62 GENERAL BIOLOGY 



fats in the blood, or are laid down in various parts of 

 the body, unchanged. The carbohydrates are all split 

 into simple sugars (e.g. glucose) before being absorbed, 

 and possibly circulate in the blood loosely combined 

 with the serum-proteid base in the same way that 

 oxygen combines with haemoglobin. 



The proteins follow a more complicated path, al- 

 though our knowledge of them is confined almost 

 wholly to what we know of the metabolism of warm- 

 blooded animals. Using the digestive fluids of the 

 alimentary canal, we can split up protein (a strip of lean 

 meat, for example) into smaller and smaller bodies 

 until we reach the amino-acids, which are the units out 

 of which the proteid molecule is built up. These are ab- 

 sorbed through the blood-vessels of the alimentary canal 

 and are apparently split further into urea, CO(NH 2 )2. 

 on the one hand, and on the other hand a residue 

 which is then resynthesized into complex albumens 

 that circulate in the blood stream or are built up into 

 the protoplasm of the tissues. Although an absolute 

 essential for the maintenance of life, nitrogen is not ac- 

 cumulated in the body ; the greater the amount of ni- 

 trogenous food ingested, the greater the amount of urea 

 eventually excreted, a condition known as nitrogenous 

 equilibrium. 



Role of Oxygen in Metabolism. --It is a matter of 

 familiar experience that all animals require dftifmndant 

 supply of oxygen in order to live. The air-breathing 

 vertebrates are especially sensitive to the lack of 

 this element, and if deprj^ed^of a supply of oxygen 

 soon succumb with characteristic symptoms of 

 asphyxiation. If we boil water and thus drive out 

 the air dissolved in it, fishes and other aquatic 

 animals soon die. But plants are no less dependent 



