236 PHYSIOLOGY OF NUTRITION 



The nature of the material consumed in respiration largely determines the value of 

 the respiration ratio. For germinating starchy seeds this value is about unity, but for 

 germinating fatty seeds it is much lower. The following equations, for the complete 

 oxidation of starch [(CeHioOs)*] and for that of triolein (glycerine trioleate, the main 

 fat in cotton seed, for instance), serve to show why oxygen accumulates in germinating 

 fatty seeds and does not do so in germinating starchy seeds. Starch contains much 

 more oxygen, in proportion to its carbon content, than does fat. 



Carbon 

 Starch Oxygen Dioxide Water 



i. C 6 H ]0 O 5 + 6 2 = 6 CO, + 5 H 2 0. In this case ~ = \ = 1.00 



U2 



Carbon 

 Triolein Oxygen Dioxide Water 



2. C 3 H503(C 18 H330) 3 +So 2 = 57 COo + 52 H 2 0. In this case ~ = | 7 - = 71 



Ripening fruits in which fat is being formed from carbohydrates exhibit a respiration- 

 ratio value much greater than unity. 



5. The Measurement of Aerobic Respiration. — The rate of aerobic respiration may 

 be measured in terms of the rate of oxygen absorption or in terms of the rate of carbon- 

 dioxide elimination. The value of the respiration ratio is derived from the magnitudes 

 of these two rates. 



6. Respiration Water. — Water is produced by aerobic respiration, as has been said, 

 but the rate of water formation is very difficult to measure and but few studies on 

 respiration water have been reported. It will be remembered that hydrolytic processes 

 (controlled by hydrolytic enzymes; see Chapter VII, Section 3) consume water and that 

 the reverse processes liberate this substance. For example, saccharose combines with 

 water and forms dextrose (glucose) and levulose; but if a molecule of dextrose and one 

 of levulose unite to form a molecule of saccharose, a molecule of water is produced. 



Saccharose Water Glucose 



Ci 2 H 22 0n + H 2 *± 2 C 6 H 12 G 



The formation of carbohydrates by photosynthesis consumes large amounts of water, 

 one molecule of water for each atom of carbon fixed in the process. These, as well as 

 other water-consuming or water-producing processes that occur in the plant, tend to 

 hide the production of water by respiration. 



7. Liberation of Heat During Aerobic Respiration. — The temperature of the plant 

 body is generally very nearly the same as that of the surroundings, simply because the 

 periphery conducts and radiates heat with so little resistance that no considerable 

 temperature gradient between the environment and the tissues is generally maintained. 

 The processes of respiration, including the final oxidations, liberate considerable 

 amounts of energy within the active tissues, and much of this takes the form of heat, 

 which tends to raise the tissue temperature. But this excess of heat is generally 

 conducted to the surroundings about as rapidly as it is formed; consequently the 

 internal temperature is usually only slightly, if at all, higher than the external. As has 

 been noted, the absorption of solar radiation tends to raise the temperature of plant 

 parts that are exposed to sunlight, during the periods of such exposure, and this is 

 another source of increased heat within these parts. But this heat also is usually lost 



