Structure of Plasma Membranes

The present model of structure of the plasma membrane takes into account EM images and biochemical data gathered so far. It is known as the fluid mosaic model of the plasma membrane and it was first proposed by Singer and Nicholson in 1972. According to this model the plasma membrane consists of a lipid bilayer with proteins inserted in it.

Cell membrane and associated proteins

This scheme summarizes the current model of membrane structure and mebrane proteins.

The proteins can be either integral proteins or peripheral proteins. Integral proteins span the membrane in its entire width while peripheral proteins do not penetrate into the lipid bilayer. Some membrane proteins have carbohydrate molecules attached to their extracellular domain. These proteins are known as glycoproteins.

Lipids of the cell membrane can contain fatty acids or not. Phospholipids consist of a glycerol molecule (CH2OH-CHOH-CH2OH) binding (esterified with) two fatty acids in C1 and C2. In C3 the glycerol binds (is esterified with) phosphoric acid (H3PO4). A fatty acid molecule consist of a long hydrocarbon chain (CH3-CH2-CH2-…) terminating with a carboxyl group (-COOH). The hydrocarbon chain is described as hydrophobic while the hydrophilic head (phosphoric acid) is described as hydrophilic. Sphingolipids are membrane lipids similar to phospholipids except for the glycerol molecule substituted by a molecule of sphingosine.

Membrane fluidity and cholesterol

Cholesterol plays an important role in regulating membrane fluidity which in turn is fundamental for functions such as the binding of ligands by plasma membrane receptors.

Cholesterol is another lipidic component of the plasma membrane. It has a stabilizing effect upon the plasma membrane because it reduces the lateral mobility in the membrane plane of the membrane lipidic complexes. The proportion of cholesterol of the plasma membrane varies from 10 to 50% of total lipids. Some cell functions such as the capacity of some receptor during an immune response to bind an antigen depend upon the fluidity of the membrane. Increase in the concentration of cholesterol will impair this function of the plasma membrane.

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