PHYSIOLOGICAL 341 



possesses, and to its likewise limited power of deriving energy from 

 chemical compounds. The last point may be considered first. There 

 are many simple organic compounds, containing oxidisable carbon 

 and hydrogen, which nevertheless cannot act as sources of energy, 

 for the mammal at least. Methane or marsh gas, CH^, may be taken 

 as an example ; it could easily be used to furnish energy in an internal- 

 combustion engine, and certain bacteria are able to live upon it; 

 but the animal body is unable to oxidise it or to convert it into any 

 oxidisable substance or energy-yielding compound. The supply 

 of such compounds is dependent in the long run on the power 

 of the green plants to form them by synthesis from simpler 

 substances of lower energy content. In the same way, the nitrogen 

 compounds required to form new tissue materials must be supplied 

 in a sufficiently complex form; there are bacteria which can "fix" 

 nitrogen gas from the air in the soil and soil-water, while the higher 

 plants absorb nitrogen chiefly in the form of nitrates from the soil ; 

 animals, however, require still more complex nitrogen-compounds, 

 for growth, at least, if not for maintenance, in the form of amino-acids 

 (of which the simplest is glycine, C2H5NO2) which they are able to 

 bind together to form proteins. Animal and plant proteins break 

 up to form simple nitrogenous compounds, such as urea (CH4N2O), 

 which in turn are converted by soil bacteria to ammonia (NH3). 

 Other bacteria oxidise the ammonia, first to nitrites (as in the case 

 of Nitrosomonas) and then to nitrates (as in the case of Nitrobacter). 

 From these nitrates, such as saltpetre, the plants, starting the 

 "nitrogen cycle" afresh, are able to form proteins. 



Not only are animals unable to form amino-acids to any great 

 extent, they are also unable, except in a limited way, to transform 

 one amino-acid into another. As the proteins of the animal body are 

 made up of many different amino-acids — glycine, alanine, leucine, 

 cystine, tyrosine, proline, histidine, tryptophane, and so forth — it 

 follows that nearly all or all of these compounds must be 

 supplied in the food, either free or combined together as proteins, 

 which can be split down in the process of digestion and built up 

 again by the tissues. The protein of maize, for example (zein), is 

 remarkable in that it contains no tryptophane, an amino-acid of 

 great importance to the animal. Hopkins and others have shown 

 that zein alone does not suffice as the protein element of the diet 

 (of mice, in this particular experiment), but becomes adequate if 

 tryptophane be added. From similar experiments has arisen the 

 conception of the "biological value of proteins" — that some are more 

 efficient foods than others because they cope more adequately with 

 the special needs of the body. 



There remains to be added to this account of the chemical 

 requirements of the body one very important point already referred 

 to. There are certain substances, the so-called "vitamins" or 



