Definition:
Outer membrane of cell composed of phospholipid and protein molecules; gives form to cell and controls passage of materials into and out of cell.

Description:
Ranges from 6.5 to 10 nm in thickness, and is composed primarily of phospholipids, proteins, and carbohydrates. The phospholipid molecules are arranged in two parallel rows, forming a phospholipid bilayer. Phospholipid molecules consist of polar phosphate heads that are hydrophilic (mixing with water) facing outward, and nonpolar fatty acid tails that are hydrophobic (not mixing with water) facing inward. The protein portions of the membrane are peripheral--loosely bound to the surface, or they are integral--embedded in the phospholipid bilayer. Generally located on the inner surface, peripheral proteins serve as enzymes. Integrated proteins provide membrane pores which are involved in active transport.

Functions: Provide structural support to the cell and to maintain selective permeability. The lipid bilayer forms a major barrier to the vast number of water soluble substances. The proteins in the cell membrane serve a variety of functions, including: 1) structural support; 2) transport of molecules across the membrane; 3) enzymatic control of chemical reactions; 4) receptors for hormones and other regulatory molecules that arrive at the outer surface; and 5) cellular markers (antigens), which identify the blood and tissue type. The carbohydrate containing molecules function to: 1) repel negative objects due to their negative charge; 2) act as receptors for hormones and other regulatory molecules; 3) form specific cell markers which enable like cells to attach and aggregate; and 4) enter into immune reactions. The permeability of a cell membrane is a function of: 1) size of molecules; 2) solubility in lipids; 3) ionic charge of molecules; and 4) the presence of carrier molecules.