550 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



by Timiriazev), but should be restored to its original value immediately 

 after the cessation of illumination. In other words, chlorophyll in 

 plants should behave in the same way as dissolved chlorophyll did in 

 the experiments of Porret and Rabinowitch and of Livingston, described 

 in chapter 18 (pages 486 et seq.). 



As stated previously, the extent of such reversible changes in illumi- 

 nated systems depends on the relative rates of two reactions — the one 

 causing the change, and the other restoring the original state. 



If chlorophyll is changed reversibly during photosynthesis, the 

 quantum yield of its change must be at least equal to that of photo- 

 synthesis, that is, it must approach unity at low light intensities, and 

 decline to about 0.1 in strong light. In direct sunlight, each chlorophyll 

 molecule may absorb light ten times each second, and thus must be 

 changed at least once in a second. Assuming that it stays in the changed 

 state for t seconds before being restored by a back reaction, the proportion 

 of changed molecules present in the stationary state will be r (as long as 

 T <$C 1 second) . If the back reaction is nonphotochemical, r can have 

 almost any value imaginable, depending on the concentration of the 

 molecules participating in the back reaction and on its energy of activa- 

 tion. If T is very small, (e. g., < 0.001 second), it is quite possible for 

 chlorophyll molecules to be chemically changed every time they initiate 

 photosynthesis, and to be restored to the original state with such prompt- 

 ness that only one chlorophyll molecule in a thousand or more will be 

 present in the changed (and probably discolored) state at any time, even 

 in the most intense light. 



We have deduced, on page 546, from the concentration dependence 

 of the rate of photoxidation, that the lifetime of the ''long-lived" activa- 

 tion state of chlorophyll in vivo is much shorter than in organic solutions. 

 Thus, the reversible bleaching of chlorophyll in vivo is likely to be much 

 weaker than the effects observed by Porret and Rabinowitch and by 

 Livingston in chlorophyll solutions in methanol. 



The situation changes if we assume that the back reaction too is 

 photochemical, i. e., that chlorophyll uses photons, first for a (direct or 

 indirect) reduction of {CO2} (oxidizing itself in this process), and then 

 in the oxidized state, for a (direct or indirect) oxidation of water and its 

 own reduction (as was assumed in the theory of Franck and Herzfeld 

 1941). Under these conditions, a stationary state of photosynthesis can 

 be maintained only if the two photochemical reactions proceed with 

 equal velocity. If we assume that the two modifications of chlorophyll 

 are (about) equally intensely colored, and react with (approximately) 

 the same quantum yield, an equality of the two reaction rates is only 

 possible if the two forms are present, in the stationary state, in approxi- 

 mately equal quantities. 



