Under normal conditions, cells are dynamic structures existing in fluid environment. A cell is enclosed by cell membrane that extends internally to enclose nucleus and various subcellular organelles suspended in cytosol
Electron microscopy has shown that cell membrane or plasma membrane has a trilaminar structure having a total thickness of about 7.5 nm and is known as unit membrane. The three layers consist of two electron-dense layers separated by an electronlucent layer. Biochemically, the cell membrane is composed of complex mixture of phospholipids, glycolipids, cholesterol, proteins and carbohydrates.
These layers are in a gel-like arrangement and are in a constant state of flux. The outer surface of some types of cells shows a coat of mucopolysaccharide forming a fuzzy layer called glycocalyx. Proteins and glycoproteins of the cell membrane may act as antigens (e.g. blood group antigens), or may form receptors (e.g. for viruses, bacterial products, hormones, immunoglobulins and many enzymes). The cell receptors are probably related to the microtubules and microfilaments of the underlying cytoplasm.
The microtubules connect one receptor with the next. The microfilaments are contractile structures so that the receptor may move within the cell membrane. Bundle of microfilaments along with cytoplasm and protein of cell membrane may form projections on the surface of the cell called microvilli. Microvilli are especially numerous on the surface of absorptive and secretory cells (e.g. small intestinal mucosa) increasing their surface area.