NUTRITION 93 



amino-acids are partially decomposed with liberation of 

 ammonia. The latter combines with CO2 in the blood to be 

 transformed into urea, which is ultimately excreted ; the 

 residual portion of the amino-acid molecule is a keto-acid 

 which forms a common link in the intermediate metabolism 

 of carbohydrates and fats. This process is known as deamina- 

 tion. The importance of this lies in the fact that in some 

 carnivorous animals there is very little carbohydrate present 

 in the food, though, as we have seen, the chemical energy 

 of carbohydrates is the ultimate source of the mechanical 

 energy of molecule contraction. Thus while most animals 

 feed predominantly on nitrogenous food, the nitrogenous part 

 of the protein molecule is of little constructive importance 

 except in growing animals, where new protoplasm is being 

 formed. We have to distinguish between endogenous meta- 

 bolism which is concerned with growth and tissue waste on the 

 one hand, and exogenous metabolism which is concerned with 

 effector activities and the maintenance of body heat on the other. 



Observations have been made concerning the deamination 

 of amino-acids in the blow-fly (Calliphora) by Weinland 

 (1908), who showed that both the larvae and a pulp made by 

 crushing them had the power, in the absence of oxygen, to 

 split peptones into amino-acids, deaminise them with evolu- 

 tion of ammonia, and produce higher fatty acids with 

 evolution of CO 2 — presumably by synthesis from the nitrogen- 

 free remainder of the amino-acids. The particular enzyme 

 reactions which occur on a large scale in the digestive 

 processes are not to be regarded as special properties of the 

 alimentary secretions, but rather as characteristic of what is 

 involved, to a greater or less extent, in the metabolism of all 

 cells in the body. Among the hydrolysis-products of one 

 important class of proteins, the nucleo-proteins, purine bases 

 are found in addition to amino-acids. Purine bases derived 

 from food or tissue waste are excreted in many animals as 

 uric acid. But enzymes are known to exist which oxidise 

 purines with formation of urea ; and uric acid is not an 

 invariable excretory product. 



The extent to which fats contribute to the diet varies 



