114 PHYSIOLOGY 



we obtain two molecules of gly eerie aldehyde, which then by a further 

 shifting of the OH and H groups becomes 



CH 3 



I 

 CH.OH 



I 

 COOH 



lactic acid 



Lactic acid with ammonia and some dehydrating agent will give amino- 

 propionic acid or alanine. The formation of the higher amino-acids in- 

 volves a process of reduction of the sugar first formed in the chlorophyll 

 granules. It is possible however that the starting-point for the amino-acid 

 synthesis may be, not a hexose itself, but some other substances, formed, 

 so to speak, as by-products in the assimilation of sugar from carbon dioxide. 

 We have seen reason to believe that the first result of the action of the sun's 

 rays within the chlorophyll corpuscle is formaldehyde. This substance in 

 the presence of calcium carbonate when exposed to the light gives a mixture 

 of glyceryl aldehyde and dihydroxyacetone. If we can assume that acetone 

 is formed from the latter by a process of reduction, we might possibly derive 

 leucine from an interaction of this substance with lactic acid and ammonia. 

 Thus: 



CH 3 CH 3 



CH 3 CH 3 \CH/ 



I I I 



CO + CH.OH + NH 3 + H 2 = CH 2 -f 2H 2 O 



I I I 



CH 3 COOH CH.NH 2 



I 

 COOH 



As an intermediate product in the synthesis of starch, glyoxylic acid 

 CHO 



has been described as occurring in the green parts of plants. This 

 COOH 



substance with ammonia gives formyl gly cine, and by the splitting off of 

 formic acid, glycine or amino-acetic acid. Why nitrates are necessary for 

 certain forms of plants is not at present understood. In the proteins nitrogen 

 always occurs in an unoxidised form as NH or NH 2 , and the nitrates taken 

 up from the soil must therefore undergo reduction before they can be built 

 into the protein molecule. It is supposed that they may pass through a series 

 of reductions, namely : 



HN0 3 HN0 2 HNO H 2 N OH 



nitric acid nitrous acid hyponitrous acid hydroxylamine 



and that the latter substance then reacts with formaldehyde or other sub- 

 stance derived from the carbon dioxide assimilation to form amino-roin- 

 pounds. In general we may say that the probable mechanism of formation 

 of arnino-acids is the production of a-oxyacids, which then react with 

 ammonia to form the amino-acids of the protein molecule ; but of the 



