212 PLANT BIOCHEMISTRY 



supposition also suggests the occurrence of a number of reactions with 

 the energy derived from the light divided into suitable parcels. Com- 

 plexity is normally associated with loss of efficiency. Hence there has 

 been interest in learning how much of the light absorbed is converted 

 to chemical energy and how much is lost as heat. Although higher 

 efficiencies may be attained by cells under special conditions, 25 to 30 

 per cent seems to represent a more normal conversion of light to 

 chemical energy. This value represents the ideal of conversion while 

 the factor under field conditions is much smaller, in the neighborhood 

 of 1 per cent. In the field much of the incident light is not absorbed 

 by the plants and much is absorbed at intensities beyond the maximum 

 rate of photosynthesis and is thus lost as heat. On cloudy days effi- 

 ciency is higher than in full sun, but the total photosynthetic product 

 of plants actually may be reduced because the lower sheltered leaves 

 do not receive enough light to permit operation at full chemical 

 capacity. 



Fixation of Carbon Dioxide 



There is more concrete information about the fixation and reduction 

 of carbon dioxide than about any other aspect of photosynthesis. The 

 overall reaction 



CO2 + 2AH2 -^ (GH2O) + 2A + H2O 



is known to rc(juire several steps, and there are certainly others still 

 unknown. The present conception of the reaction pathway coupled 

 to photosynthesis is represented schematically in Figure 9-3. 



Ribulose-l,5-diphosphate is believed to be the carbon dioxide 

 acceptor and to go through the six-carbon acid to two molecules of 

 3-phosphoglycerate. The next step, transformation to 3-phospho- 

 glyceraldehyde, requires energy in the form of TPNH. This high- 

 energy compound and the ATP needed for the formation of the 

 ribulose-l,5-diphosphate presumably arise from energy transfers follow- 

 ing the absorption of light. Although the details of the relationship 

 of ATP and TPNH to chlorophyll a are unknown, the energy from 

 chlorophyll functions by way of these cofactors. 



The role of the erythrose-4-phosphate is inferred from the other 

 components of the reactions, but the actual participation of this par- 

 ticular compound has not been established in cells. Further work 

 will probably aher this scheme somewhat by deleting or adding steps 

 with the possibility of specific systems for different species. It is 

 unlikely that the ultimate pathway will be simpler. Probably it will 

 be more complex. 



