PHOTOSYNTHESIS 



to dialysis. Thus some type of metalloprotein might carry the 

 reducing power. Another possibility is that reduced thioctic 

 acid, a dithiol, might be a carrier of reducing power and might 

 be attached by its carboxyl group to form a part of a lipid. It 

 may be simply that the enzymes which reduce CO2 in the chloro- 

 plast are so active that the reducing power is largely used before 

 it can diffuse out of the chloroplast. 



The other reagent requirement for the reduction of CO2 at 

 photosynthetic rates seems to be a high level of ATP, as will be 

 seen later in this discussion. Bradley (D. F. Bradley, private 

 communication) finds that the level of ATP is higher in the dark 

 than in the light if the plant is in an atmosphere of 4% CO2 in 

 air, lower in the dark than in the light if the atmosphere is 

 nitrogen. Strehler (54) finds that there is an increase in the level 

 of ATP in the cell upon turning on the light after a period of 

 darkness. This increase occurred during the induction period 

 when the rate of carbon reduction had not yet reached its 

 maximum value. These facts might be explained in the following 

 way. In the dark with O2, respiration and the production of 

 ATP by some of the energy available from respiration proceed 

 in both the chloroplast and the space outside. When the light 

 is turned on the rate of production outside the chloroplast is not 

 immediately affected, but inside the chloroplast there is a com- 

 bination of several effects. Normal dark respiration ceases owing 

 to the production of reducing power and in particular reduced 

 thioctic acid (18), while at the same time the production of ATP 

 through energetic coupling of the recombination of some photo- 

 chemically produced oxidizing and reducing agents begins (4). 

 The result of these various rate changes is an initial increase in 

 the level of ATP. As the rate of CO2 reduction increases, the 

 demand for ATP increases. This results in a decrease in the 

 level of ATP in the chloroplast, perhaps to a value much less 

 than during the dark time, and ATP will begin to flow into the 

 chloroplast from the cellular space outside. In an atmosphere 

 of N2, on the other hand, respiration will cease in the dark, lead- 

 ing to a lower level of ATP in both chloroplast and cytoplasm, 



33 



