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.

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.

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).

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 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.



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.

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.

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.



Antibiotics: Substances produced by some microorganisms, plants, and vertebrates that kill or inhibit the growth of bacteria.

Antibiotic resistance: Tendency of certain bacteria to develop a resistance to commonly over-used antibiotics.

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): and released by the A hormone produced by the hypothalamuspituitary 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.

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.