Abdomen (ab´ duh mun) [L. belly] • In arthropods, the posterior portion of the body; in mammals, the part of the body containing the intestines and most other internal organs, posterior to the thorax.
Abscisic acid (ab sighs´ ik) [L. abscissio: breaking off] • A plant growth substance having growth-inhibiting action. Causes stomata to close.
Abscission (ab sizh´ un) [L. abscissio: breaking off] • The process by which leaves, petals, and fruits separate from a plant.
Absolute temperature scale • Also known as the Kelvin scale. A temperature scale in which zero is the state of no molecular motion. This "absolute zero" is –273° on the Celsius scale.
Absorption • (1) Of light: complete retention, without reflection or transmission. (2) Of liquids: soaking up (taking in through pores or cracks).
Absorption spectrum • A graph of light absorption versus wavelength of light; shows how much light is absorbed at each wavelength.
Abyssal zone (uh biss´ ul) [Gr. abyssos: bottomless] • That portion of the deep ocean floor where no light penetrates.
Accessory pigments • Pigments that absorb light and transfer energy to chlorophylls for photosynthesis.
Acetylcholine • A neurotransmitter substance that carries information across vertebrate neuromuscular junctions and some other synapses.
Acetylcholinesterase is an enzyme that breaks down acetylcholine.
Acetyl CoA (acetyl coenzyme A) • Compound that reacts with oxaloacetate to produce citrate at the beginning of the citric acid cycle; a key metabolic intermediate in the formation of many compounds.
Acid [L. acidus: sharp, sour] • A substance that can release a proton in solution. (Contrast with base.)
Acid precipitation • Precipitation that has a lower pH than normal as a result of acid-forming precursors introduced into the atmosphere by human activities.
Acidic • Having a pH of less than 7.0 (a hydrogen ion concentration greater than 10-7 molar).
Acoelomate • Lacking a coelom.
Acquired Immune Deficiency Syndrome • See AIDS.
Acrosome (a´ krow soam) [Gr. akros: highest or outermost + soma: body] • The structure at the forward tip of an animal sperm which is the first to fuse with the egg membrane and enter the egg cell.
ACTH (adrenocorticotropin) • A pituitary hormone that stimulates the adrenal cortex.
Actin [Gr. aktis: a ray] • One of the two major proteins of muscle; it makes up the thin filaments. Forms the microfilaments found in most eukaryotic cells.
Action potential • An impulse in a neuron taking the form of a wave of depolarization or hyperpolarization imposed on a polarized cell surface.
Activating enzymes (also called aminoacyl-tRNA synthetases) • These enzymes catalyze the addition of amino acids to their appropriate tRNAs.
Activation energy (Ea) • The energy barrier that blocks the tendency for a set of chemical substances to react.
Active site • The region on the surface of an enzyme where the substrate binds, and where catalysis occurs.
Active transport • The transport of a substance across a biological membrane against a concentration gradient—that is, from a region of low concentration (of that substance) to a region of high concentration. Active transport requires the expenditure of energy and is a saturable process. (Contrast with facilitated diffusion, free diffusion; see primary active transport, secondary active transport.)
Adaptation (a dap tay´ shun) • In evolutionary biology, a particular structure, physiological process, or behavior that makes an organism better able to survive and reproduce. Also, the evolutionary process that leads to the development or persistence of such a trait.
Adenine (a´ den een) • A nitrogen-containing base found in nucleic acids, ATP, NAD, etc.
Adenosine triphosphate • See ATP.
Adenylate cyclase • Enzyme catalyzing the formation of cyclic AMP from ATP.
Adrenal (a dree´ nal) [L. ad-: toward + renes: kidneys] • An endocrine gland located near the kidneys of vertebrates, consisting of two glandular parts, the cortex and medulla.
Adrenaline • See epinephrine.
Adrenocorticotropin • See ACTH.
Adsorption • Binding of a gas or a solute to the surface of a solid.
Aerobic (air oh´ bic) [Gr. aer: air + bios: life] • In the presence of oxygen, or requiring oxygen.
