cellulose A complex carbohydrate that is the largest single component of plant cell walls and is also found in the cell walls of many other organisms. Cellulose is dense and rigid and provides structure for much of the natural world. Mammals cannot digest cellulose, so it serves as insoluble dietary fiber that helps move food through the digestive tract.
central cell Cell with two nuclei in the embryo sac (mature female gametophyte) in a flower. Upon fertilization, this cell will develop into the endopserm, the food for the embryo.
central nervous system (CNS) The part of the nervous system that integrates and coordinates sensory data and sends out commands. This division consists of the brain and the spinal cord.
central vacuole An organelle in a plant cell, containing mostly water but also other substances, that may occupy up to 90 percent of the volume of the cell. The vacuole stores nutrients, retains or degrades waste, and is involved in metabolism.
centrosome A cellular structure that acts as an organizing center for the assembly of microtubules. A cell's centrosome duplicates prior to mitosis and plays an important part in the development of the cell's mitotic spindle.
cephalopods A class of molluscs that includes squid, octopus, and nautilus, and cuttlefish.
cerebellum Region of the vertebrate brain that adjusts ongoing movements on the basis of sensory information and memory of past movements. The cerebellum helps us both maintain balance and remember how to ride a bicycle.
cerebral cortex The highly convoluted layer of gray matter at the surface of the brain of vertebrates. This layer consists largely of cell bodies of sensory and motor neurons.
cerebral hemispheres The two halves (left and right) of the cerebrum of the vertebrate brain. This is the site of many functions, including conscious thought, sensations, and memory.
cerebrospinal fluid The fluid that circulates in the spine and brain, supplying nutrients, hormones, and white blood cells, and cushioning the nervous tissue from jarring injury.
cerebrum The top portion of the vertebrate brain, divided into two cerebral hemispheres. Here, conscious thought, memory, sensations, and complex motor patterns originate.
cervix The lower part of the uterus, a narrow neck which opens into the vagina.
character An attribute of an organism, such as stem length, seed color, or seed shape in Mendel's pea plants. Mendel studied the traits of these characters (green or yellow seeds, for example) in offspring of crosses he controlled.
chemical bonding General term for a bond created when electrons of two atoms interact and rearrange into a new from that allows the atoms to become attached to each other. Ionic, covalent, and hydrogen bonds are all chemical bonds. Chemical bonding is what enables atoms to stick together and form visible substances and structures.
chemoautotrophy Nutritional mode in which an organism gets its carbon from carbon dioxide, but gets the energy to make food from this carbon by oxidizing inorganic materials. Some bacteria and some archaea are chemoautotrophs.
chemoheterotrophy Nutritional mode in which an organism gets its carbon and its energy from organic materials, referred to as food. Animals are chemoheterotrophs.
CFCs(chlorofluorocarbons) A class of human-made chlorine compounds that destroys the atmospheric ozone that protects life on land from damaging ultraviolet radiation.
chitin A complex carbohydrate that gives shape and strength to the external skeleton of arthropods, including insects, spiders, and crustaceans.
chlorofluorocarbons(CFCs ) A class of human-made chlorine compounds that destroys the atmospheric ozone that protects life on land from damaging ultraviolet radiation.
chlorophyll a The primary pigment of chloroplasts, found embedded in its membranes. Together with the accessory pigments, chlorophyll a absorbs some wavelengths of sunlight in the first step of photosynthesis.
chloroplast A type of plastid (an organelle) in a plant cell that contains chlorophyll and is the site of photosynthesis. Chloroplasts are power plants in plant cells.
chorionic villus sampling (CVS) A medical procedure to test for genetic abnormalities in a fetus. Cells derived from the embryo are suctioned from the villi (extensions of the chorionic membrane surrounding the placenta) and examined. This alternative to amniocentesis can be done somewhat earlier and results are faster, but it is riskier for the fetus.
chromatid One of the two identical strands of chromatin (DNA plus associated proteins) that make up a chromosome in its duplicated state.
chromatin Substance consisting of DNA plus the protein material around which DNA is wrapped. Chromatin is the material in chromosomes.
chromosome Package made up of chromatin, which is DNA and associated proteins. All the genes of organisms are in the DNA of chromosomes.
