104 BIOLOGICAL CHEMISTRY 



Synthesis of amino acids requires the introduction of 

 nitrogen, but the energy value of protein is only slightly 

 greater than that of carbohydrate.* One would expect to 

 find that ammonia would be the most easily available supply 

 of nitrogen for the amine group, but higher plants absorb 

 nitrates and use them for synthesis. 



The inter-conversion of carbohydrates, fats and proteins is 

 part of the subject of metabolism which deals with the 

 chemical changes that occur in cells. It is dealt with here 

 because it is a synthetic process by which a higher concentra- 

 tion of energy is produced by the formation of fat. 



Most of the evidence to be adduced is furnished from animal 

 sources, but the steps in the process are probably alike in 

 the animal and vegetable kingdoms. 



In order to follow these changes we must know something 

 about the intermediate stages. We can assume that the 

 various steps are reversible, thus the changes observed in 

 the decomposition of the different substances can be con- 

 sidered as steps in the processes of synthesis. 



The great difficulty of the problem of metabolism is that 

 the intermediate stages are difficult to demonstrate. The 

 carbohydrates and fats are completely decomposed into water, 

 and carbon dioxide and the proteins to the same substances, 

 with the addition of nitrogenous waste products, such as 

 ammonia. 



METHODS USED TO DETERMINE INTERMEDIATE STAGES 

 IN METABOLISM 



The experimental methods are various, but the following 

 are the general methods employed. Some substance is 

 administered to an animal and the result observed. Thus 

 feeding animals on a diet rich in carbohydrate leads to the 

 formation of fat, hence we must look for some means by 

 which carbohydrate can be turned into fat. 



By a comparison of the structural formulae of carbohydrate 

 and of fats, and by a study of the reactions of these substances, 

 a chemical analogy is made out indicating some possible 

 line of conversion of the one into the other. 



After some plausible hypothesis has been developed, it 

 is tested experimentally. If the results of these experiments 

 are not explicable on the hypothesis which prompted the 

 experiments the hypothesis must be altered and new experi- 

 ments performed. 



* 5-3 Calories as contrasted with 4-1 Calories. In animal cata- 

 bolism protein yields only 4-1 Calories as it is not completely oxidised. 



