Page:Popular Science Monthly Volume 82.djvu/142

138 acids may be lessened or counteracted by neutral salts like sodium or calcium chloride. Such antagonistic actions between acids and salts, while not shown by colloids in general, are peculiarly characteristic of certain proteins. Thus the rate of swelling of gelatine (a typical scleroprotein) in water is greatly increased by the addition of a little acid; this effect is prevented by the addition of neutral salts, and the basis of this form of anti-cytolytic action may possibly lie here—i. e., the disruptive action of the acid on the proteins of the membrane is checked or prevented by the salt. Yet the plasma-membrane undoubtedly contains other constituents, and among these the substances belonging to the group of lipoids appear to be fundamentally important. These substances, fat-like in their solubilities and colloidal in their physico-chemical character, are always present in cells. Much light has been thrown on their physiological significance by the investigations of Overton and his successors, which have shown that ready permeability to lipoid-solvents is highly characteristic of both animal and plant cells. Alcohols, esters, ethers, hydrocarbons and similar compounds, all of which are soluble in lipoids, enter living cells rapidly, in contrast to neutral salts, sugars, amino-acids—the chief crystalloidal constituents of protoplasm—which diffuse into resting cells (with unmodified plasma-membrane) either imperceptibly or with extreme slowness. Overton's results thus indicate that lipoids enter into the composition of the plasma-membrane. This is to be expected. The structure probably consists of a mixture of all those protoplasmic constituents which have marked effect in lowering the surface-tension of the cell-boundary. Lipoids are conspicuous among this group of substances. That they form part of the plasma-membrane is also indicated by the readiness with which the permeability and other properties of this structure may be altered by lipoid-modifying substances. Lipoid-solvents as a class, when present in certain concentrations, have a specific action in increasing, often irreversibly, the permeability of the plasma-membrane. In lower concentrations many appear to decrease this permeability. Their influence on irritability, which is probably a function of the condition of this membrane, also indicates their importance as membrane-constituents. Narcotic action is highly characteristic of lipoid-solvents, and there is good evidence that this action depends on an alteration of the plasma-membrane. I shall refer to this possibility later, in connection with the problem of the relation of membranes to stimulation. All of these facts taken together indicate very clearly that the colloids composing the semi-permeable surface-film of living cells consist of