PHOTOSYNTHESIS 



energy requirements of photosynthesis, that a minimum of 160,000 

 calories is needed per mole of carbon dioxide. Ten to twelve light 

 quanta (let alone eight) transformed into phosphate bonds at 12,000 

 calories each would mean that the latter would have to be used with 

 120% efficiency. Actually, the efficiency with which the energy-rich 

 phosphate bond can be used for synthetic reactions appears to be around 

 60%. If we postulate two phosphate bonds per quantum (that is, 

 twenty bonds altogether), tliere would be enough energy. However, 

 a complicated mechanism must be provided to divide the energy of 

 the excited molecule between the two phosphate bonds. Finally, the 

 resulting phosphorylated compounds should be stable enough to 

 survive the end of an illumination period and cause the reduction 

 of carbon dioxide for some time afterward in the dark. As said 

 above, nothing of that kind has ever been observed, though many 

 an investigator has looked for it. Hence, it is simpler to assume that 

 Nature makes use of the particular advantages inherent in photo- 

 chemical reactions and produces intermediate hydrogen donors which 

 are capable of a one-step gain in free energy larger than those possible 

 by way of energy-rich phosphate bonds. 



A question quite different from that discussed is whether carbon 

 dioxide becomes reduced to carbohydrate in the form of a phos- 

 phorylated compound. It is known that plant constituents combining 

 with carbon dioxide in the dark are half saturated at a carbon dioxide 

 partial pressure of less than 0.1 mm. mercury. The nature of the 

 very first fixation of carbon dioxide preceding the reaction with excited 

 chlorophyll is still under investigation. Suggestions like "reversal of 

 a decarboxylation" are no more helpful than the dictum that the 

 liberation of oxygen is "the reversal of respiration." What is needed 

 is a working model fulfilling the energy requirements. A carboxyla- 

 tion reaction as effective as that taking place in the first step of photo- 

 synthesis needs the coupling with a reaction releasing some free energy 

 {ca. 10,000 cal.). Ruben (15) suggests that the carboxylation occurs 

 with the aid of an energy-rich phosphate bond present in the molecule 

 which takes up carbon dioxide. Lipmann (11) recently has shown 

 that, in the presence of certain enzyme preparations, acetyl phosphate, 

 carbon dioxide, and molecular hydrogen can condense to pyruvate. 

 By proposing that the pyruvate is removed quickly through further 

 reduction and a new energy-rich phosphate furnished by an oxida- 



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