io HUMAN PHYSIOLOGY 



in their constitution. All are insoluble in alcohol and 

 ether. 



When in solution, or apparent solution, many of the pro- 

 teids do not dialyse through an animal membrane, and they 

 are hence called Colloids. Other colloidal bodies reacting 

 much like the proteids have been prepared synthetically by 

 chemists e.g. by heating together amido-benzoic acid and 

 phosphoric anhydride. Like other colloids they tend to 

 coagulate, forming a clot just as, for instance, silicic acid 

 may clot when carbon dioxide is passed through its solution. 



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



(B) Chemistry. Proteids 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 proteids 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 proteid of the 

 white of egg is given simply to show how complex these 

 substances are : C^Hg^N^OgA. 



Our knowledge of the constitution of the molecule is still 

 very imperfect. 



The simplest bodies having the characters of proteids are 

 the Protamines, basic substances which are found in the heads 

 of spermatozoa, combined with nucleic acid (p. 12). When 

 they are broken down they yield chiefly Hexone Bases, so 

 called from having six atoms of carbon in their molecule. 

 These are 



DIAMIDO ACIDS acids having two ainidogens NH 2 

 in their molecule. The most important is : 



(1) Arginin, diamido-valerianic acid linked to guani- 



din, and therefore allied to creatin. (See p. 43.) 



(2) Lysin, diamido-caproic acid, and 



(3) Histidin, a substance of unknown constitution. 



