CRYSTALLOID IN LIVING CELLS 7 



and, by the synthesis of the polypeptides, has already shown the 

 lines on which proteins must be built together. 



Under the influence of hydrolytic agents, such as heating with 

 either alkalies or acids under pressure, the proteins take up the 

 elements of water and yield a large number of simpler organic 

 substances ; and conversely by the action of dehydrating or con- 

 densing agencies these simpler organic substances or organic 

 radicles of the proteins can again be made to unite. In the latter 

 direction the process cannot be carried back quite to that degree 

 of complexity which yields the naturally occurring protein, mainly 

 because at that level the degree of chemical association of the 

 constituents is so delicately balanced that the chemical manipula- 

 tions cause splitting off and decomposition. 



The synthesised products are in fact beginning to possess that 

 delicately balanced power of associating and dissociating which is 

 characteristic in still higher degree of living matter, and it is for 

 .this reason that only the living cell has hitherto been able to put 

 the finishing touches upon the delicate unions which finally yield 

 proteins, and beyond these up to living protoplasm, where the 

 complexity and corresponding instability reach their acme. 



The organic radicles, which form the building stones, so to 

 speak, for the structure of the protein molecules, may be divided 

 into three classes, viz. those which are purely organic bases, those 

 which are entirely organic acids, and a third and most characteristic 

 class which possess both the properties of organic acids and organic 

 bases in modified degree. The compounds of this third class are 

 known as the amido-acids, and it is to them that the proteins owe 

 their peculiar property of building up into such complex bodies 

 of high molecular weights. 1 The simplest type of amido-acid con- 

 tains one organic acid radicle and one basic radicle, the acid 

 character being given by the carboxyl group (COOH) and the 

 basic character by the amidogen group (NH 2 ). As the simplest 

 example, glycocoll or glycene, which is the amido-acid of acetic 

 acid, may be quoted. Acetic acid is CH 3 .COOH, and is purely 

 acid in its properties, combining with bases such for example as 

 ammonium to form ammonium acetate (CH 8 .COO.NH 4 ). If the 

 ammonia, instead of neutralising the carboxyl group, becomes 

 attached, with the loss of one atom of hydrogen, as the group 



1 The terms amido-acid and amino-acid have the same meaning, and are used 

 indiscriminately in describing members of this class of compounds. 



