic, thin soils.
  • Tap root A primary root that grows vertically downward and gives off small lateral roots; occurs in dicots. Root system in plants characterized by one root longer than the other roots. Example: carrot.
  • Target cell A cell that a particular hormone effects by its direct action (either passing through the membrane or binding to a surface receptor). |
  • Tarsals The bones that make up the ankle joint.
  • Taxis The behavior when an animal turns and moves toward or away from an external stimulus (pl.: taxes).
  • Taxon Term applied group of organisms comprising a given taxonomic category
  • Taxonomy A systematic method of classifying plants and animals. Classification of organisms based on degrees of similarity purportedly representing evolutionary (phylogenetic) relatedness.
  • Tectonic plates Segments of the lithosphere that comprise the surface of the Earth much the way a turtle shell is composed of its plates.
  • Telophase The Þnal stage of mitosis in which the chromosomes migrate to opposite poles, a new nuclear envelope forms, and the chromosomes uncoil. The last phase of nuclear division in eukaryotes when the segregated chromosomes uncoil and begin to reform nuclei. This is immediately followed (in most cases) by cytokinesis.
  • Temperate forest biome Extends across regions of the northern hemisphere with abundant rainfall and long growing seasons. Deciduous, broad-leaved trees are the dominant plants.
  • Template strand The strand of DNA that is transcribed to make RNA.
  • Temporal lobe The lobe of the cerebral cortex that is responsible for processing auditory signals.
  • Tendons Bundles of connective tissue that link muscle to bone. Fibrous connective tissue that connects muscle to bone.
  • Terminal buds Buds located at the end of a plant shoot.
  • Termination The end of translation; occurs when the ribosome reaches the stop codon on the messenger RNA molecule and the polypeptide, the messenger RNA, and the transfer RNA molecule are released from the ribosome.
  • Termination codon One of three three-base sequences that initiate termination of the protein synthesis process. See stop codon.
  • Tertiary structure The folding of a protein's secondary structure into a functional three-dimensional conþguration. Shape assumed by protein due to interactions between amino acids far apart on the chain.
  • Test cross Genetic crossing of an organism with known genotype (one that exhibits the recessive phenotype) with an individual expressing the dominant phenotype but of unknown heritage.
  • Testes The male gonad; produce spermatozoa and male sex hormones. Male gonads in mammals. Singular, testis. Paired organs that contain seminiferous tubules in which sperm are produced. |
  • Testosterone Male sex hormone that stimulates sperm formation, promotes the development of the male duct system in the fetus, and is responsible for secondary sex characteristics such as facial hair growth.
  • Tetrad The four chromatids in each cluster during synapsis; formed by the two sister chromatids in each of the two homologous chromosomes.
  • Thalamus The brain region that serves as a switching center for sensory signals passing from the brain stem to other brain regions; part of the diencephalon.
  • Thecodonts An informal term for a variety of Permian and Triassic reptiles that had teeth set in individual sockets. Small, bipedal thecodontians are the probable ancestors of dinosaurs.
  • Theory A hypothesis that has withstood extensive testing by a variety of methods, and in which a higher degree of certainty may be placed. A theory is NEVER a fact, but instead is an attempt to explain one or more facts.
  • Thermacidophiles A group of archaebacteria that are able to tolerate high temperatures and acidic ph.
  • Thermiogenesis The generation of heat by raising the body's metabolic rate; controlled by the hypothalamus.
  • Thermoregulation The regulation of body temperature.
  • Thigmotropism Plants' response to contact with a solid object; e.g., tendrils' twining around a pole. Plant response to touch.
  • Thoracic cavity The chest cavity in which the heart and lungs are located.
  • Thorax In many arthropods, one of three regions formed by the fusion of the segments (others are the head and abdomen).
  • Thorns Stems modified to protect the plant.
  • Thoroughfare channels Shortcuts within the capillary network that allow blood to bypass a capillary bed.
  • Thylakoids The specialized membrane structures in which photosynthesis takes place. Internal membranes in the chloroplast where the light reaction chemicals are embedded. Collections of thylakoids form the grana.
