J. A. BASSHAM AND M. CALVIN 



position of water by this energy, and the formation of oxygen 

 and reducing power are all rather resistant to inactivation, 

 provided relatively simple buffer solutions are employed in the 

 isolation of the chloroplasts. This seems to indicate the presence 

 of an organized enzymatic apparatus, perhaps with microscopi- 

 cally large and intricate structure and with all necessary cofactors 

 and prosthetic groups firmly attached. Such a view is con- 

 sistent with the apparent structure of the grana and lamellae as 

 revealed by electron microscope studies (40,57). 



The carbon reduction cycle, however, must be easily 

 inactivated or separated during preparation of isolated chloro- 

 plasts. This may indicate that the enzymes involved in this cycle 

 are not part of an organized structure, except insofar as the 

 chloroplast structure tends to enclose these enzymes and retain 

 within itself a space with high reducing potential. 



It seems likely that this reducing power does not diffuse out 

 of the chloroplast. If it did, one might expect greater inhibition 

 of respiration in the light than is actually observed in cells which 

 contain chloroplasts (12,31). It is significant that inhibition of 

 respiration during photosynthesis is most pronounced in some 

 organisms which do not contain organized chloroplasts (11,13) 

 and this fact again indicates that one role of the chloroplast 

 structure may be to retain reducing power at a high level for 

 photosynthesis. Another bit of evidence relating to this possi- 

 bility is the observation that the conversion of photosynthetic 

 intermediates to respiratory intermediates is inhibited in the 

 light (18). This might be attribtued to the reducing condition 

 within the chloroplast. In this case, the inhibition is believed to 

 be due to the reduction of a specific cofactor rather than to a 

 general reduction of metabolic intermediates which might other- 

 wise enter the oxidative pathway. 



The mechanism by which the chloroplast might retain the 

 reducing power is not entirely clear, since such carriers of 

 reducing power as TPNH might be expected to diffuse out of the 

 chloroplast rather freely. One possibility is that the primary 

 carrier of the reducing power is itself a protein complex resistant 



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