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synthesis involving as many as eight hypothetical steps. Although 

 completely valueless today, these models emphasized the complexity of 

 photosynthesis and inspired the experiments which led to the unraveling 

 of the carbon cycle. Another interest in photosynthetic efficiency has 

 been to predict the maximum yield obtainable and hence the maximum 

 number of living organisms (humans) that can be sustained on earth. 

 Perhaps most important of all, describing photosynthesis in terms of 

 its over-all energy efficiency is using the language which appeals to the 

 physicist and the chemist. 



In a previous section, it was noted that to convert 1 mole of C0 2 to 

 hexose, 1 1 7 kcal of free energy were needed. With the scheme shown 

 in Figure 4, 135 kcal of free energy were necessary. The last number 

 would represent 3 red photons per C0 2 molecule or 2 blue ones. The 

 various numbers of photons per C0 2 molecule, observed experimentally, 

 have varied from 1 to 1 3 . Most recent values for prolonged photosynthesis 

 range from 4 to 8 photons per C0 2 ; that is, 30-60 per cent efficiency. 



These measurements have been plagued by a variety of errors. Many 

 of the earlier determinations were based on manometry and short 

 experiments, although the manometers could not respond sufficiently 

 rapidly to make this meaningful. Also, it was necessary to know the 

 respiration in the light, which was determined eventually by means of 

 tracer techniques. The tracer techniques used the stable isotopes 

 Q18Q16 m t h e g as phase and H 2 16 in the liquid. By following the 

 uptake of O 18 , it was shown that the rate of respiration was the same in 

 the light as it had been in the dark. However, after rapid photosyn- 

 thesis, the respiratory rate increased in the dark owing presumably to 

 the ease of oxidizability of some of the photosynthetic intermediates 

 shown in Figure 4. 



Taking account of respiration, measuring C0 2 in terms of its infra- 

 red absorption and 2 in terms of its magnetic moment (with a Pauling 

 oxygen meter), and measuring light intensity with a bolometer gave 

 values for photosynthetic efficiency which bridged the gap between the 

 various other investigators whose results appear to conflict. Data 

 obtained in this fashion for very short flashes gave apparent efficiencies 

 of 200-300 per cent. (This can be interpreted to indicate that ATP 

 and PNH produced outside the chloroplasts can be used for short periods 

 to assist photosynthesis.) At low light intensities, a sustained rate of 

 about 4 photons per C0 2 molecule (that is, about 60 per cent efficiency) 

 was observed. At very high light intensity, a rate of 7.4 photons per 

 C0 2 molecule (that is, about 30 per cent efficiency) was indicated. 



This last variation with intensity is in accord with the model of the 

 excitation energy of the pigment molecules being transferred to charge 

 separation and the charges being trapped (or held as stable free radicals) 