  • Thymine One of the pyrimidine bases in DNA, thymine is replaced by uracil in RNA.
  • Thyroid-stimulating hormone A hormone produced by the anterior pituitary that stimulates the production and release of thyroid hormones.
  • Tight junctions Junctions between the plasma membranes of adjacent cells in animals that form a barrier, preventing materials from passing between the cells.
  • Tissues Groups of similar cells organized to carry out one or more speciþc functions. Groups of cells performing a function in a multicellular organism.
  • Toxins Term applied to poisons in living systems.
  • Trace fossil Any indication of prehistoric organic activity, such as tracks, trails, burrows, or nests.
  • Trachea In insects and spiders, a series of tubes that carry air directly to cells for gas exchange; in humans, the air-conducting duct that leads from the pharynx to the lungs. |
  • Tracheids Long, tapered cells with pitted walls that form a system of tubes in the xylem and carry water and solutes from the roots to the rest of the plant. One type of xylem cells. Tracheids are long and relatively narrow, and transport materials from the roots upward. Tracheids are dead at maturity and have lignin in their secondary walls. |
  • Transcription The synthesis of RNA from a DNA template. The making of RNA from one strand of the DNA molecule.
  • Transfer rnas (trnas) Small, single-stranded RNA molecules that bind to amino acids and deliver them to the proper codon on messenger RNA. The trucks of protein synthesis that carry the specified amino acid to the ribosome. Abbreviated trna.
  • Transformation In grifþth's experiments with strains of pneumonia bacterium, the process by which hereditary information passed from dead cells of one strain into cells of another strain, causing them to take on the characteristic virulence of the Þrst strain.
  • Transforming factor grifþth's name for the unknown material leading to transformation; later found to be DNA.
  • Transition reaction Biochemical process of converting 3-carbon pyruvate into 2-carbon acetyl and attaching it to coenzyme A (coa) so it can enter Kreb's cycle. Carbon dioxide is also released and NADH is formed (from NAD and H) in this process.
  • Translation The synthesis of protein on a template of messenger RNA; consists of three steps: initiation, elongation, and termination. Making of a polypeptide sequence by translating the genetic code of an mrna molecule associated with a ribosome.
  • Translocation 1) The movement of a segment from one chromosome to another without altering the number of chromosomes. 2) the movement of þuids through the phloem from one part of a plant to another, with the direction of movement depending on the pressure gradients between source and sink regions.
  • Transpiration The loss of water molecules from the leaves of a plant; creates an osmotic gradient; producing tension that pulls water upward from the roots. | |
  • Triassic Period The first period of the Mesozoic Era between 225 and 185 million years ago. Pangaea began to breakup during this time. The ancestors of dinosaurs were present, as were early mammals and mammal-like reptiles.
  • Trichocysts Barbed, thread-like organelles of ciliated protozoans that can be discharged for defense or to capture prey.
  • Trichomes Extensions from the epidermis of the plant that provide shade and protection for the plant.
  • Trilobites A group of benthonic, detritus-feeding, extinct marine invertebrate animals (phylum Arthropoda), having skeletons of an organic compound called chitin. Trilobites appear in abundance early in the Cambrian period and were dominant animals in the Burgess Shale fauna, before finally becoming extinct at the end of the Permian period.
  • Triplet Three-base sequence of mrna that codes for a specific amino acid or termination codon.
  • Trisomy A condition where a cell has an extra chromosome.
  • Trophoblast The outer layer of cells of a blastocyst that adhere to the endometrium during implantation.
  • Tropic hormone Hormone made by one gland that causes another gland to secrete a hormone.
  • Tropical rain forest biome The most complex and diverse biome; found near the equator in South America and Africa; characterized by thin soils, heavy rainfall, and little þuctuation in temperature.
  • Tropism The movement of plant parts toward or away from a stimulus in the plant's environment. Plant movement in response to an environmental stimulus.
  • True-breeding Occurs when self-fertilization gives rise to the same traits in all offspring, generation after generation. Now interpreted as equivalent to homozygous.
  • Trypanosomes A type of roundworm, responsible for human disease associated with eating raw or undercooked pork.
