Phospholipid Headgroups on Membrane Structure and Function

Influence of Phospholipid Headgroups on Membrane Structure and Function 

In addition to acyl chains, the phospholipid headgroups should contribute to the stability of phospholipid bilayers as reflected by the considerably higher melting points (200°C) of pure phospholipids than of fatty acids (60°C). Also, the type of headgroup is important for the properties of a phospholipid membrane. For example, monolayers of phosphatidylethanolamine are less expanded than those derived from phosphatidylcholine and the melting point of the solid, pure phospholipid is lower for the former than for the latter, irrespective of the length or unsaturation of the fatty acids (Williams and Chapman, 1970). 

Cholesterol also decreases the heat absorbed in the transition from the gel to the liquid-crystalline state (melting) of a phospholipid. cholesterol membrane, such that by 33 mole% cholesterol, no transition is apparent (Oldfield and Chapman, 1971 Ladbrooke and Chapman, 1969). Apparently, a cholesterol molecule is able to complex with a phospholipid molecule (Dark et al., 1972) to yield a complex which is more fluid than the gel state and less fluid than the liquid-crystalline state. 

Inhomogeneous Nature of the Lipid Phase of Biological Membranes 

However, microorganisms do adjust their lipid composition in response to changes in growth temperature in order to maintain the fluidity of their membranes in some constant range. Complete solidifi- 

---