Cambell's Vocabulary

5' cap

A modified form of guanine nucleotide added onto the 5' end of a pre-mRNA molecule.

ABC hypothesis

A model of flower formation identifying three classes or organ identity genes that direct formation of the four types of floral organs.

Abiotic

Nonliving; referring to the physical and chemical properties of an environment.

Abortion

The termination of a pregnancy in progress.

Abscisic Acid (ABA)

A plant hormone that slows growth, often antagonizing the actions of growth hormones. Two of its many effects are to promote seed dormancy and facilitate drought tolerance.

Absorption

Third stage of food processing in animals: the uptake of small nutrient molecules by an organism's body.

Absorption spectrum

The range of a pigment's ability to absorb various wavelengths of light; also a graph of such a range.

Abyssal Zone

The part of the ocean's benthic zone between 2,000 and 6,000 m deep.

Acanthodian

Any of a group of ancient jawed aquatic vertebrates from the Silurian and Devonian periods.

Accessory fruit

A fruit, or assemblage of fruits, in which the fleshy parts are derived largely or entirely from tissues other than the ovary.

Acclimatization

Physiological adjustment to a change in an environmental factor.

Acetyl CoA

Acetyl coenzyme A; the entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme.

Acetylcholine

One of the most common neurotransmitters; functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ions, either depolarizing or hyperpolarizing the membrane.

Acid

A substance that increases the hydrogen ion concentration of a solution.

Acquired Immunodeficiency Syndrome (AIDS)

The symptoms and signs present during the late stages of HIV infection, defined by a specified reduction in the number of T cells and the appearance of characteristic secondary infections.

Acrosomal Reaction

The discharge of hydrolytic enzymes from the acrosome, a vesicle in the tip of a sperm, when the sperm approaches or contacts an egg.

Acrosome

A vesicle in the tip of a sperm containing hydrolytic enzymes and other proteins that help the sperm reach the egg.

Actin

A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.

Action Potential

An electrical signal that propagates along the membrane of a neuron or other excitable cell as a nongraded (all-or-none) depolarization.

Action Spectrum

A graph that profiles the relative effectiveness of different wavelengths of radiation in driving a particular process.

Activation Energy

The amount of energy that reactants must absorb before a chemical reaction will start; also called free energy of activation.

Activator

A protein that binds to DNA and stimulates gene transcription. In prokaryotes, activators bind in or near the promoter; in eukaryotes, activators generally bind to control elements in enhancers.

Active immunity

Long-lasting immunity conferred by the action of B cells and T cells and the resulting B and T memory cells specific for a pathogen. Active immunity can develop as a result of natural infection of immunization.

Active Site

The specific region of an enzyme that binds the substrate and that forms the pocket in which catalysis occurs.

Active Transport

The movement of a substance across a cell membrane against its concentration or electrochemical gradient, mediated by specific transport proteins and requiring an expenditure of energy.

Adaptation

Inherited characteristic of an organism that enhances its survival and reproduction in a specific environment.

Adaptive Evolution

A process in which traits that enhance survival or reproduction tend to increase in frequency over time, resulting in a better match between organisms and their environment.

Adaptive Immunity

A vertebrate-specific defense that is mediated by B lymphocytes (B cells) and T lymphocytes (T cells) and that exhibits specificity, memory, and self-nonself recognition; also called acquired immunity.

Adaptive Radiation

Period of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles in their communities.

Addition Rule

A rule of probability stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities.

Adenylyl Cyclase

An enzyme that converts ATP to cyclic AMP in response to an extracellular signal.

Adhesion

The clinging of one substance to another, such as water to plant cell walls by means of hydrogen bonds.

Adipose Tissue

A connective tissue that insulates the body and serves as a fuel reserve; contains fat-storing cells called adipose cells.

Adrenal Gland

One of two endocrine glands located adjacent to the kidneys in mammals. Endocrine cells in the outer portion (cortex) respond to ACTH by secreting steroid hormones that help maintain homeostasis during long-term stress. Neurosecretory cells in the central portion (medulla) secrete epinephrine and norepinephrine in response to nervous inputs triggered by short-term stress.

