The basic unit of life, capable of growing and multiplying. All living things are either single, independent cells or aggregates of cells. A cell is composed of cytoplasm and a nucleus, and is surrounded by a membrane or wall. Cells can be categorized by the presence of specific cell surface markers called clusters of differentiation .

Cell Membrane . A two-layered structure, composed of fatty material, that occurs within and surrounds all cells. Most natural membranes have various proteins, such as receptors and enzymes, embedded in them.