520 



Destructive metabolism 



All of these complex organic substances 

 absorbed by animal cells are capable of 

 delivering a vast amount of energy if de- 

 graded to the simpler substances from which 

 they came. The plants built them up into 

 complex molecules by utilizing energy re- 

 ceived from the sun, and all of this energy 

 is available when the reverse process oc- 

 curs. The animal cell complements the 

 work of the plant cell. The latter builds up 

 the energy, whereas the former releases it, 

 all for the sake of perpetuating animal life 

 on the earth. 



In bringing about the degradation of 

 complex molecules to simple ones, animal 

 cells have employed a long list of chemical 

 reactions, some of which are still only 

 vaguely understood, if at all. The chief 

 reaction employed by animals to release en- 

 ergy is oxidation, a familiar process in the 

 world outside of cells as well. This may be 

 defined as ( 1 ) the addition of oxygen, ( 2 ) 

 the loss of hydrogen, or (3) both. 



In the burning of glucose the following 

 initial and final products are indicated: 



CeHisOe + 6O2 -> 6CO2 + 6H2O + energy 



This reaction involves many steps between 

 the initial and final stages, in fact, over 

 twenty-five enzymes are essential, meaning 

 there must be at least that many separate 

 steps in this degradation in which energy 

 is released at almost every step. 



This oxidation process is little different, 

 as far as the energy release is concerned, 

 from similar burning in a test tube. The one 

 significant difference is the temperature at 

 which the oxidation proceeds. It occurs at 

 body temperatures in the cell, which is pos- 

 sible again only by the presence of the 

 numerous enzymes. 



Fats, likewise, can be burned to release 

 energy. Ordinarily they are not drawn upon 

 as a source of energy and it is only after 

 days of starvation that they do come into 

 the picture. Then they burn to carbon 



ORGAN SYSTEMS OF MAN 



dioxide and water, but in a slightly differ- 

 ent manner from glucose. 



2C61H98O6 -f I45O2 -> IO2CO2 + 98H2O + energy 



Needless to say, enzymes also play their 

 indispensable role in this reaction. One pe- 

 culiar thing about the oxidation of fats is 

 that they will not burn well except in the 

 presence of glucose. In other words, they 

 must burn simultaneously, though the rea- 

 son why is not clear. 



Proteins can also be utilized for energy, 

 although their chief function is in the con- 

 struction of protoplasm. Amino acids do not 

 burn directly but must first be stripped of 

 their amino groups (NHo). This is done in 

 the liver by the addition of water and the 

 formation of ammonia and a deaminated 

 compound. The process is known as deami- 

 nation: 



R 



R 



H— C— NH2 -f H2O -> H— C— OH + NH3 



COOH 



COOH 



The ammonia forms ammonium hydroxide 

 immediately by uniting with water. This 

 is a strong alkali which is normally pro- 

 duced in considerable quantities and if 

 allowed to accumulate would upset the 

 acid-base level of the blood, causing a 

 serious condition that could not be toler- 

 ated by the organism. Therefore, the am- 

 monia must be removed from the blood as 

 fast as it forms; this is also done in the liver 

 where it is converted into urea which is 

 neutral in its reaction. This is accomplished 

 in the following manner: 



2NH3 + CO2 -^ H2N— C— NH2 + H2O 



II 

 



(urea) 



The kidneys then remove the urea from the 

 blood and pass it into the urine of which it 

 makes up a considerable part. The more 

 protein that is taken into the body, the 

 greater is the urea output in the urine, so 

 that the examination of the urine of an ani- 



