PEOTOPLASM 7 



are usually amorphous, but many have been prepared in a 

 crystalline condition, and it is probable that all may take a 

 crystalline form. The crystals vary in shape, being usually 

 small and needle-like, but sometimes forming larger rhombic 

 plates. Some proteins form apparent solutions in water, 

 others require the presence of neutral inorganic salts, others 

 of an acid or alkali, while some are completely insoluble 

 without a change in their constitution. All are insoluble in 

 alcohol and ether. 



When in solution, or apparent solution, the native pro- 

 teins do not dialyse through an animal membrane. They 

 are colloids. Other colloidal bodies reacting much like the 

 proteins have been prepared synthetically by chemists e.g. 

 by heating together amido-benzoic acid and phosphoric an- 

 hydride. Like other colloids proteins tend to coagulate, form- . 

 ing a clot just as, for instance, silicic acid may clot when 

 carbon dioxide is passed through its solution. The native 

 proteins are coagulated by simply heating their solution. 



All proteins rotate the plane of polarised light to the left. 



(B) Chemistry. Proteins contain the following chemical 

 elements: carbon, hydrogen, oxygen, nitrogen, and sulphur, 

 in about the following percentage amounts: 



c. H. N. s. 0. 



52 7 16 1 24 



It is important to remember the amounts of nitrogen and 

 carbon, since proteins are the sole source of the former 

 element in the food and an important source of the latter. 



As regards the number of atoms of these elements which 

 go to form a single molecule, information has been obtained 

 by studying compounds with various metals. The following 

 probable formula of the molecule of the chief protein of the 

 white of egg is given simply to show how complex these 

 substances are : C^H^N^OggSg. 



The constitution of the protein molecule has been investi- 

 gated first by studying the products of the decomposition 

 of the molecule by various agents, and second by attempting 

 to build up the molecule by the synthesis of the products of 

 disintegration. 



