CHAPTER XIII 



THE NITROGEN CYCLE 



We have seen in the chapter on the chemistry of albumins 

 that substances comprised under this term constitute the 

 basis of both animal and vegetable living matter. We know 

 that the nitrogen in our food stuffs occurs mainly in the form 

 of albumin, either animal or vegetable. The vegetarian, if lie 

 does not consume eggs, must at any rate add to his diet a 

 considerable proportion of beans and peas, which are rich in 

 vegetable albumin. The actual amount of nitrogenous food 

 needed for useful work is a vexed question and need not here 

 be considered, our object being confined to following out the 

 chemical history of the nitrogen whether large or small 

 in quantity. Used as food we have already learned that 

 peptic and tryptic digestion of albumin leads by gradual 

 stages to the formation of end products, largely consist- 

 ing of amino acids. These one would not expect to be 

 excreted as such from the body ; they are built up again 

 into the body substance through the biotic energy of the 

 cells, and a portion also wiU be used up as fuel for main- 

 taining that energy ; consequently, therefore, we do not find 

 in the products of excretion of the animal body just those 

 amino acids and polypeptides which are formed when 

 albumin is digested by pepsin or trypsin, under laboratory 

 conditions. 



Some of these substances, it is true, are found amongst the 

 products of excretion ; thus leucin and tyrosin have been 