  • Tubal ligation A contraceptive procedure in women in which the oviducts are cut, preventing the ova from reaching the uterus.
  • Tubal pregnancy Occurs when the morula remains in the oviduct and does not descend into the uterus.
  • Tube nucleus One of the cells in the male gametophyte in seed plants. The tube nucleus grows through the stigma, style, and into the ovule, clearing the way for the sperm nuclei to enter the embryo sac.
  • Tubers Swollen underground stems in plants that store food, such as the irish potato.
  • Tube-within-a-tube system A type of body plan in animals. The organism has two openings&emdash;one for food and one for the elimination of waste&emdash;and a specialized digestive system.
  • Tubular secretion The process in which ions and other waste products are transported into the distal tubules of the nephron.
  • Tubulins The protein subunits from which microtubules are assembled.
  • Tumor suppressor genes Genes that normally keep cell division under control, preventing the cell from responding to internal and external commands to divide.
  • Tundra biome Extensive treeless plain across northern Europe, Asia, and North American between the taiga to the south and the permanent ice to the north. Much of the soil remains frozen in permafrost, and grasses and other vegetation support herds of large grazing mammals.
  • Turgor pressure Pressure caused by the cytoplasm pressing against the cell wall.
  • Turner syndrome In humans, a genetically determined condition in which an individual has only one sex chromosome (an X). Affected individuals are always female and are typically short and infertile.
  • U
  • Umbilical cord The structure that connects the placenta and the embryo; contains the umbilical arteries and the umbilical vein.
  • Unicellular Single-celled.
  • Uniformitarianism The idea that geological processes have remained uniform over time and that slight changes over long periods can have large-scale consequences; proposed by James Hutton in 1795 and reþned by Charles Lyell during the 1800s. The principle on which modern geology was founded: processes operating today on the earth operated in much the same way in the geologic past. Sometimes expressed as "the present is the key to the past".
  • Uninucleate Term applied to cells having only a single nucleus.
  • Unsaturated fat A triglyceride with double coavent bonds between some carbon atoms.
  • Uracil The pyrimidine that replaces thymine in RNA molecules and nucleotides.
  • Ureter A muscular tube that transports urine by peristaltic contractions from the kidney to the bladder.
  • Urethra A narrow tube that transports urine from the bladder to the outside of the body. In males, it also conducts sperm and semen to the outside.
  • Urine Fluid containing various wastes that is produced in the kidney and excreted from the bladder.
  • Uterus The organ that houses and nourishes the developing embryo and fetus. The womb. Female reproductive organ in which the fertilized egg implants.
  • V
  • Vaccination The process of protecting against infectious disease by introducing into the body a vaccine that stimulates a primary immune response and the production of memory cells against the disease-causing agent.
  • Vaccine A preparation containing dead or weakened pathogens that when injected into the body elicit an immune response.
  • Vacuoles Membrane-bound þuid-Þlled spaces in plant and animal cells that remove waste products and store ingested food.
  • Vagina The tubular organ that is the site of sperm deposition and also serves as the birth canal.
  • Vas deferens The duct that carries sperm from the epididymis to the ejaculatory duct and urethra. The tube connecting the testes with the urethra.
  • Vascular bundle Groups of xylem, phloem and cambium cells in stems of plants descended from the procambium embryonic tissue layer.
  • Vascular cambium A layer of lateral meristematic tissue between the xylem and phloem in the stems of woody plants. Lateral meristem tissue in plants that produces secondary growth.
  • Vascular cylinder A central column formed by the vascular tissue of a plant root; surrounded by parenchymal ground tissue.
  • Vascular parenchyma Specialized parenchyma cells in the phloem of plants.
  • Vascular plants Group of plants having lignified conducting tissue (xylem vessels or tracheids).
  • Vascular system Specialized tissues for transporting þuids and nutrients in plants; also plays a role in supporting the plant; one of the four main tissue systems in plants.
  • Vasectomy A contraceptive procedure in men in which the vas deferens is cut and the cut ends are sealed to prevent the transportation of sperm. Surgical separation of the vas deferens so that sperm, while still produced, do not leave the body.
  • Vasopressin See antidiuretic hormone.
