CHAPTER 4 



Energy Efficiency in Photosynthesis 



Farrington Daniels 

 Department of Chemistry, University of Wisconsin, Madison 



The process of photosynthesis. Principles of photochemistry. Measurement of energy. 

 Measurement of chemical change in photosynthesis by nonmanometric methods. Meas- 

 urement of chemical change in photosynthesis by manometric methods. Biological factors. 

 Practical applications. Discussion. References. 



What is the theoretical maximum utiHzation of solar energy in growing 

 plants? This is an important question both in the laboratory, where 

 different mechanisms for photosynthesis are proposed, and in the eco- 

 nomic world, where geographical agricultural expansion is reaching its 

 limit and where the exhaustion of fuel reserves is foreseeable. 



Any proposed mechanism for photosynthesis must fall within the 

 energy balance of the observed over-all process. A low maximum 

 efficiency of energy conversion will permit a wide variety of exothermic 

 reaction steps in the photosynthetic process. On the other hand, if the 

 conversion of radiant energy into chemical energy is very high, 70 per 

 cent, for example, only a limited number of exothermic reactions can be 

 fitted into the photosynthetic mechanism. 



Different workers in the field of photosynthesis do not agree concern- 

 ing the maximum efficiency of energy storage in photosynthesis, and it is 

 the purpose of this chapter to examine the various ways of measuring 

 the energy absorbed and the chemical products in photosynthesis. 



1. THE PROCESS OF PHOTOSYNTHESIS 



The primary reaction of photosynthesis is given by the equation 



CO2 + H2O -t- (chlorophyll) + light -> (H2CO) + O2 + (chlorophyll), 



where (H2CO) represents a unit of a carbohydrate associated with a 

 gram-atom of carbon. At the time of this writing (1952) it has not been 

 possible to produce this reaction in the absence of chlorophyll or in any 

 way outside the living plant, but there seems to be no theoretical reason 

 why photosynthesis cannot at some time be accomplished in vitro as well 



as in vivo. 



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