PHOTOSYNTHESIS BY ISOLATED CHLOROPLASTS 



297 



Light + ADP + P, 

 Light + H,,0 + CO2 



-^ ATP 



-* (CH2O) + O2 



(2) 

 (3) 



As discussed more extensively elsewhere (3-5) chloroplasts appear 

 in the light of present evidence as remarkably complete and auton- 

 omous cytoplasmic structures which contain all the enzyme systems 

 needed for photosynthesis. These enzymes are divided into three 

 main groups, each controlling an increasingly complex phase of 

 photosynthesis: photolysis of water, photosynthetic phosphorylation, 

 and CO2 fixation. Whole chloroplasts contain all three groups of en- 

 zymes. Chloroplasts broken by treatment with water contain only 



t 



LIGHT 



'^2 0,^1 1 



I 



.H,0 



cytochromes' 



rij'-' oscorbo 



— r" ♦ 



chloroplost VI tK 



I FMN 



-po: 



-AMP 



CO, 



sugor phosphates 



I 



STARCH 



Fig. 1. Scheme for photosynthesis by isolated chloroplasts. Photolysis of water 

 (center) leading either to ATP sj-nthesis and the reconstitution of water (right) 

 or to CO2 reduction (below) linked to oxygen evolution (left) (4). 



two; the group of C02-fixing enzj^mes is leached out. The phosphoryl- 

 ating enzymes, which are not water-soluble, remain bound to the 

 particles, as do the enzymes of photolysis. 



It is visualized that in vivo photolysis is linked (Fig. 1) either with 

 phosphorylation, resulting in the production of ATP and the re- 

 constitution of water, or with CO2 fixation, resulting in the evolution 

 of oxygen and the reduction of CO2 to the level of carbohydrate 

 (ecjuation 3). CO2 fixation requires the participation of all three groups 

 of enzymes, phosphorylation requires two, whereas photolysis of water 

 can proceed without the others provided an artificial hydrogen ac- 

 ceptor is supplied. The last process, the well-known Hill reaction 

 (equation 1), provides a convenient method for measuring the ac- 

 tivity of the enzymes concerned in the photolysis of water, under non- 