  • Vectors Self-replicating DNA molecules that can be joined with DNA fragments to form recombinant DNA molecules.
  • Veins Thin-walled vessels that carry blood to the heart. Units of the circulatory system that carry blood to the heart.
  • Ventilation The mechanics of breathing in and out through the use of the diaphragm and muscles in the wall of the thoracic cavity.
  • Ventral Term applied to the lower side of a fish, or to the chest of a land vertebrate.
  • Ventricle The chamber of the heart that pumps the blood into the blood vessels that carry it away from the heart. The lower chamber of the heart through which blood leaves the heart.
  • Venules The smallest veins. Blood þows into them from the capillary beds. Small veins that connect a vein with capillaries.
  • Vernalization artiþcial exposure of seeds or seedlings to cold to enable the plant to þower.
  • Vertebrae The segments of the spinal column; separated by disks made of connective tissue (sing.: vertebra).
  • Vertebrate Any animal having a segmented vertebral column; members of the subphylum Vertebrata; include reptiles, Þshes, mammals, and birds.
  • Vesicles Small membrane-bound spaces in most plant and animal cells that transport macromolecules into and out of the cell and carry materials between organelles in the cell.
  • Vessel elements Short, wide cells arranged end to end, forming a system of tubes in the xylem that moves water and solutes from the roots to the rest of the plant. Large diameter cells of the xylem that are extremely specialized and efficient at conduction. An evolutionary advance over tracheids. Most angiosperms have vessels.
  • Vestigial structures Nonfunctional remains of organs that were functional in ancestral species and may still be functional in related species; e.g., the dewclaws of dogs.
  • Villi Finger-like projections of the lining of the small intestine that increase the surface area available for absorption. Also, projections of the chorion that extend into cavities Þlled with maternal blood and allow the exchange of nutrients between the maternal and embryonic circulations. Projections of the inner layer of the small intestine that increase the surface area for absorbtion of food.
  • Viroids Infective forms of nucleic acid without a protective coat of protein; unencapsulated single-stranded RNA molecules. Naked RNA, possibly of degenerated virus, that infects plants.
  • Virus Infectious chemical agent composed of a nucleic acid (DNA or RNA) inside a protein coat.
  • Vitamins A diverse group of organic molecules that are required for metabolic reactions and generally cannot be synthesized in the body.
  • Vulva A collective term for the external genitals in women.
  • W X Y Z
  • White blood cell Component of the blood that functions in the immune system. Also known as a leukocyte.
  • Wood The inner layer of the stems of woody plants; composed of xylem.
  • X-chromosome One of the sex chromosomes.
  • Xerophytic leaves The leaves of plants that grow under arid conditions with low levels of soil and water. Usually characterized by water-conserving features such as thick cuticle and sunken stomatal pits.
  • X-ray diffraction Technique utilized to study atomic structure of crystalline substances by noting the patterns produced by x-rays shot through the crystal.
  • Xylem Tissue in the vascular system of plants that moves water and dissolved nutrients from the roots to the leaves; composed of various cell types including tracheids and vessel elements. Plant tissue type that conducts water and nutrients from the roots to the leaves.
  • Z lines Dense areas in myoþbrils that mark the beginning of the sarcomeres. The actin Þlaments of the sarcomeres are anchored in the Z lines.
  • Zebroid A hybrid animal that results from breeding zebras and horses.
  • Zone of differentiation Area in plant roots where recently produced cells develop into different cell types.
  • Zone of elongation Area in plant roots where recently produced cells grow and elongate prior to differentiation.
  • Zone of intolerance The area outside the geographic range where a population is absent; grades into the zone of physiological stress.
  • Zone of physiological stress The area in a population's geographic range where members of population are rare due to physical and biological limiting factors.
  • Zygomycetes One of the division of the fungi, characterized by the production of zygospores; includes the bread molds.
  • Zygospore In fungi, a structure that forms from the diploid zygote created by the fusion of haploid hyphae of different mating types. After a period of dormancy, the zygospore forms sporangia, where meiosis occurs and spores form.
  • Zygote A fertilized egg. A diploid cell resulting from fertilization of an egg by a sperm cell.


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