FUNCTION OF FATS 135 



Strictly speaking, a substance which is adsorbed under certain 

 circumstances will be set free when the circumstances are reversed. 

 Many substances, however, undergo alteration in physical state 

 on adsorption. For example, in the formation of meringues, the 

 egg white becomes coagulated and so becomes incapable of re- 

 entering the liquid state. Such an irreversible reaction is termed 

 pseudo-adsorption. 



Adsorption (including pseudo-adsorption) is, as we have seen 

 in a previous chapter, dependent on surface forces. Now, from its 

 very nature, surface tension has a negative temperature coefficient. 

 Increase of temperature lowers surface tension. It follows that 

 increase of temperature will diminish the amount of material 

 adsorbed, and, conversely, a decrease in temperature will increase 

 the adsorbability of substances in solution. Can we associate with 

 this fact the \'arying thickness of animal membranes according to 

 their degree of exposiu'e to cold ? 



There is no doubt that material adsorbed to a surface can in 

 turn take up other matter from the body of the solution. In fact 

 it might completely remove one component. (Cf. emulsions and 

 emulsifying agents.) Surface concentration alone cannot account 

 for all the properties of the plasmahaut. 



Composition of Membranes. 



The exact chemical composition of animal cell membranes is 

 not known, but modern research tends to show that it is similar 

 to that of the cell as a wdiole. This is particularly true when 

 attention is directed mainly to the lipide content of protoplasm. 

 One is struck first of all with the way in which under physiological 

 conditions the amount of tissue lipide is kept constant in all cells 

 (except those of the liver). These unsaturated fats seem insepar- 

 able from the life of the cell. We have seen (Chap. VI,) how fats 

 spread over a surface in a layer one molecule thick, and how slight 

 alterations in the hydrogen ion concentration on either side of this 

 fatty layer may make extensive alterations in the structure and 

 intimate composition of the layer, and how fats solid at a particular 

 temperature become apparently fluid when adsorbed. 



Couple with their insolubility in water, the chemical inertness of 

 the fats and their substitution products, and one can see how 

 suitable they are as building stones for the house of the cell. If 

 this view is correct, then fat solvents should cause disruption of 

 animal membranes. This is easily shown to be the case (cf. 

 Haemolysis by ether and by soaps, Chap. XXII.), 



Cholesterol and lecithin are general constituents of cell membranes 

 and their relative proportions play an important part in controlling 