Aerobic Respiration

A catabolic pathway for organic molecules, using oxygen (O2) as the final electron acceptor in an electron transport chain and ultimately producing ATP. This is the most efficient catabolic pathway and is carried out in most eukaryotic cells and many prokaryotic organisms.

Age Structure

The relative number of organisms of each age within a population.

Aggregate Fruit

A fruit derived from a single flower that has more than one carpel.

AIDS (acquired immune deficiency syndrome)

The symptoms and signs present during the late stages of HIV infection, defined by a specified reduction in the number of T cells and the appearance of characteristic secondary infections.

Alcohol Fermentation

Glycolysis followed by the reduction of pyruvate to ethyl alcohol, regenerating NAD+ and releasing carbon dioxide.

Alga (Algae)

A general term for any species of photosynthetic protist, including both unicellular and multicellular forms. Algal species are included in three eukaryote supergroups (Excavata, SAR, and Archaeplastida).

Alimentary Canal

A complete digestive tract, consisting of a tube running between a mouth and an anus.

Alkaline Vent

A deep-sea hydrothermal vent that releases water that is warm (40-90 C) rather than hot and that has a high pH. These vents consist of tiny pores lined with iron and other catalytic minerals that some scientists hypothesize might have been the location of the earliest abiotic synthesis of organic compounds.

Allele

Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.

Allopatric Speciation

The formation of a new species in populations that are geographically isolated from one another.

Allopolyploid

A fertile individual that has more than two chromosome sets as a result of two different species interbreeding and combining their chromosomes.

Allosteric Regulation

The binding of a regulatory molecule to a protein at one site that affects the function of a protein at a different site.

Alpha Helix

A coiled region constituting one form of the secondary structure of proteins, arising from a specific pattern of hydrogen bonding between atoms of the polypeptide backbone.

Alternation of generations

A lifecycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of plants and some algae.

Alternative RNA splicing

A type of eukaryotic gene regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns.

Altruism

Selflessness; behavior that reduces an individual's fitness while increasing the fitness of another individual.

Alveolates

One of the three major subgroups for which the SAR eukaryotic supergroup is named. This clade arose by secondary endosymbiosis, and its members have membrane-enclosed sacs located just under the plasma membrane.

Alveolus (Alveoli)

One of the dead-end air sacs where gas exchange occurs in a mammalian lung.

Alzheimer's disease

An age-related dementia characterized by confusion and memory loss.

Amino Acid

A chemical group consisting of a nitrogen atom bonded to two hydrogen atoms; can act as a base in solution, accepting a hydrogen ion and acquiring a charge of +1.

Aminoacyl-tRNA synthase

An enzyme that joins each amino acid to the appropriate tRNA.

Ammonia

A small, toxic molecule (NH3) produced by nitrogen fixation or as a metabolic waste product of protein and nucleic acid metabolism.

Ammonite

A member of a group of shelled cephalopods that were important marine predators for hundreds of millions of years until their extinction at the end of the Cretaceous period.

Amniocentesis

A technique associated with prenatal diagnosis in which amniotic fluid is obtained by aspiration from a needle inserted into the uterus. The fluid and the fetal cells it contains are analyzed to detect certain genetic and congenital defects in the fetus.

Amniote

A member of a clade of tetrapods named for a key derived character, the amniotic egg, which contains specialized membranes, including the fluid-filled amnion, that protect the embryo. Amniotes include mammals as well as birds and other reptiles.

Amniotic Egg

An egg that contains specialized membranes that function in protection, nourishment, and gas exchange. The amniotic egg was a major evolutionary innovation, allowing embryos to develop on land in a fluid-filled sac, thus reducing the dependence of tetrapods on water for reproduction.

Amoeba

A protist characterized by the presence of pseudopodia.

Amoebocyte

An amoeba-like cell that moves by pseudopodia and is found in most animals. Depending on the species, it may digest and distribute food, dispose of wastes, form skeletal fibers, fight infections, or change into other cell types.

Amoebozoan

A protist in a clade that includes many species with lobe- or tube-shaped pseudopodia.

