136 PHOTOSYNTHESIS 



Chloroplasts and mitochondria have several points in 

 common and biochemical evidence suggests that the process 

 of photosynthesis may quite closely resemble a reversal of 

 respiration. 



The reversibility of enzyme systems concerned in respiration 



In heterotrophic organisms respiration can in a sense 

 reverse itself; carbon dioxide can be re-assimilated and 

 appear even in carbohydrate. This has been shown for 

 mammalian liver both in tissue slices and in the intact 

 animal. Naturally, of course, there can be no net assimilation 

 of carbon because the reincorporation of the COg requires 

 energy derived from respiration itself. In the green plant 

 however there is a supply of energy from the photochemical 

 transformation. 



The process of respiration considered as a whole is to be 

 regarded as an irreversible reaction because the over-all 

 change in free energy is large. If this is represented by the 

 complete oxidation of carbohydrate the value is about 

 115,000 cals. per CHOH group. Thus the amount of carbo- 

 hydrate theoretically in equilibrium with CO2 and Og at 

 atmospheric pressure would be iq-s*-* molar, an amount 

 equal to zero for all practical purposes. The respiration of 

 carbohydrate takes place in many stages and each stage will 

 correspond to a smaller energy step. When in a given step 

 the concentrations of both an initial and a final product 

 remain large enough to be within the range of an enzyme 

 system concerned in the catalysis of the reaction the process 

 can become reversible from a practical standpoint. For 

 example, in the presence of inorganic phosphate starch may 

 break down so completely that no residual starch may be 

 detected. This process consists of two steps each associated 

 with a small free energy change. For each step, in presence 

 of the appropriate enzyme, equilibrium may be approached 

 from either side. 



Starch +phosphate -^ glucose- 1 -phosphate (i) 



Glucose- 1 -phosphate -^ hexose-6-phosphate (2) 



If now, starting with hexose-6-phosphate, some means of 



