ENERGY-RICH PHOSPHATE BONDS 



.H,0 



33 



2 O2 



AMP 



Sugar phosphates 



Starch 



Fig. 14. Scheme for photosynthesis by isolated chloroplasts. Photolysis of water (center) 



leading either to ATP synthesis and the reconstitution of water (right) or to CO2 reduction 



(below) linked to oxygen evolution (left). 



enzymes of photosynthetic phosphorylation and the enzymes of reductive carboxylation 

 is shown in Fig. 14. In broken chloroplast preparations, CO2 fixation is greatly stimulated 

 by the addition oi pyridine nucleotides, ATP, phosphorylated sugars, and a soluble enzyme 

 system which is obtained by treating intact chloroplasts with water (Whatley et al., 1956). 



The components of the chloroplast phosphorylation system have not as yet been fully 

 established. However, it is known that flavin mononucleotide, ascorbate, Mg^"^, and 

 vitamin K, when added in catalytic amounts, greatly stimulate photosynthetic phosphory- 

 lation. Possibly, the cytochrome pigments also participate in the process (Arnon, 1955). 

 TPN^ is a component of the chloroplast phosphorylation system. 



Photosynthetic phosphorylation may be distinguished from oxidative phosphorylation in the 

 following respects: 



i) Oxidative phosphorylation occurs in mitochondria in the dark; photosynthetic 

 phosphorylation takes place in illuminated chloroplasts. 



2) The latter process is anaerobic whereas the former occurs in an oxygen atmosphere 

 and oxygen is consumed in the process. 



It is to be emphasized that aerobic phosphorylation also takes place in plant tissues and 

 is indispensable for the maintenance of cellular function during periods of darkness. 



Indeed, a light dependent reduction of DPN^ or TPN"^ by plant chloroplasts can be 

 coupled with the reoxidation of the DPNH2 (TPNH2) by plant mitochondria with the 

 resulting generation of ATP (Vishniac and Ochoa, 1952) or with other reductive processes 

 of plant cells (Jagendorf, 1956). 



4. The storage of high-energy phosphate bonds 



ATP generated by the mechanisms discussed above may react with creatine 

 in the presence of the enzyme, creatine kinase: 



i) Creatine + ATP 



Literature p. 124 



creatine phosphate + ADP 



text continued on p. 37 



