J. A. BASSHAM AND M. CALVIN 



whether the number of ATP molecules formed per RH2 burned 

 is 1, 2, 3, or 4, respectively. 



The controversy regarding the minimal requirement of 

 photosynthesis has not been settled. The recent experiments 

 reported by Warburg et al. (60) are very convincing, since 

 quantum yields of four and even three are reported with high 

 light intensities for long periods of time and without corrections 

 for respiration, thus effectively answering criticisms based on the 

 possibility that the reported quantum requirements are due to 

 contribution of respiratory energy to photosynthesis. 



This leaves only criticisms based on the evaluation of the 

 manometric technique. No such evaluation will be attempted 

 here, but it may be worth while to point out one possible diffi- 

 culty. Calculations of oxygen evolution and carbon dioxide 

 uptake by the two-vessel method depend on the assumption of 

 constant solubilities of these gases. However, the solubility of 

 carbon dioxide may change significantly if the j&H of the medium 

 changes, and this in turn could be influenced by the secretion of 

 acid by the algae. For example, it has been observed in this 

 laboratory that in high light intensities, algae produce glycolic 

 acid. Tolbert (N. E. Tolbert, private communication) has 

 found that glycolic acid formed in strong light by algae is secreted 

 into the medium. One might speculate that perhaps blue light 

 might activate some acid-secreting enzyme system, though there 

 is at present no evidence for this. 



In view of this and many other difficulties inherent in the 

 manometric determination of quantum yields, it has seemed 

 desirable to try other methods of measuring oxygen liberation 

 for quantum requirement calculations. One such study is that 

 of Brackett et al. (8), who used a polarographic determination of 

 oxygen and calculated quantum requirement as low as six. A 

 relatively simple and straightforward experiment has now been 

 carried out using an oxygen analyzer employing paramagnetic 

 measurement of oxygen (5). 



A suspension of algae was placed in a thin plastic cell of 

 large area. A mixture of 4% COo was passed through this 



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