cotyledon An embryonic leaf. A major division in plants is between the monocots, which have one embryonic leaf, and the dicots, which have two embryonic leaves.

coupled reaction The situation in which an exergonic and an endergonic reaction are brought together, with the exergonic reaction releasing energy that is used to power the endergonic reaction.

covalent bond A type of chemical bond in which two atoms are linked because they share electrons.

Cretaceous Extinction A mass extinction event that occurred at the boundary between the Cretaceous and Tertiary periods. This event, which included an asteroid impact, resulted in the end of the dinosaurs along with many other organisms.

cross-pollinate Pollinate one plant with pollen of another plant. Mendel used this technique in conducting his experiments to uncover rules of heredity.

crossing over The exchange of pieces of chromosomes that occurs when homologous chromosome pairs intertwine during prophase I. This process, also called recombination, accounts for some of the diversity in individual gametes.

cuticle In plants, the waxy outer covering of leaves and shoots. The cuticle forms a protective layer over the dermal tissue, keeping water in and infecting bacteria and fungi out.

cytokinesis The stage of cell division in which a cell's cytoplasm divides.

cytoplasm The region inside the plasma membrane and outside the nucleus. Usually, this region is filled with the jelly-like cytosol containing the cell's extra-nuclear organelles.

cytoskeleton The set of protein strands that function as internal scaffolding for a cell. These protein strands give the cell shape, anchor its structures, act as monorails for particles moving within the cell, and allow whole cells to move. Microfilaments, intermediate filaments, and microtubules are all parts of the cytoskeleton.

cytosol The protein-rich, jelly-like fluid in which a cell's organelles that are outside the nucleus are immersed.

cytotoxic T cell A type of T cell, active in cell-mediated immunity, that attaches to infected cells in the body and punctures their outer membranes, causing them to rupture. Also known as killer T cells.