MECHANISM OF PHOTOSYNTHESIS I3I 



of how the absorption of light energy and its conversion 

 into a chemical form is to be explained in terms of the 

 details of molecular structure in the photocatalytic system. 

 Here the principles are largely derived from the successful 

 application of quantum theory to atoms and to very simple 

 molecules. More recently however the theory has been 

 applied to certain types of organic substances with con- 

 siderable success. 



The discovery of the 'triplet state' in the photochemistry 

 of fluorescein by G. N. Lewis and Kasha has had an 

 important bearing on theory in this branch of photo- 

 synthesis. The triplet state of a molecule differs from the 

 fluorescent state in giving a more delayed light emission, 

 known as phosphorescence. The process is not influenced 

 by temperature in the same way as is fluorescence. Chemi- 

 cally a molecule in the triplet state should have the pro- 

 perties of a biradical and thus exhibit a definite para- 

 magnetic property. This was proved in the case of fluor- 

 escein and further the triplet state could be recognized by a 

 characteristic absorption spectrum. In passing from the 

 fluorescent state of the molecule to a triplet state some 

 energy has to be lost, and with fluorescein the energy level 

 is appreciably below that of the fluorescent state. So far 

 evidence for the existence of triplet states has been obtained 

 with only a limited number of organic compounds. Hence 

 whilst the existence of the triplet state would seem to offer 

 the key to determining the mechanism of conversion of light 

 into chemical energy we have as yet no conclusive evidence 

 that such a state is possible in the case of chlorophyll a. 

 Even so the conception of a 'biradical' formed from a mole- 

 cule consequent on light absorption is helpful in considering 

 the impact of physical theory upon our more specialized 

 biochemical field. Such a biradical would have both oxidiz- 

 ing and reducing properties. Thus perhaps it could easily 

 lose the extra energy by chemical reaction with both an 

 electron donor and an electron acceptor. Also the molecule 

 might react with itself to give two products the reduced and 

 the oxidized form of the dye. Hence we may see how a 

 general postulate has come to be accepted by many: that the 



