74 GENERAL CONCEPTS 



cule such as glucose. The carbon and oxygen atoms are removed from a 

 substrate molecule together, by a process known as decarboxylation. 

 There is one such decarboxylation process as pyruvic acid (three car- 

 bons) is converted to acetyl coenzyme A (two carbons) and two more in 

 the Krebs cycle as citric acid (six carbons) is converted to oxaloacetic 

 acid (lour carbons). 



The conversion of glucose to pyruvic acid in the absence of air, 

 sometimes referred to as fermentation, extracts only a small portion of 

 the energy of the glucose molecule. When yeast cells ferment glucose, 

 they convert the pyruvic acid formed to alcohol and carbon dioxide. 

 The souring of milk by bacteria involves the conversion of milk sugar 

 (lactose) through the glycolytic cycle to pyruvic acid, and finally the 

 conversion of the pyruvic acid to lactic acid. 



Further examination of Figure 4.2 will show that the Krebs cycle is 

 the final common pathway for the oxidation of fatty acids and amino 

 acids as well as for carbohydrates. It is the chief source of chemical 

 energy in the cell. The fatty acids most commonly found in tissues are 

 ones containing 16 and 18 carbons in a long chain. These long chains 

 are chopped into two carbon pieces, as acetyl coenzyme A, and these 

 pieces enter the Krebs cycle by uniting with oxaloacetic acid. Certain 

 amino acids can be transformed enzymatically into pyruvic acid and 

 others are converted to other members of the Krebs cycle. By a variety 

 of different pathways, the amino groups are removed and the carbon 

 chains of the amino acids finally enter the Krebs cycle and are oxidized 

 to yield carbon dioxide, water and energy. 



Some interesting calculations of the over-all energy changes in- 

 volved in metabolism in the human body have been made by E. G. Ball 

 of Harvard University. Since the conversion of oxygen to water in- 

 volves the participation of hydrogen atoms and electrons, the total flow 

 of electrons in the human body can be calculated and expressed in 

 amperes. From the oxygen consumption of an average 70 kg. man at 

 rest— 264 ml. per minute— and the fact that each oxygen atom requires 

 two hydrogen atoms and two electrons to form a molecule of water. 

 Dr. Ball calculated that 2.86 X 10"" electrons flow from foodstuff, via 

 dehydrogenases and the cytochromes, to oxygen each minute. Since an 

 ampere equals 3.76 X 10"" electrons per minute, this flow of electrons 

 amounts to 76 amperes. This is quite a bit of current, for an ordinary 

 100 watt light bulb uses just a little less than 1 ampere. Then, from the 

 number of calories used by this 70 kg. man at rest— 1.27 calories per 

 minute— Dr. Ball calculated that 88.7 watts were being used. Since, in 

 electrical units, watts divided by amperes equals volts, 88.7 divided by 

 76 equals 1.17 volts. The body, then, uses energy at about the same 

 rate as a 100 watt light bulb, but differs from it in having a much 

 larger flow of electrons passing through a much smaller voltage change. 



23. The Dynamic State of Protoplasm 



The body of an animal or man appears to be unchanging as days and 

 weeks go by and it would seem reasonable to infer that the component 

 cells of the body, and even the component molecules of the cells, are 



