60 THE MECHANISM OF LIFE 



and we get an idea of the exceedingly complex chemical structure 

 of the proteids — a structure upon which, in some way or other, the 

 phenomena of life depend. Proteids are therefore combinations 

 of amino- acids, and they contain, generally, about 16 per cent, of 

 nitrogen. 



Carbohydrates contain carbon, hydrogen, and oxygen, but no 

 nitrogen. They are also very complex chemical substances; 

 thus " grape-sugar " (dextrose, or glucose) has the formula: 



CH.OH— CH(OH)— CH(OH)— CH(OH)— CH(OH)— CHO. 



Many other sugars are more complicated, and the starches and 

 gums are more complicated still. 



Fats are still more complicated in chemical structure than the 

 carbohydrates. What we usually call an " oil " or a " fat " is a 

 combination of one or more fatty acids with glycerine. A fatty 

 acid consists of carbon, hydrogen, and oxygen atoms united 

 together to form a chain; thus stearic acid is: 



CH3— CH — 15 other CH^ links— COOH, 



and may be written in short by R — COOH, where the R stands 

 for the chain CHj — CH^ — to seventeen terms. Now three 

 molecules of stearic acid combine with glycerine to give us — 



R— CO— 0— CH2 



1 

 R— CO— 0— CH 



I 

 R_C0— 0— CH2 



which is tristearin, the principal constituent of mutton fat. 

 Since there are a great number of different fatty acids there 

 .must also be many kinds of fats. 



Now all this seems very technical, but it ntust be clearly under- 

 stood that no one can hope to get even a slight acquaintance with 

 the mechanism of life without a knowledge of at least such detail 

 as we have set out above. And, after all, it is not very difficult ! 



What we eat is therefore a mixture of proteids, carbohydrates, 

 and fats. The proteids, especially when cooked, are insoluble in 

 water, and so are the fats. We can take cane-sugar, and other 

 sugars, into the alimentary canal, but they must be converted 

 into dextrose before they can be used by the tissues. The starches 

 which form the bulk of potatoes, rice, and similar foods, as well 

 as a large part of cereals and the breads that are made from these 



