PROTOPLASM 15 



plates. In protoplasm proteins form part of the emulsoid 

 complex. But some purified proteins form colloidal suspensions 

 — hydrosols — in water, some require the presence of neutral 

 inorganic salts, others of an acid or alkali, while some are 

 completely insoluble and uususpendable without a change in 

 their constitution. The animal proteins are insoluble in 

 alcohol ; all proteins are insoluble in ether. 



All proteins rotate the plane of polarised light to the left 

 on account of the action of the amino acids of which they 

 are composed (p. 16). 



2. Chemistry.— (1) Percentage Composition. — Proteins 

 contain the following chemical elements : — Carbon, hydrogen, 

 oxygen, nitrogen, and sulphur, in about the following 

 percentage amounts : — 



c. H. N. s. o. 



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. 



(2) Size of Molecules.— As regards the number of 

 atoms of these elements which go to form a single molecule, 

 information is afforded by the percentage of sulphur in the 

 molecule, and by the number of atoms of various metals 

 which combine with a molecule of the protein. The follow- 

 ing probable formula for the molecule of the chief protein of 

 the white of egg is given simply to show how complex these 

 substances are : — C204H322N52O66S2. 



(3) Constitution. — The constitution of the protein 

 molecule has been investigated (A) by studying the products 

 of the decomposition of the molecule by various agents, and 

 (B) by attempting to build up the molecule by the synthesis 

 of the products of disintegration, 



(A) The native proteins have very large molecules, and 

 they tend to break down into simpler proteins. This 

 decomposition is accompanied by hydration, and it is 

 accelerated by the action of acids, and by a group of 

 enzymes which may be named proteolytic enzymes. 



Under the action of alkalies and under the influence of 



