Describe the structure and function of plasma membranes.

All plasma membranes are composed of phospholipids, cholesterol, proteins, glycoproteins and glycolipids. The structure of the membrane can be described by the fluid mosaic model, where phospholipids are arranged to form a continuous bilayer, with their hydrophobic lipid tails at the centre and their hydrophilic phosphate heads facing outwards. The phospholipids are constantly moving, giving the membrane its ‘fluidity’. This allows a mosaic of proteins, glycoproteins and glycolipids to be scattered throughout the bilayer, and able to move around within it, whilst cholesterol is present between phospholipid tails to regulate the fluidity of the membrane.
Plasma membranes can be found both at the cell surface and as intracellular membranes surrounding organelles. Depending on the location of the membrane, its function, and therefore molecular composition, will differ. Cell surface membranes function to control the passage of materials in and out of cells, recognise surrounding cells, receive chemical signals and transmit nerve impulses. Intracellular membranes provide separate compartments to isolate the different processes occurring within a cell, control the intracellular transport between them and can also act as the site of chemical reactions, such as the inner mitochondrial membrane, which contains respiratory enzymes.