Afferent (af´ ur unt) [L. ad: to + ferre: to bear] • To or toward, as in a neuron that carries impulses to the central nervous system, or a blood vessel that carries blood to a structure. (Contrast with efferents.)
AIDS (acquired immune deficiency syndrome) • Condition caused by a virus (HIV) in which the body’s helper T lymphocytes are reduced, leaving the victim subject to opportunistic diseases.
Aldehyde (al´ duh hide) • A compound with a -CHO functional group. Many sugars are aldehydes. (Contrast with ketone.)
Aldosterone (al dahs´ ter own) • A steroid hormone produced in the adrenal cortex of mammals. Promotes secretion of potassium and reabsorption of sodium in the kidney.
Alga (al´ gah) (plural: algae) [L.: seaweed] • Any one of a wide diversity of protists belonging to the phyla Pyrrophyta, Chrysophyta, Phaeophyta, Rhodophyta, and Chlorophyta.
Allele (a leel´) [Gr. allos: other] • The alternate forms of a genetic character found at a given locus on a chromosome.
Allele frequency • The relative proportion of a particular allele in a specific population.
Allergy [Ger. allergie: altered reaction] • An overreaction to an antigen in amounts that do not affect most people; often involves IgE antibodies.
Allometric growth • A pattern of growth in which some parts of the body of an organism grow faster than others, resulting in a change in body proportions as the organism grows.
Allopatric speciation (al´ lo pat´ rick) [Gr. allos: other + patria: fatherland] • Also called geographical speciation, this is the formation of two species from one when reproductive isolation occurs because of the the interposition of (or crossing of ) a physical geographic barrier such as a river. (Contrast with parapatric speciation, sympatric speciation.)
Allopolyploid • A polyploid in which the chromosome sets are derived from more than one species.
Allostery (al´ lo steer´ y) [Gr. allos: other + stereos: structure] • Regulation of the activity of a protein by the binding of an effector molecule at a site other than the active site.
Alpha helix • Type of protein secondary structure; a right-handed spiral.
Alternation of generations • The succession of haploid and diploid phases in some sexually reproducing organisms, notably plants.
Altruistism • A behavior whose performance harms the actor but benefits other individuals.
Alveolus (al ve´ o lus) (plural: alveoli) [L. alveus: cavity] • A small, baglike cavity, especially the blind sacs of the lung.
Amensalism (a men´ sul ism) • Interaction in which one animal is harmed and the other is unaffected. (Contrast with commensalism, mutualism.)
Amine • An organic compound with an amino group (see Amino acid).
Amino acid • An organic compound of the general formula H2N-CHR-COOH, where R can be one of 20 or more different side groups. An amino acid is so named because it has both a basic amine group, -NH2, and an acidic carboxyl group, -COOH. Proteins are polymers of amino acids.
Ammonotelic (am moan´ o teel´ ic) [Gr. telos: end] • Describes an organism in which the final product of breakdown of nitrogen-containing compounds (primarily proteins) is ammonia. (Contrast with ureotelic, uricotelic.)
Amniocentesis • A medical procedure in which cells from the fetus are obtained from the amniotic fluid. The genetic material of the cells is then examined. (Contrast with chorionic villus sampling.)
Amniote • An organism that lays eggs that can be incubated in air (externally) because the embryo is enclosed by a fluid-filled sac. Birds and reptiles are amniotes.
Amphipathic (am´ fi path´ ic) [Gr. amphi: both + pathos: emotion] • Of a molecule, having both hydrophilic and hydrophobic regions.
Amylase (am´ ill ase) • Any of a group of enzymes that digest starch.
Anabolism (an ab´ uh liz´ em) [Gr. ana: up, throughout + ballein: to throw] • Synthetic reactions of metabolism, in which complex molecules are formed from simpler ones. (Contrast with catabolism.)
Anaerobic (an ur row´ bic) [Gr. an: not + aer: air + bios: life] • Occurring without the use of molecular oxygen, O2.
Anagenesis • Evolutionary change in a single lineage over time.
