48 



OVER-ALL REACTION OF PHOTOSYNTHESIS 



CHAP. 3 



reduction (cf. Chapter 9). Table 3. IV shows how rapidly the heats of 

 combustion rise with the progress of reduction. Photosynthesis lifts the 

 stable "food" of the plants, CO2 + H2O, to the carbohydrate level, as 

 indicated by the arrow on the left side of table 3. IV. Starting from this 



Table 3.IV 

 The Four Reduction Levels of Carbon Dioxide 



level, the plants produce compounds whose energy content is higher 

 than that of the carbohydrates (e. g., alcohols and fats) without fur- 

 ther supply of external energy, by dismutations, that is, reactions in 

 which one part of the carbohydrates is oxidized enabling another part to 

 be reduced. 



If formaldehyde were the first product of photosynthesis (as suggested 

 by Baeyer in 1870), the heat effect of this process would be close to 135 

 kcal per gram atom of assimilated carbon. However, the "formaldehyde 

 hypothesis" has never been proved and is now considered improbable 

 {cf. Chapter 10). Whether photosynthesis involves the formation of 

 another reduction intermediate with an energy content as high as that 

 of formaldehyde is unknown. The same can be said of the often pos- 

 tulated formation of a peroxide as precursor of free oxygen {cf. Chapter 11), 

 which would add approximately another 45 kcal to the chemical energy 

 accumulated in the first stage of photosynthesis. If both an unstable 

 aldehyde and an unstable peroxide were among the immediate products 

 of photosynthesis, the true heat effect of this process would be as high 

 as 180 kcal per mole of reduced carbon dioxide. However, by the time 

 the synthesis reaches an analytically recognizable stage — that of sugar 

 and oxygen — the energy accumulation has been stabilized at about 112 

 kcal per mole. As shown by table 3.V, this value does not depend greatly 

 on the exact nature ofithel^firstlsugar." Formaldehyde is less stable 

 by about 23 kcal than an HCOH link in a long-chain carbohydrate; 

 when we pass from this "monose" to a "biose" (glycolaldehyde), and 

 further to "trioses" (for example, glyceraldehyde), tetroses, pentoses and 



