ABSTRACT
Although cell membrane fluidity can be regulated by altering the phospholipid fatty acid content, this is not the organism’s only means of control. Most cell membranes contain varying amounts of sterols. In plants the primary membrane sterol is sitosterol; in animals, cholesterol, and in fungi ergosterol, although many other similar compounds also occur. Sterols alter the fluidity of the cell membrane by ‘broadening’ the melting transition so that the membrane melts over a much wider temperature range than that observed for the lipid alone. This is illustrated in Figure 1.3, which is a thermogram for the melting of a typical lipid membrane, in this case dipalmitoyl phosphatidylcholine. A thermogram is a plot of the energy absorbed as the temperature of the system is raised; the peak is caused by the absorption of energy required to melt the lipid bilayer. In the absence of sterols the bilayer melts over a small temperature range, causing a sharp peak in the thermogram. In the presence of cholesterol the melting transition is much broader, and the thermogram peak spans several degrees. The membrane begins to melt at a lower temperature than in the absence of sterol, and retains some structure up to a temperature above the transition temperature of the pure lipid. The effect of this is to ‘smear out’ the melting of the membrane, so that the fluidity is not so dependent on temperature. Obviously, this is of considerable importance in allowing the cell to function over a range of temperatures.