Analogy (a nal´ o jee) [Gr. analogia: resembling] • A resemblance in function, and often appearance as well, between two structures which is due to convergence in evolution rather than to common ancestry. (Contrast with homology.)
Anaphase (an´ a phase) [Gr. ana: indicating upward progress] • The stage in nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologues) occurs. Anaphase lasts from the moment of first separation to the time at which the moving chromosomes converge at the poles of the spindle.
Anaphylactic shock • A precipitous drop in blood pressure caused by loss of fluid from capillaries because of an increase in their permeability stimulated by an allergic reaction.
Ancestral trait • Trait shared by a group of organisms as a result of descent from a common ancestor.
Androgens (an´ dro jens) • The male sex steroids.
Aneuploidy (an´ you ploy dee) • A condition in which one or more chromosomes or pieces of chromosomes are either lacking or present in excess.
Angiosperm (an´ jee oh spurm) [Gr. angion: vessel + sperma: seed] • One of the flowering plants; literally, one whose seed is carried in a "vessel," which is the fruit. (See fruit.)
Angiotensin (an´ jee oh ten´ sin) • A peptide hormone that raises blood pressure by causing peripheral vessels to constrict; maintains glomerular filtration by constricting efferent glomerular vessels; stimulates thirst; and stimulates the release of aldosterone.
Animal [L. animus: breath, soul] • A member of the kingdom Animalia. In general, a multicellular eukaryote that obtains its food by ingestion.
Animal hemisphere • The metabolically active upper portion of some animal eggs, zygotes, and embryos, which does not contain the dense nutrient yolk. The animal pole refers to the very top of the egg or embyro. (Contrast with vegetal hemisphere.)
Anion (an´ eye one) • An ion with one or more negative charges. (Contrast with cation.)
Anisogamy (an´ eye sog´ a mee) [Gr. aniso: unequal + gamos: marriage] • The existence of two dissimilar gametes (egg and sperm).
Annual • Referring to a plant whose life cycle is completed in one growing season. (Contrast with biennial, perennial.)
Anterior pituitary • The portion of the vertebrate pituitary gland that derives from gut epithelium and produces tropic hormones.
Anther (an´ thur) [Gr. anthos: flower] • A pollen-bearing portion of the stamen of a flower.
Antheridium (an´ thur id´ ee um) (plural: antheridia) [Gr. antheros: blooming] • The multicellular structure that produces the sperm in bryophytes and ferns.
Antibody • One of millions of proteins, produced by the immune system, that specifically recognizes a foreign substance and initiates its removal from the body.
Anticodon • A "triplet" of three nucleotides in transfer RNA that is able to pair with a complementary triplet (a codon) in messenger RNA, thus aligning the transfer RNA on the proper place on the messenger. The codon (and, reciprocally, the anticodon) codes for a specific amino acid.
Antidiuretic hormone • A hormone that controls water reabsorption in the mammalian kidney. Also called vasopressin.
Antigen (an´ ti jun) • Any substance that stimulates the production of an antibody or antibodies in the body of a vertebrate.
Antigen processing • The breakdown of antigenic proteins into smaller fragments, which are then presented on the cell surface, along with MHC proteins, to T cells.
Antigenic determinant • A specific region of an antigen, which is recognized by and binds to a specific antibody.
Antiport • A membrane transport process that carries one substance in one direction and another in the opposite direction. (Contrast with symport.)
Antisense nucleic acid • A single-stranded RNA or DNA complementary to and thus targeted against the mRNA transcribed from a harmful gene such as an oncogene.
Anus (a´ nus) • Opening through which digestive wastes are expelled, located at the posterior end of the gut.
Aorta (a or´ tuh) [Gr. aorte: aorta] • The main trunk of the arteries leading to the systemic (as opposed to the pulmonary) circulation.
Apex (a´ pecks) • The tip or highest point of a structure, as the apex of a growing stem or root.
Apical (a´ pi kul) • Pertaining to the apex, or tip, usually in reference to plants.
Apical dominance • Inhibition by the apical bud of the growth of axillary buds.
