418 



George Hoch and Olga v. H. Owens 



may have also an origin in respiration. The chromatic M 

 transients have been ascribea by Blinks to changes in respi- m 

 ration ^^^\ Above all they indicate that photosynthesis ^ 

 and respiration are not mutally exclusive processes within 

 the cell but rather are interrelated, allowing the cell to 

 function as an entity. 



Brown and iVeiss (13 il^) have found carbon dioxide evolu- 

 tion to be suppressed during illumination of both a green 

 and a brown alga. Furthermore, this suppression was inde- 

 pendent of intensity of illumination over a wide range. If 

 this is also the case in Anacystis , which seems likely, then 

 we can assume dark respiration to be inhibited by light. 

 The increased oxygen uptake which occurs when accessory pig- 

 ments are illuminated may be possibly ascribed to an oxida- 

 tion of a photosynthetic reductant proauced in photosystem 

 II. The mechanism by which chlorophyll a light suppresses 

 respiration is likely to be similar to other systems for 

 control of competing, energy supplying processes. The 

 Pasteur effect has been thought to exert its control over 

 fermentation through the phosphorylating components common 

 to both systems. This may be the case in photosynthesis 

 also. If we were to assume that oxygen uptaKe is inhibited 

 by the abstraction of reducing equivalents from respiration 

 by an oxidized moiety produced in photosynthesis, then CO2 

 evolution would have to continue unabated in the light. It 

 seems more probable that control is exerted through high 

 energy phosphate. 



In this regard, it is well to consider the total energy 

 requirements for the growth of micro-organisms or plants at 

 the expense of light energy. Reduction of carbon dioxide to 

 the level of carbohydrate apparently requires two reduced 

 pyridine nucleotides and 3 ATP or 12 PNH and I8 ATP per 

 hexose. From, a variety of organisms the ATP aemand for 

 synthesis of cell stuff from a supplied hexose is I8 ATP. 

 Hence, per 6 CO2 fixed the light must supply 12 PNH and 36 

 ATP or two and 6 respectively per CO2. Present schemes for 

 photosynthesis (as Hill and Bendall '^15) envisage two quanta 

 per electron and that is what is measured for the reduction 

 of TPN in the Hill reaction. However, this supplies only 

 2 ATP per O2 instead of 6. The missing k ATP could come 

 from, a recycling of P700 but at the expense of at least 2 

 more quanta. Hence, from present data the requirements for 

 growth are 10 hv per CO2 (uncorrected for respiration). If 

 the lov/er values found in the literature (as for instance 



