346 LIFE : OUTLINES OF GENERAL BIOLOGY 



and the niicleoproteins. In this last case the protein is united with 

 the compk'x nucleic acid, which contains phosphoric acid, a sugar, 

 and four different nitrogenous substances, built out of the ring — 



— N C = 



I I 



=C C— 



I II 



— X c— 



Much more is known of the metabolism of the absorbed amino- 

 acids that are destined to yield energy, though even here a great 

 deal of uncertainty remains. The first change which thej' undergo 

 is that they are deaminated, that is, the — NHj or amino group is 

 split off to form ammonia (XH3), while the rest, containing carbon, 

 hydrogen, and oxygen only, is oxidised to carbon dioxide and water, 

 >aelding energy. It is possible to imitate these reactions in part at 

 least in the laboratory, for example: 



CH CH, 



I I 



CH.NH, ^ NH, + CO 



I I 



COOH CHO 



Alanine -> Ammonia + Pyruvic aldehyde 



P\TUvic aldehyde is readily oxidised to pjTuvic acid, an important 

 substance (CH3— CO— COOH), which in turn yields carbon 

 dioxide and water by way of acetaldehyde. It is not very certain 

 whether this process of deamination is carried out in the liver only, 

 or in all organs of the body, nor is it certain whether the liver is the 

 chief site of the transformation which changes the harmful ammonia 

 into the innocuous urea, 



/NH, 



co( 



^NH, 



which is one of the chief waste products of the body. 



The sugars are oxidised, for the most part, in a very similar way, 

 since they readily yield lactic acid and p\Tuvic acid; but some 

 fraction is jwssibly diverted in other directions. Less is known of 

 the oxidation of the fats, but it probably follows a similar course. 



Two important features remain to be considered, the first being 

 the question of storage. Carbohydrate is stored in large quantities 

 in the liver and the muscles, in the form of glycogen, a complex 

 substance similar to starch, while the existence of stores or depots 

 of fat is familiar to everyone. The second point is that the animal 

 body has a great power of converting one type of substance into 