Apical meristem • The meristem at the tip of a shoot or root; responsible for the plant’s primary growth.
Apomixis (ap oh mix´ is) [Gr. apo: away from + mixis: sexual intercourse] • The asexual production of seeds.
Apoplast (ap´ oh plast) • in plants, the continuous meshwork of cell walls and extracellular spaces through which material can pass without crossing a plasma membrane. (Contrast with symplast.)
Apoptosis (ay´ pu toh sis) • A series of genetically programmed events leading to cell death.
Aquaporin • A transport protein in plant and animals cells through which water passes in osmosis.
Archegonium (ar´ ke go´ nee um) [Gr. archegonos: first of a kind] • The multicellular structure that produces eggs in bryophytes, ferns, and gymnosperms.
Archenteron (ark en´ ter on) [Gr. archos: beginning + enteron: bowel] • The earliest primordial animal digestive tract.
Arteriosclerosis • See atherosclerosis.
Artery • A muscular blood vessel carrying oxygenated blood away from the heart to other parts of the body. (Contrast with vein.)
Ascus (ass´ cuss) [Gr. askos: bladder] • In fungi belonging to the phylum Ascomycota (the sac fungi), the club-shaped sporangium within which spores (ascospores) are produced by meiosis.
Asexual • Without sex.
Assortative mating • A breeding system in which mates are selected on the basis of a particular trait or group of traits.
Atherosclerosis (ath´ er oh sklair oh´ sis) • A disease of the lining of the arteries characterized by fatty, cholesterol-rich deposits in the walls of the arteries. When fibroblasts infiltrate these deposits and calcium precipitates in them, the disease become arteriosclerosis, or "hardening of the arteries."
Atmosphere • The gaseous mass surrounding our planet. Also: a unit of pressure, equal to the normal pressure of air at sea level.
Atom [Gr. atomos: indivisible] • The smallest unit of a chemical element. Consists of a nucleus and one or more electrons.
Atomic mass (also called atomic weight) • The average mass of an atom of an element on the amu scale. (The average depends upon the relative amounts of different isotopes of an element on Earth.)
Atomic number • The number of protons in the nucleus of an atom, also equal to the number of electrons around the neutral atom. Determines the chemical properties of the atom.
ATP (adenosine triphosphate) • A compound containing adenine, ribose, and three phosphate groups. When it is formed, useful energy is stored; when it is broken down (to ADP or AMP), energy is released to drive endergonic reactions. ATP is an energy storage compound.
ATP synthase • An integral membrane protein that couples the transport of proteins with the formation of ATP.
Atrium (a´ tree um) • A body cavity, as in the hearts of vertebrates. The thin-walled chamber(s) entered by blood on its way to the ventricle(s). Also, the outer ear.
Autoimmune disease • A disorder in which the immune system attacks the animal’s own antigens.
Autonomic nervous system • The system (which in vertebrates comprises sympathetic and parasympathetic subsystems) that controls such involuntary functions as those of guts and glands.
Autosome • Any chromosome (in a eukaryote) other than a sex chromosome.
Autotroph (au´ tow trow´ fik) [Gr. autos: self + trophe: food] • An organism that is capable of living exclusively on inorganic materials, water, and some energy source such as sunlight or chemically reduced matter. (Contrast with heterotroph.)
Auxin (awk´ sin) [Gr. auxein: increase] • In plants, a substance (indoleacetic acid) that regulates growth and various aspects of development.
Auxotroph (awks´ o trofe) [Gr. auxanein: to grow + trophe: food] • A mutant form of an organism that requires a nutrient or nutrients not required by the wild type, or reference, form of the organism. (Contrast with prototroph.)
Axon [Gr.: axle] • Fiber of a neuron which can carry action potentials. Carries impulses away from the cell body of the neuron; releases a neurotransmitter substance.
Axon hillock • The junction between an axon and its cell body; where action potentials are generated.
Axon terminals • The endings of an axon; they form synapses and release neurotransmitter.
Axoneme (ax´ oh neem) • The complex of microtubules and their crossbridges that forms the motile apparatus of a cilium.