ENERGY EFFICIENCY IN PHOTOSYNTHESIS 261 



Another fundamental law of photochemistry accepted by scientists is 

 that each unit, or photon, of light absorbed can activate one molecule. 

 This one-to-one relation, which was established by Einstein, refers to the 

 number of molecules activated by the light, and not to the number of 

 molecules reacting chemically. A large number of different secondary 

 reactions can follow the primary activation of the absorbing molecule. ^ 



Ordinary chemical reactions obtain the large activation energies that 

 they need from collisions between rapidly moving molecules, and these 

 violent, chemically significant collisions increase greatly with an increase 

 in the temperature of the reacting system. In photoactivation, however, 

 the energy of activation of the primary step comes from hght from an 

 outside source and is not influenced appreciably by the temperature of 

 the reacting system. The secondary, thermally activated reactions can, 

 of course, be changed by changing the temperature. The extent to which 

 an over-all photochemical reaction, or photosynthesis, depends on tem- 

 perature offers a means for distinguishing between the thermal and photo- 

 chemical steps. Inasmuch as photosynthesis involves secondary thermal 

 reactions, better efficiencies are obtained at higher temperatures, pro- 

 vided they are not so high as to destroy some of the biological processes. 



It is helpful to consider the minimum energy requirements for photo- 

 synthesis. The primary reaction of photosynthesis, 



CO2 + H2O -^ (H2CO) + O2 (4-1) 



can be reversed. When a carbohydrate, represented by H2CO, is burned 

 in oxygen to give carbon dioxide and water, heat is evolved, as deter- 

 mined with a combustion calorimeter, amounting to 112,000 cal per mole 

 of 6.02 X 10'-^ molecules. The reverse reaction is written 



(H2CO) +02^ CO2 + H2O -K 112,000 cal. (4-2) 



By a well-known principle of thermochemistry, it is necessary to supply 

 at least as much as the 112,000 cal in order to make the reverse of reaction 

 (4-1) take place. In fact, it will be necessary to supply more than the 

 112,000 cal, because an energy of chemical activation must be supplied 

 in addition to the thermodynamic heat of reaction in order to make the 

 reaction go with measurable speed. Accordingly, if carbohydrates are 

 to be produced from carbon dioxide and water by the absorption of hght, 

 the light must be energetic enough to be the equivalent of more than 

 112,000 cal/mole. 



The energy contained in light is easily calculated from the quantum 

 theory, according to which the energy of one unit of radiation, 1 photon, 



1 These secondary reactions are ordinary thermal reactions governed by the laws of 

 thermodynamics and chemical kinetics. Early workers sometimes misinterpreted 

 Einstein's law of photochemistry to indicate that there should be one molecule of 

 product for each photon absorbed. 



