Structure of cell membranes

y The thickness of cell membranes ranges from 6-10 nm, typically being about

o y * One nanometre is equal to 10~9 metre.

Cell membranes are composed primarily of lipids and proteins. Lipids are the major components of membranes, including glycerophospholipids (phospho-glycerides), sphingolipids (sphingomyelin) and cholesterol. Cephalin (phospha-tidylethanolamine) and lecithin (phosphatidylcholine) are the most common glycerophospholipids in membranes. Membrane lipids form self-sealing bilayers. They are amphipathic molecules, with hydrophobic and hydrophilic moieties. The hydrophobic groups, the long fatty acyl side chains, form the core, with the polar hydrophilic groups lining both surfaces.

Carbohydrates comprise 5%-10% of cell membranes. They consist of glyco-lipids and glycoproteins and form the glycocalyx coat on the surface of the plasma membrane. This layer is responsible for the immunological characteristics of the cell and carries surface receptors that are involved in molecular recognition.

According to the fluid mosaic model of Singer and Nicolson cell membranes possess fluid structures, being considered as two-dimensional solutions of oriented globular proteins and lipids. They take the form of a continuous fluid but stable lipid bilayer, studded with an array of membrane-associated or membrane-spanning proteins. The fluidity of the membrane is determined by the degree of unsaturation of the constituent fatty acids. The lipids and proteins can undergo rotational and lateral movement.Membranes are structurally and functionally asymmetrical. This is due to asymmetrical orientation of integral and peripheral membrane proteins, laterally and transversely. Membranes are also electrically polarised, with the inside being negative with respect to the exterior.

On electron microscopy, a trilaminar structure is evident. This consists of two dark outer bands, representing the polar heads of the membrane phospholipids and protein molecules on the inner and outer surfaces of the membrane, and an inner lighter band due to the nonpolar tails of the lipid molecules.

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