Fluid mosaic membranes

The structure of cell surface membranes allows movement of substances between cells and their surroundings and allows cells to communicate with each other by cell signalling.

1. Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins

• The fluid mosaic model describes the plasma membrane of animal cells
• The 'fluid' part refers to the fact that some parts of the membrane can move freely, if they are not attached to other parts of the cell
• The 'mosaic' part illustrates the 'patchwork' of proteins that is found in the phospholipid bilayer
• The membrane consists of a phospholipid bilayer and the phospholipids can move about by diffusion within their own monolayers
• The hydrophobic tails of the phospholipids point inwards, facing each other to form a non-polar hydrophobic interior while the hydrophilic heads point outwards, facing the aqeuous environments
• The phosphlipid bilayer is semi-permeable; it is permeable to small and non-polar molecules
• It is impermeable to large molecules and ions
• There are various protein molecules floating around within the plasma membrane
• Proteins which are found embedded within the membrane (inner and outer layers) are called intrinsic proteins, and when they span the whole membrane, they are known as transmembrane proteins
• Proteins which are found on the inner or outer surface of the membrane are called extrinsic proteins and many of these are bound to intrinsic proteins
• Many proteins and lipids have short, branching carbohydrate chains attached to the part of them which faces the outside of the membrane, thus forming glycoproteins and glycolipids
• Because the tails of the phospholipids are non-polar and hydrophobic, they act as a barrier to most water-soluber substances 
• Glycolipids act as signalling molecules, activating other molecules in the membrane such as enzymes
• They may also be hydrolysed to release small, water-soluble, glycerol-related molecules
• At low temperatures, cholesterol increases the fluidity of the membrane, preventing it from being too rigid
• The increased fluidity means that cells can survive in colder temperatures
• The interaction between the cholesterol and the tails of the phospholipids helps to stabilise cells at higher temperatures when the membrane could otherwise be too fluid
• Cholesterol is also important for the mechanical stability of…