H2 BIOLOGICAL CHEMISTRY 



Glucose 



ti 



CH 3 CH 3 CH 3 



I H 2 NH 3 | 



HC OH < C=O + CHNH 2 



I I I 



CpOH CHO COOH 



Lactic acid. Pyruvic aldehyde. Alanine. 



. 



pyruvic acid 



fats 



The horizontal line composed of lactic acid, pyruvic aldehyde 

 and alanine is an interesting example of a reversible reaction. 

 The formation of lactic acid causes an increase in acidity and 

 this reaction is inhibited by acid. Thus it is necessary to 

 neutralise the acid when one wishes to get a good yield of lactic 

 acid by the action of glyoxalase on glyoxal (pyruvic aldehyde). 



The removal of ammonia from alanine furnishes a base cap- 

 able of neutralising acids. Dakin points out that these changes 

 are of importance in maintaining the neutrality of the cells. 



The principle underlying these reactions seems to be con- 

 tained in Le Chatelier's Theorem (p. 59). An increase in 

 acidity would tend to prevent a reaction which takes place 

 with the formation of acid. We therefore see that these 

 interconversions are governed by principles which apply in 

 general chemistry : the actual amount of change being 

 regulated by the law of mass action and by chemical affinity. 



Not all amino acids can be shown to be formed from 

 carbohydrate and fat. The diagram given above is an in- 

 stance of one group where conversion of amino acid to fat or 

 carbohydrate can be proved by a series of experiments. 



Although we cannot say that all the reactions are reversible 

 the subjects discussed in this chapter indicate the way in which 

 the plant may form fats and proteins from the carbohydrates. 



The carbohydrates can be turned into pyruvic aldehyde, 

 pyruvic aldehyde can condense through pyruvic acid to form 

 fatty acids or in the presence of ammonia it can form alanine. 

 There is one curious point, namely, that plants absorb nitrogen 

 in the form of nitrates and many of them cannot thrive on 

 ammonia as a sole source of nitrogen, yet the formation of 

 amino acids seems to require the presence of ammonia. 



GENERAL REFERENCE 



H. D. DAKIN : Oxidations and Reductions in the Animal Body. Longmans, 

 Green & Co., 1012. 



