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Polar head group

Glycerol backbone

Fatty acid chains

Polar head group

Glycerol backbone

Fatty acid chains

Scheme 9.1 Structures of phospholipids found in membranes. R1 and R2 may vary in length. R1 is usually saturated and R2 is usually unsaturated.

phospholipids (see Scheme 9.1) with two hydrophobic chains and cholesterol or other related structures, which associate into lipoidal bilayers in aqueous media. Embodied in the matrix of lipid molecules are proteins, which are generally hydrophobic in nature, embedded in the matrix of lipid molecules. Thus the membrane has a hydrophilic exterior and a hydrophobic interior.

Cholesterol is a major component of most mammalian biological membranes; its removal causes the membrane to lose its structural integrity and to become highly permeable. Cholesterol complexes with phospholipids and its presence reduces the permeability of phospholipid membranes to water, cations, glycerol and glucose. The shape of the cholesterol molecule allows it to fit closely in bilayers with the hydrocarbon chains of unsaturated fatty acids (Fig. 9.1). The present consensus of opinion is that cholesterol condenses and rigidifies membranes without solidifying them. The flexibility of biological membranes is an important feature, giving them their ability to re-form and to adapt to changed environments, a vital part of their function. The dynamic characteristics of biological membranes are due to their unique construction from small amphipathic molecules. On stretching (as with a soap film), the molecules at the surfaces become less concentrated in the bilayer but are replenished from the bulk phase to maintain the original tension.

Figure 9.2 shows a diagram of the 'fluid mosaic' model of a biological membrane.

Polar

Choline i

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