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at certain sites. At low intensities, the probability of the sites being 

 available is high and hence, more of the absorbed energy should be 

 available for photosynthesis. In contrast, at higher intensities fewer 

 trapping sites would be available and more of the absorbed energy 

 would be converted to heat. 



The models presented m this chapter, particularly those in Figures 3 

 and 4, suggest that one should be able to do many kinetic experiments 

 and determine many different types of rates. This is indeed the case. 

 Each experiment must, if these models are valid, involve the interplay 

 of several rate constants and concentrations. The results are somewhat 

 frustrating in that no one has really succeeded in disentangling the 

 various constants. Such steps in building a better model of photo- 

 synthesis still lie in the future. 



7. Summary 



Photosynthesis is the trapping of the free energy- of the photons of visible 

 light, converting the energy into stable chemical forms. The process of 

 photosynthesis makes life as it exists on earth possible, both by pro- 

 ducing carbohydrates, the ultimate source of food energy for almost all 

 organisms, and also by liberating molecular oxygen into the atmosphere. 

 Photosynthesis is catalyzed by all green plants, by the green protozoan, 

 euglena, by the blue-green algae, and by a variety of pigmented bacteria. 



In all of the higher forms, photosynthesis is catalyzed by intracellular 

 organelles called chloroplasts. Within the chloroplasts there are smaller 

 organelles called grana which contain the pigments necessary for photo- 

 synthesis. The reactions can be divided for convenience into three 

 parts: (a) a light reaction or quantum conversion which occurs in the 

 grana and leads to the dissociation of H s O ; (b) the phosphorylation of 

 ADP to ATP ; and (c) C0 2 conversion to carbohydrate. Of these, only 

 the last is understood in detail. C0 2 fixation and conversion to hexose 

 can occur in all types of tissues, although it follows somewhat different 

 pathways. Similarly, phosphorylation is a concomitant of oxidation in 

 known living cells. The light reaction, however, is unique to photosynthesis. 



In the light reaction, the incoming photon is first absorbed by any 

 of a variety of pigments in the grana, including chlorophyll a, chloro- 

 phyll b, carotenoid pigments, and phycobilins. By a mechanism not 

 clearly understood, the electronic excitation can be passed from one 

 molecule to another. This occurs with very high efficiency and accord- 

 ingly must be very rapid. In some fashion, the electronic excitation 

 produces a charge separation, the resulting unpaired electrons being 

 trapped or stored at certain sites where they may be regarded as stable 

 free radicals. These then react to drive the phosphorylation chain of 