Amphibian

A member of the clade of tetrapods that includes salamanders, frogs, and caecilians.

Amphipathic

Having both of hydrophilic region and a hydrophobic region.

Amplification

The strengthening of stimulus energy during transduction.

Amygdala

A structure in the temporal lobe of the vertebrate brain that has a major role in the processing of emotions.

Amylase

An enzyme that hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose.

Anabolic Pathway

A metabolic pathway that consumes energy to synthesize a complex molecule from simpler molecules.

Anaerobic Respiration

A catabolic pathway in which inorganic molecules other than oxygen accept electrons at the "downhill

end of electron transport chains.

Analogous

Having characteristics that are similar because of convergent evolution, not homology.

Analogy

Similarity between two species that is due to convergent evolution rather than to descent from a common ancestor with the same trait.

Anaphase

The fourth stage of mitosis, in which the chromatids of each chromosomes are moving to the poles of the cell.

Anatomy

The structure of an organism.

Anchorage Dependence

The requirement that a cell must be attached to a substratum in order to initiate cell division.

Androgen

Any steroid hormone, such as testosterone, that stimulates the development and maintenance of the male reproductive system and secondary sex characteristics.

Aneuploidy

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.

Angiosperm

A flowering plant, which forms seeds inside a protective chamber called an ovary.

Anhydrobiosis

A dormant state involving loss of almost all body water.

Animal Pole

The point at the end of an egg in the hemisphere where the least yolk is concentrated; opposite of vegetal pole.

Anion

A negatively charged ion.

Anterior

Pertaining to the front, or head, of a bilaterally symmetrical animal.

Anterior Pituitary

A portion of the pituitary gland that develops from nonneural tissue; consists of endocrine cells that synthesize and secrete several tropic and nontropic hormones.

Anther

In an angiosperm, the terminal pollen sac of a stamen, where pollen grains containing sperm-producing male gametophytes form.

Antheridium (Antheridia)

In plants, the male gametangium, a moist chamber in which gametes develop.

Anthropoid

A member of a primate group made up of the monkeys and the apes.

Antibody

A protein secreted by plasma cells (differentiated B cells) that binds to a particular antigen; also called immunoglobulin. All antibodies have the same Y-shaped structure and in their monomer form consist of two identical heavy chains and two identical light chains.

Anticodon

A nucleotide triplet at one end of a tRNA molecule that base-pairs with a particular complementary codon on an mRNA molecule.

Antidiuretic Hormone (ADH)

A peptide hormone, also called vasopressin, that promotes water retention by the kidneys. Produced in the hypothalamus and released from the posterior pituitary, ADH also functions in the brain.

Antigen

A substance that elicits an immune response by binding to receptors of B or T cells.

Antigen presentation

The process by which an MHC molecule binds to a fragment of an intracellular protein antigen and carries it to the cell surface, where it is displayed and can be recognized by a T cell.

Antigen-presenting cell

A cell that upon ingesting pathogens or internalizing pathogen proteins generates peptide fragments that are bound by class II MHC molecules and subsequently displayed on the cell surface to T cells. Macrophages, dendritic cells, and B cells are the primary antigen-presenting cells.

Antigen Receptor

The general term for a surface protein, located on B cells and T cells, that binds to antigens, initiating adaptive immune responses. The antigen receptors on B cells are called B cell receptors, and the antigen receptors on T cells are called T cell receptors.

Antiparallel

Referring to the arrangement of the sugar-phosphate backbones of DNA double helix (they run in the opposite 5'-3' directions).

Aphotic zone

The part of an ocean or lake beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.

Apical Bud

A bud at the tip of a plant stem; also called a terminal bud.

Apical Dominance

Tendency for growth to be concentrated at the tip of a plant shoot, because the apical bud partially inhibits axillary bud growth.

Apical Ectodermal Ridge (AER)

A thickened area of ectoderm at the tip of a limb bud that promotes outgrowth of the limb bud.

Apical Meristem

A localized region at a growing tip of a plant body where one or more cells divide repeatedly. The dividing cells of an apical meristem enable the plant to grow in length.