Cell membrane entropy as regulator of receptor submergence.

An entropic mechanism, previously worked out to explain the low adhesiveness of fluid phospholipid, also seems to explain some effects of membrane fluidity on the "embedding" of receptor proteins. Water, at the phospholipid interface, has to compete with protein for the available area but is impeded in its Brownian motion by hydrogen bonding to the slow-moving phosphatide. Hence, any increase in phosphatide mobility (e.g. by lipid melting) is entropically favourable to water, which spreads by displacing receptor particles from the interface into the hydrocarbon phase. This reconciles the kinetic viewpoint of Shintzky (experimental correlation of submergence with microviscosity) with the thermodynamic theory of Gerson (submergence increases with phospholipid hydrophilicity). Because Brownian motion increases water entropy, it follows that a more fluid interface must have a more negative free energy i.e. become more hydrophilic.