238 PHYSIOLOGY OF NUTRITION 



the aerobic phase. It is supposed that sugar and water unite and form carbon dioxide 

 and hydrogen, that the hydrogen unites with respiration pigments and thus forms chro- 

 mogens, and that the chromogens are directly oxidized by free oxygen, forming water 

 and the pigments. According to this hypothesis, as long as an unsatisfied acceptor of 

 hydrogen is present, alcohol and similar substances are not formed, and the acceptor 

 (pigment) is kept unsatisfied through the oxidation of the chromogen as long as free 

 oxygen is adequately supplied. On the other hand, in the absence of free oxygen the 

 pigment soon becomes satisfied and ceases to be able to absorb hydrogen, after which 

 alcohol, etc., arise from the original decomposition of sugar, along with carbon dioxide. 



11. Materials Consumed in Respiration. — In ordinary plants it is generally true 

 that carbohydrates are the substances consumed by respiration. The respiration 

 processes are 'controlled by enzymes, and these, in turn, are formed in the protoplasm. 

 On the amount of protoplasm in a tissue depends the amount of enzymes present, and 

 the latter determine the rate of respiration as long as the supply of carbohydrates is 

 adequate. With inadequate supply of carbohydrates the respiration rate is low, 

 even with plenty of protoplasm and enzymes; on the other hand, an excess of carbo- 

 hydrates exerts no influence on the respiration rate. With plenty of carbohydrates 

 it appears that the respiration rate varies with the amount of nucleins in the respiring 

 tissue, the amount of nucleins being considered as proportional to the amount of com- 

 plex proteins, insoluble in gastric juice. It may be supposed that the amount of 

 complex proteins present is proportional to the amount of respiration enzymes, and this 

 supposition may explain the apparent relation between the respiration rate and the 

 supply of complex proteins. Neither simple nor complex proteins are generally used 

 in respiration; carbohydrates may sometimes result from protein decomposition, 

 however, and these may be used. 



12. Special Cases of Respiration in Lower Plants. — In many lower forms, sub- 

 stances other than carbohydrates are decomposed in respiration. In acetic-acid 

 bacteria, for example, respiration is the simple oxidation of ethyl alcohol (by means of 

 free oxygen), with the formation of acetic acid and water. When the supply of 

 alcohol has been consumed, however, these bacteria oxidize acetic acid and thus form 

 carbon dioxide and water. Many other alcohols are similarly oxidized by micro- 

 organisms. 



As already mentioned (Chapter II, Section 3), many bacteria obtain energy from 

 inorganic substances, and it should be remarked that the decompositions thus brought 

 about are respiration processes. Hydrogen sulphide, ammonia, hydrogen, etc., are 

 thus oxidized. 



13. Circulation of Energy in Nature. — While the amount of matter in and on the 

 earth remains always practically the same, almost no material being now given off to, 

 or received from, the rest of the universe, the energy exchange between the earth and 

 its surroundings is very rapid and exceedingly important. Energy is continually and 

 rapidly radiated from the earth's surface into sidereal space, and the supply available 

 to organisms would soon be practically exhausted if it were not for the fact that new 

 radiant energy is continuously being supplied from the sun. A very small part of the 

 radiant energy emanating from the sun is intercepted by the earth, and a small portion 

 of what is intercepted is rendered potential by the photosynthetic formation of 

 carbohydrates in chlorophyll-bearing plant tissues. The potential energy of the 

 carbohydrates thus formed becomes again kinetic through the processes of respira- 

 tion, oxidation, and combustion. Most of this energy from carbohydrates is quickly 

 radiated from the earth into the surrounding universe, and the remainder goes the 



