PLANT METABOLISM 395 



The important consideration is that the reactions involve the capture of 

 radiant energy by chlorophyll and use of the energy in the reduction 

 of coenzymes, which can serve in further reductions. These should be 

 viewed as model reactions useful in the study of portions of the photo- 

 synthetic process rather than as proved partial reactions of photosynthesis 

 as they actually take place in normal plants. 



Storage of energy 



The formulation of the photosynthetic reaction (p. 388) indicated that 

 119.6 Cal. were stored for each mole of carbon dioxide reduced to the 

 level of carbohydrate; this is equivalent to 717.6 Cal. per mole of glu- 

 cose formed. These values are in terms of change in free energy (aF, 

 p. 414), when glucose, at molar concentration, is formed in aqueous solu- 

 tion at 25°C. from liquid water and carbon dioxide at a concentration 

 of 0.03 per cent. That the value for AF differs from that given for 

 glucose oxidation in Chap. 16 (683 Cal.) can be attributed to the differ- 

 ence in concentration of reactants and products in photosynthesis and 

 in animal respiration. 



The energy stored in photosynthetic products represents radiant energy 

 which has been captured and converted to chemical energy. Light 

 energy is absorbed or radiated in discrete units, or quanta, which are 

 equal to hv, when h is Planck's constant (6.547 X 10~-^ ergs per sec), 

 and V is the frequency [frequency is calculated by dividing the wave- 

 length of light (cm.) into the velocity of light, 3 X 10^*^ cm. per sec.]. 

 The total amount of energy in 6.06 X 10^^ quanta is called one Einstein. 

 It is evident that the higher the frequency (shorter the wavelength), 

 the greater is the energy of each quantum of light. Einstein's theory 

 of photochemical equivalence indicates that a photochemical reaction 

 is induced when one molecule absorbs on© quantum of light of charac- 

 teristic frequency. The question of the quantum efficiency of photosyn- 

 thesis may be stated as follows: How many quanta are required for the 

 reduction of one molecule of carbon dioxide to the level of carbohydrate? 



In 1922 Warburg and Negelein determined the quantum efficiency of 

 photosynthesis by the alga Chlorella in light of various frequencies. 

 They found that 4 to 5 quanta were required for each molecule of carbon 

 dioxide. The value of 4 quanta was readily accepted, because it was 

 attractive to think that one quantum was required to activate each of 

 the 4 hydrogens necessary to reduce carbon dioxide to the carbohydrate 

 level. Some 16 years later. Manning, Stauffer, Duggar, and Daniels 

 employed a variety of methods but were unable to reproduce the high 

 efficiency reported by Warburg and Negelein. Several other investigators 

 likewise were unsuccessful and found that 8 to 12 quanta were required 

 rather than 4. Recently Warburg has repeated his earlier experiments 



