370 



Photosynthesis /20 : 5 



steps at which ATP and PNH are used are the ones utilizing free energy, 

 ultimately derived from the absorption of visible light. Each PN + 

 reduction requires about 8 kcal/mole. The cycle of Figure 4 uses two 



Fructose-6-P 

 H 2 COH 



HOCH 



Glucose-6-P 

 HC=0 



Starch 



A polymer of 



glucose 



Sucrose 

 Figure 5. The conversion of fructose-6-phosphate to sucrose and starch. 



reduced pyridine nucleotides and three ATP's per molecule of C0 2 . 

 Thus, the total free energy required for this scheme is about 135 kcal/mole 

 of C0 2 , just a little higher than the 116 kcal/mole necessary to form 

 hexose. The pigments responsible for this conversion are described in 

 the next section; their operation and efficiency are discussed in 

 Section 6. 



5. The Photosynthetic Pigments 



The grana within the chloroplasts contain the pigments responsible for 

 the absorption of light and its conversion into the forms useful for 

 photosynthesis. These pigments may be grouped in three classes: the 

 chlorophylls, the carotenoids, and the phycobilins. All photosynthetic 

 cells contain chlorophyll. Carotenoids are likewise found in grana from 



