The cell membrane and receptors

The plasma membrane forms the interface between the cell and its environment. It is composed essentially of a phospholipid matrix and many different types of protein molecules which may be embedded within the matrix (integral proteins) or more loosely associated with the cytoplasmic ‘face’ of the membrane (peripheral proteins). The passage of essential ions and molecules across the membrane is controlled by integral proteins acting as channels or transporters. Intercellular communication is mediated by protein receptors, which are activated by signalling molecules such as hormones and neurotransmitters. Physical contact between the cell and its environment (and between cells) is mediated by membrane adhesion proteins. The plasma membrane is a highly dynamic structure in terms of molecular composition and topological configuration. The linkage of the internal cytoskeleton to the plasma membrane (via peripheral and integral proteins) expedites cellular shape changes or amoeboid motion of some types of cell. The process of endocytosis enables the cell to internalize small volumes of extracellular fluid (pinocytosis) by invagination and formation of intracellular vesicles or to engulf entire cells by phagocytosis. Secretion of molecules, such as hormones, is accomplished by fusion of intracellular vesicles with the plasma membrane. This mechanism of exocytosis is mediated by another type of protein: the SNARE proteins.