PHOTOSYNTHESIS 



energy transfer reactions. We shall return to a discussion of the 

 energy requirements for these other processes later. 



The relation between photosynthesis and respiration is 

 shown in Figure 2, where respiration is indicated by dotted lines 

 and photosynthesis by solid lines. The points at which reducing 

 agents and ATP are utilized in photosynthesis or produced in 

 respiration are indicated. 



It can be seen that the path of carbon reduction in photo- 

 synthesis is far from a simple reversal of its path in respiration. 

 However, the conversion of PGA to hexose in photosynthesis and 

 the reverse in respiration appear to follow siinilar if not identical 

 paths. Moreover, some of the pentose cycle path seems to have 

 some steps in common with photosynthesis. 



THE LIGHT REACTION 



In considering the most characteristic reaction of photo- 

 synthesis, the light reaction, it is necessary to keep in mind the 

 physical arrangement of the chloroplast structure as it is now 

 thought to exist. The fine structure of the chloroplast (or of the 

 grana into which some chloroplasts seem to be divided) is 

 believed to be laminar, with very thin, perhaps monomolecular 

 layers of chlorophyll alternating with thicker layers of proteins 

 and lipoproteins. Although the organization and thickness of 

 these layers seem to vary with the species, particularly in some 

 algae as compared with higher plants, it seems likely that the 

 electrochemical fields that exist at the chlorophyll-protein and 

 lipoprotein interfaces are similar in all cases. Thomas et al. (56) 

 have recently studied the Hill reaction in particles of sublaminar 

 size. Measurement of oxygen evolution as a function of the 

 particle size showed an ability of particles as small as 10^ A^ in 

 volume to carry out the Hill reaction with about 50% the specific 

 activity of intact grana, but a rapid decrease in such ability with 

 smaller particles, and no activity with particles with volumes of 

 less than 2 X 10^ A^ at a given light intensity. However, higher 

 light intensities resulted in Hill reaction activity with even 

 smaller particles, so it was concluded that if there is a physical 



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