The Cell Membrane

All cell are bounded by a thin membrane called the plasmalemma. This membrane is not visible under the light microscope. The structure seen under the light microscope is the cell membrane. This consists of the plasmalemma along with surrounding cell cement. In the broad sense the term ‘cell membrane’ also includes the limiting membrances of cell organelles like mitochondria and lysosomes, and other membrances like those of the nuclear envelope, the endoplasmic reticulum and the Golgi complex.

The plasma membrane may be protected by other coverings. The plasma membrane of the egg is surrounded by other coverings. The plasma membrane of the egg is surrounded by the vitelline membrane and the jelly layer, both of which are secreted by the ovary. In some tissues the cell cements fills the gap between adjacent cells. Fusion of the cell cement forms a supporting layer, the basement membrane, for a sheet of cells.

Chemical Composition of Cell Membranes

Membranes from different sources vary in their composition. They essentially consists of lipoproteins which are special non-bonded combinations of lipids with proteins. In general, membranes contain about 60% protein and 40% carbohydrate by dry weights. Carbohydrate is usually present to the extent of 1-10% of the total dry weight is glycoprotein and glycolipids. In membrane of some cell organelles the nucleic acids DNA and RNA have also been reported, but are most likely contamination.

Proteins

Membranes contain three different classes of proteins, structural proteins, enzymes and carrier proteins. Structural proteins form the ‘backbone’ of the cell membrane. They have little catalytic activity and are extremely lipophilic. Their amino acid composition is nearly the same. The plasma membrane consists largely of structural proteins.

The plasma membrane proteins fall in two main categories, intrinsic or integral proteins and extrinsic or peripheral proteins. Human erythrocyte glycoprotein consists of three chemically distinct regions: (1) an N-terminal part external to the membrane and containing all the carbohydrate, (2) a middle hydrophphobic regions located within the membrane and (3) a hydrophilic C-terminal portion, rich in praline but lacking carbohydrate, on the internal side of the membrane. The middle intramembranous part appears to be intimately associated with the membrane phospholipids.