366 Photosynthesis /20 : 3 



DPN, or triphosphopyridine nucleotide, TPN. The structure of DPN 

 is shown in Chapter 18. 



If chloroplasts are separated from other cellular debris and washed 

 with water, they lose the ability to fix C0 2 but can still split water 

 according to the scheme 



nhv + A + H 2 chloroplast ^ AH 2 + \0 2 (6) 



where A may be any of a wide variety of substances which can be 

 oxidized. This is known as the Hill reaction. Because it is easy to 

 demonstrate, it was used for many years to study the activity of chloro- 

 plast preparations. At one time, it was believed that the Hill reaction 

 was just a fluke or an indication of an improperly functioning chloro- 

 plast. When A is oxidized pyridine nucleotide, the Hill reaction is 

 currently regarded as an essential part of photosynthesis. 



C. Photosynthetic Phosphorylation 



Besides the energy used to split H 2 0, additional energy is necessary 

 to convert C0 2 into hexose. This energy is supplied by the splitting 

 of the coenzyme ATP (adenosine triphosphate) to ADP and (P) , 

 (adenosine diphosphate and inorganic phosphate). As mentioned in 

 Chapter 18, many oxidations lead to the formation of ATP, the over-all 

 process being called oxidative phosphorylation. 



It would seem possible that under some circumstances the ATP 

 necessary to form hexoses could come from respiratory oxidation. As 

 will be discussed further, this appears to be the case. However, most 

 of the ATP comes from the chloroplasts themselves. They catalyze 

 phosphorylation (that is, the formation of ATP from ADP and (P)) in 

 the presence of light, even if no molecular 2 is present. By analogy 

 with oxidative phosphorylation, this last process is called photosynthetic 

 phosphorylation. 



Photosynthetic phosphorylation differs from oxidative phosphoryla- 

 tion in that it does not involve molecular oxygen. However, there are 

 a number of cytochromes as well as flavins and pyridine nucleotides 

 within the chloroplast. Photosynthetic phosphorylation does involve 

 a series of oxidations and reductions. The initial step may be regarded 

 as the formation of two separate compounds which can serve as the 

 oxidized [OH] and reduced [H] ends of the phosphorylating chain. 



To recapitulate, the chloroplast not only catalyzes the splitting of 

 water to form the reduced compounds necessary for C0 2 fixation but 

 also effectively splits water to drive the photosynthetic phosphorylation 

 chain. If the latter is limited by the ADP available, then the two uses 

 of "split water" must keep in step. For if the C0 2 fixation goes faster, 



