Structure and Dynamics of Biomembranes

Biological membranes are complex heterogeneous nanostructures containing a variety of compounds. The main ones are phospholipids and proteins. The protein/lipid ratio varies strongly between, say, 0.25 and 2.5 depending on the species and, for a given species, the type of cell or organelle and, therewith, the functions. 

Another class of proteins that occur in many, but not all, membranes are cytoplasmic proteins located at the inner surface. These mainly rod-shaped proteins are connected to each other and are firmly anchored to intrinsic membrane proteins. They thus form a strong, dynamic fibrous network, the so-called cytoskeleton, that controls the lateral diffusion of the intrinsic proteins. 

The third class is that of the peripheral or extrinsic membrane proteins. They reside entirely in the (aqueous) extracellular or intracellular space. They are physically bound to the manbrane by specific and/or nonspecific interactions. Most membrane-linked enzymes are such extrinsic proteins. 

The fluidity of the bilayer primarily depends on the nature of the aliphatic fatty acid chains in the lipids. The fluidity increases with increasing chain length beyond, say C,o. Furthermore, and more strongly, the order in the bilayer is sensitive to the degree of saturation of the chemical bonds in the chains. The smaller the number of (especially cis) double bonds, the more fluid the bilayer is. For a given composition the temperature determines the membrane fluidity. It explains why the degree 

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