THE COMPONENTS OF PLANTS AND ANIMALS 25 



The proteins in this sense are subdivided into : 



1. Simple proteins 



2. Conjugated proteins 



3. Derived proteins 



Simple proteins are denned as those yielding only a 

 amino acids or their derivatives upon hydrolysis. Conju- 

 gated proteins are those which contain the protein mole- 

 cule united to some other molecule or molecules otherwise than 

 as a salt. Derived proteins are the products of the hydrolytic 

 cleavage of the protein molecule and include a wide range of 

 substances, from slightly altered protein to the peptids. 



42. Physical properties. In the dry state, the proteins are 

 in general white or slightly tinted substances. They are usually 

 amorphous, but a number of them have also been obtained in 

 the crystalline form and some are found crystallized in nature. 

 Some of the proteins are soluble in water, others only in salt 

 solutions or in acids or alkalies. They are insoluble in most 

 other ordinary solvents. 



The proteins belong to the class of colloids, i.e., they do not 

 diffuse through membranes and are claimed to have no osmotic 

 pressure when free from electrolytes. Colloids in general exist 

 in two forms, a liquid form, technically known as a sol, and a 

 solid form called a gel, the difference being well illustrated by 

 the familiar substance gelatin. When a colloid is distributed 

 through water so as to form an apparent solution the latter is 

 known as a hydrosol. Whether the proteins are to be regarded 

 as soluble in water, or whether their apparent solution is in 

 reality a suspension, has been much discussed. It has been 

 shown, however, that these solutions are conductors of electricity 

 and it has been concluded that they are true solutions. It may 

 be said, however, that no sharp boundary exists between a 

 true solution and a suspension but that an indefinite number of 

 intermediate stages is possible. As a matter of convenience, 

 however, we may speak of solutions of the proteins. 



Different proteins may be precipitated from their solutions 

 by various reagents, particularly acids, alkalies and metallic 

 salts. Ammonium sulphate, especially, has been largely used 

 for the purpose of separating different proteins by means of 

 fractional precipitation. 



