NITROGENOUS COMPOUNDS 95 



oxygen, and nitrogen, usually sulphur, and sometimes phos- 

 phorus. They are the most complex compounds known, 

 and probably the most important, since they are a necessary 

 constitituent of every living cell, whether plant or animal, 

 and compose most of the dry matter of animals except the 

 bones. Moreover, plant proteins are important not only to 

 plants themselves, but also to animals, since the latter arc 

 dependent for the most part on the ready-made proteins 

 of plants for their own body nitrogenous compounds. The 

 name itself is significant, being derived from a Greek word 

 signifying " preeminent." 



The composition of proteins is about as follows: Carbon, 

 50 to 55 per cent.; hydrogen, 6 to 7.3 per cent.; oxygen, 

 19 to 24 per cent.; nitrogen 15 to 19 per cent.; sulphur, 

 0.3 to 2.5 per cent., and phosphorus, if present, 0.4 to 0.8 

 per cent. A protein molecule is known to be exceedingly 

 large, the atomic weight of different proteins being estimated 

 at from 4000 to 16,000 in round numbers. A formula which 

 has been proposed for zein — an example of a typical plant 

 protein — will give some idea of the complexity of the mole- 

 cule: 



C230H362O65N58S . 



Most of the knowledge of proteins is based on studies 

 of the animal proteins. Plant proteins in general, however, 

 are very similar, although not so numerous. 



The various proteins differ somewhat in solubility, some 

 of them being soluble in water, others in dilute alcohol, 

 others in salt solutions, and still others in verv dilute acids 

 and in alkalies. Strong acids and alkalies dissolve proteins 

 on heating, but with decomposition. On heating with 

 strong sulphuric acid the nitrogen of proteins is converted to 

 ammonium sulphate, from which ammonia can be evolved 

 with sodium hydroxide. This is the basis of the quantitative 

 estimation of proteins (Section 95). They are built up from 

 a series of amino-acids or their derivatives from which 

 ammonia is easily split off. On hydrolytic decomposition 

 thev generallv break down into amino-acids. 



For the most part proteins are noncrystallizable, belonging 



