ABSTRACT
The cell membrane confines the contents of the cell. It consists of a continuous bilayer of phospholipid with the polar hydrophilic ends forming the outer and inner layers and the hydrophobic tails forming the central core of the bilayer. The hydrophilic heads on the two sides of the cell membrane are of different composition, those on the outside often being modified by glycosylation: a process that involves the addition of various sugar residues. Embedded in this lipid bilayer are proteins whose function can be classified as follows:
Signal transduction (mediating the action of external ligands such as growth factors and neurotransmitters). These receptor proteins cross the cell membrane and may have intrinsic enzyme functions (such as the tyrosine kinase activity of the receptor for epidermal growth factor) or may be linked to other proteins such as G proteins (for example, the muscarinic acetylcholine receptor) (Figure 2.1).
