20 : 3/ Photosynthesis 



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then the ADP available will increase, thereby speeding up the rate of 

 phosphorylation. Likewise, if phosphorylation proceeds more rapidly 

 for a time, then the ADP supply will be depleted and the rate of phos- 

 phorylation decreased. This is a simple example of the action of nega- 

 tive feedback on a chemical scale serving to keep the two processes in 

 step. 



Input 



Output 



One 2 

 liberated 

 per CO 2 

 used 



Net Loss: 

 One H 2 

 per C0 2 

 used 



Net Gain: 

 One hexose 

 per 6 C0 2 

 used 



Figure 3. Block diagram of reactions within chloroplast. The 

 brackets around the H and O indicate that these do not imply 

 molecular or atomic hydrogen and oxygen, but rather reducing 

 and oxidizing compounds. Considerable evidence indicates 

 there is a flavin mononucleotide intermediate between (H) and 

 pyridine nucleotide. The circle for the carbon pathways and 

 the square for the phosphorylating chain are purely diagram- 

 matic. The carbon pathway is discussed in more detail in 

 Section 4. There is not an instantaneous balance between 

 C0 2 fixed and 2 released. A feedback mechanism, con- 

 trolling the rate of phosphorylation, assures that over a period 

 of time the number of moles of 2 released is equal to the 

 number of moles of G0 2 fixed. 



One may then summarize the action of the chloroplast schematically 

 as shown in Figure 3. This shows H 2 0, C0 2 , and photons being used 

 up, and H 2 0, 2 , and hexose being formed. It emphasizes the three 

 types of reactions catalyzed by the chloroplasts, the splitting of water, 

 the fixation of carbon dioxide, and photosynthetic phosphorylation. 



