ENERGY EFFICIENCY IN PHOTOSYNTHESIS 263 



If 4 photons are required to produce the reaction of one molecule, <I> = 3^^. 

 In photosynthesis it has become the habit to refer to the reciprocal of 

 the quantum yield: 



1 , No. of photons absorbed 



$ No. of molecules in chemical change 



This ratio has been given the name quantum requirement. Thus, if the 

 quantum yield is 0.1 molecule per photon, the quantum requirement is 

 10 photons per molecule. 



The energy efficiency in photosynthesis is defined as follows: 



theoretical minimum energy 

 Energy efficiency = experimentally determined energy" 



If 3 photons of red light should ever be found capable of producing a 

 molecule of photosynthetic product, the energy efficiency would be equal 

 to 112,000/(3 X 40,500), or 0.92. If 3 photons of blue light at 4000 A 

 is required, the energy efficiency is 112,000/(3 X 71,500), or 0.52. 



If 4 photons of red light at 7000 A is required, the energy efficiency is 

 112,000/(4 X 40,500), or 0.70. And if 8 is required, the efficiency is 0.35. 



3. MEASUREMENT OF ENERGY 



In photochemical research it is desirable to use essentially monochro- 

 matic light because all the photons then have the same amount of energy, 

 the photoreactions are sure to be the same, and the calculations are made 

 easier. Chlorophyll has strong absorption bands in the red and in the 

 blue, but it absorbs also to a limited extent throughout most of the visible 

 spectrum, from 4000 to 7000 A. 



It happens that in photosynthesis the quantum yield is about the same 

 in blue, green, yellow, and red light, but it is desirable, nevertheless, to 

 use monochromatic light. A discontinuous source of fight, such as a 

 mercury-vapor lamp (Daniels et al., 1949; Rabinowitch, 1951) or a cad- 

 mium arc combined with a prism or grating, gives the best source of light. 

 Light filters (Daniels et al, 1949) of colored glass or interference filters of 

 thinly deposited metals can be used. Red fight, with its lower energy 

 per photon, is used most commonly because with it greater energy effi- 

 ciency can be expected. However, metallic-vapor lamps giving red light 

 are difficult to operate, and most of the work in the red has been done 

 with tungsten lamps and optical monochromators (ibid.) or filters. 



The energy of the light in ergs per second per square centimeter is 

 usually measured with thermopiles or bolometers calibrated against a 

 standard carbon-filament lamp supplied by the U.S. National Bureau of 

 Standards (ibid.). When the surfaces are properly blackened, the bolom- 

 eter or the thermopile gives equal deflections on a sensitive galvanometer 



