A thin structure that completely surrounds the cell as a “skin.” They may be seen with the aid of an electron microscope. The entire membrane appears to be about 100 Angstroms (Å; 0.1 μm) thick and is composed of two dark lines each about 30 A thick which are, however, separated by a lighter area. This trilaminar “sandwich” structure is referred to as the unit membrane. The plasma membrane is composed of lipoidal (fat-like) material in which proteins and protein complexes and whole functional systems are embedded. In the plasma membrane are incorporated such energy-dependent transport systems as Na+ and K+ transporting AT-Pase and amino acid transport systems. Besides the cell, membranes surround such systems as the endoplasmic reticulum, vacuoles, lysosomes, golgi bodies, mitochondria, chloroplasts, and the nucleus, to mention just a few. The plasma membrane and membranes in general function in part as a permeability barrier to the free movement of substances between the inside and exterior of the cell or organelles that they surround.
The membrane that surrounds the cell. It regulates the passage of substances into and out of the cell.