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. PICTURE
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. PICTURE
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. PICTURE
cardiovascular system The human circulatory system consisting of the heart and the vessels that transport blood to and from the heart. PICTURE
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. PICTURE
carpals The bones that make up the wrist joint.
carpels The female reproductive structures of a flower; consisting of the ovary, style, and stigma. PICTURE
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. PICTURE 1 PICTURE 2
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. PICTURE 1 PICTURE 2
cell cycle The sequence of events from one division of a cell to the next; consists of mitosis (or division) and interphase. PICTURE
cell-mediated immunity Immune reaction directed against body cells that have been infected by viruses and bacteria; controlled by T cells.
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.
cells The smallest structural units of living matter capable of functioning independently. PICTURE 1 PICTURE 2
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.
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. PICTURE 1 PICTURE 2
cell wall Structure produced by some cells outside their cell membrane; variously composed of chitin, peptidoglycan, or cellulose. PICTURE
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. PICTURE
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. PICTURE
centromere A specialized region on each chromatid to which kinetochores and sister chromatids attach. PICTURE
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. PICTURE
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. PICTURE
cervix The lower neck of the uterus that opens into the vagina. PICTURE
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. PICTURE 1 PICTURE 2
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). PICTURE
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. PICTURE
chlorophyll a The green photosynthetic pigment common to all photosynthetic organisms. PICTURE
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. PICTURE
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. PICTURE
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. PICTURE
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. PICTURE
chromosomes Structures in the nucleus of a eukaryotic cell that consist of DNA molecules that contain the genes. PICTURE
chromosome theory of inheritance Holds that chromosomes are the cellular components that physically contain genes; proposed in 1903 by Walter Sutton and Theodore Boveri.
Chrysophytes Protistan division that is referred to as the golden brown algae; includes the diatoms. PICTURE 1 PICTURE 2
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. PICTURE
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. PICTURE
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. PICTURE
classes Taxonomic subcategories of phyla. PICTURE
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. PICTURE
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. PICTURE 1 PICTURE 2 PICTURE 3
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. PICTURE 1 PICTURE 2
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. PICTURE
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. PICTURE 1 PICTURE 2
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.
complementary nucleotides The bonding preferences of nucleotides, Adenine with Thymine, and Cytosine with Guanine. Also referred to as complementary base pairing. PICTURE
complement system A chemical defense system that kills microorganisms directly, supplements the inflammatory response, and works with, or complements, the immune system. PICTURE
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). PICTURE
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. PICTURE
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. PICTURE
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. PICTURE
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). PICTURE
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. PICTURE
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). PICTURE
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. PICTURE
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. PICTURE
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. PICTURE
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.
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