chyme The soupy mixture of food and gastric juices that is passed from the stomach to the duodenum (upper portion of the small intestine) during digestion.

cilia(singular, clium) Hair-like extensions of a cell, made of microtubules. Many cilia will occur on the surface of a given cell, and they move rapidly back and forth to propel the cell or to move material around the cell.

circadian rhythms Biological cycles that can function independently of environmental cues and that are roughly synchronized to Earth¡¯s 24-hour rotation.

cisternae(singular, cisterna) Flattened sacs of the rough endoplasmic reticulum (ER). The many cisternae increase the surface area of the ER so that there is more space for the ribosomes to synthesize proteins.

cisternal space Space in the middle of the cisternae (flattened sacs) of the rough endoplasmic reticulum. Amino acid chains, newly synthesized on the ribosomes, enter the cisternal space and then fold into their protein conformations.

citric acid cycle Another name for the Krebs cycle, named for the first product of the cycle, citric acid.

cladistics The branch of systematics that uses shared derived characters to determine the order of branching events in speciation and therefore which species are most closely related. Cladistics is concerned only with evolutionary relationships, not classification.

cladogenesis A form of evolution that occurs by species diverging into two species, with the original species and the new species both persisting. This form is referred to as branching evolution because the new species has branched off from the original species.

cladogram An evolutionary tree constructed using the cladistic system.

class A taxonomic grouping including several related orders. This category is below phylum and above order. Humans are in the class Mammalia.

classical conditioning A form of learning in which animals learn to respond in a customary way to a new stimulus that has been paired with an existing stimulus. In the case of Pavlov¡¯s dog, a customary response (salivation) was elicited when a new stimulus (the sound of a bell) was paired with an existing stimulus (food delivery).

cleavage The cellular division of a zygote during animal development, resulting in a ball of smaller cells which then differentiate.

climate The average weather conditions, including temperature, precipitation, and wind, in a particular region.

climax community The relatively stable community that develops at the end of any process of ecological succession.

clone An exact genetic copy. Also, used as a verb, to make one of these copies. A single gene or a whole, complex organism can be cloned.

cloning vector A self-replicating agent that, in the cloning process, serves to transfer and replicate genetic material. Cloning vectors generally are bacterial plasmids or the viruses known as bacteriophages.

closed circulation system A type of circulatory system, found in all vertebrates and in some invertebrates, in which blood stays within vessels.

clotting factor A group of substances, found in blood platelets, that are important in the process of blood-clotting.

coastal wetlands Lands adjacent to the ocean that are wet for at least part of the year. These can be fresh or salt water wetlands.

coastal zone The region lying between the point on shore where the ocean¡¯s waves reach at high tide to the point off shore where the continental shelf drops off.

cocci(singular, coccus) Spherical bacteria.

codominant The condition in which two alleles in a heterozygous organism are both expressed, so the phenotype has characteristics of both of the phenotypes of individuals homozygous for either allele. Blood type (A, B, AB, O) is an example of codominance.

codon An mRNA triplet that codes for a single amino acid or a start or a stop command in the translation stage of protein synthesis.

coelom A central body cavity, found in animals, that is lined with cells of mesodermal origin.

coenzyme A type of accessory molecule that participates at the active site of an enzyme and allows it to function. Many vitamins are important coenzymes.

coevolution The interdependent evolution of two or more species. Coevolution can benefit both species, as in flowering plants and their animal pollinators, or it can be an arms race between species, as in a plant and its predators.