PLANT CONSTITUENTS — FATS, PROTEINS 283 



Physiological Function. — Physiologically lecithin is very 

 important. We have spoken of the fact that it is hydrolyzed 

 by the enzyme lipase and in the animal body the lecithin of 

 plant food is probably digested and absorbed in much the 

 same way as fats. If the lecithin of animals, essential to the 

 living cell and nerve tissue, is derived in the case of herbivorous 

 animals entirely from plant lecithin, it thus becomes an essential 

 food constituent. The generally accepted connection between 

 organic phosphorus (lecithin compounds) in food and the 

 building of brain and nerve tissue is based upon these facts. 



We know nothing in regard to the metabolism of lecithin 

 and only very little in regard to its probable function in the 

 living cell. Recent work upon certain toxic substances, e.g. 

 the toxins of cobra poison and of tetanus, leads to the view that 

 lecithin acts as an accelerator to these toxins. Its general 

 physiological action is perhaps of a like nature, that is, it acts 

 as an accelerator to the activity of enzymes. We spoke of the 

 fact that cholesterol acts as an antitoxin. Toward lecithin it 

 acts in such a way that it prevents the accelerating action of 

 lecithin upon these toxins. We thus see how intimately these 

 two compounds are connected with each other, and though we 

 know relatively little as yet in regard to them, their importance 

 is at least indicated. 



PROTEINS 



Proteins differ from carbohydrates in not being an abundant 

 constituent of plants, though they are, nevertheless, a universal 

 and essential constituent. In animals, on the contrary, pro- 

 teins are the abundant constituent. All protoplasmic cell 

 contents of plants contain protein, and as proteins differ in 

 composition from carbohydrates and fats in containing the 

 element nitrogen this element becomes an essential plant food 

 constituent. 



Forms of Proteins. — We have previously stated that pro- 

 teins are synthesized in the leaves of plants, but they are found 



