26 PHOTOSYNTHESIS 



chlorophyll increased. For a certain range of light inten- 

 sities one molecule of oxygen is consumed under the best 

 conditions for each light quantum absorbed by the system; 

 thus in recent years Warburg has used this reaction as a light 

 meter. Protoporphyrin will also sensitize the reaction al- 

 though it may be noted that unlike its magnesium com- 

 pound or chlorophyll it does not luminesce when heated in 

 organic solutions. Porphyrins if present in the blood are 

 well known to sensitize animals to light, and this may even 

 result in the death of the organism. In vitro, the addition of 

 porphyrin causes the serum of an animal to take up oxygen 

 in the light. The light dependent oxidation of certain com- 

 pounds in organic solvents gives rise to peroxides; isoamyl- 

 amine, for example, gives an amine peroxide when illumin- 

 ated with chlorophyll in presence of oxygen. 



In the second class, the photosensitized oxidoreductions, 

 the best known reaction is the reduction of methyl red by 

 phenylhydrazine. In reactions involving photosensitization 

 by dyes in which the dye substance is itself a reactant it is 

 generally found that absorption of light increases the tend- 

 ency to accept hydrogen. When the reaction is photosensi- 

 tized by the presence of another pigment, as for example 

 chlorophyll, it may not be necessary to assume that the 

 photosensitizer directly takes part in the transfer of hydro- 

 gen. It may simply replace the direct absorption of light by 

 the reactant. The problem as to how the energy is then 

 transmitted to the reactants, especially when complex 

 organic molecules are involved, has not yet been solved. 



In autoxidation reactions such as that of thiourea no 

 observable quenching of the chlorophyll fluorescence 

 is observed. On the other hand in the oxidoreduction re- 

 action both methyl red and phenyl hydrazine independently 

 quench the fluorescence of chlorophyll. 



5. Reversible bleaching of chlorophyll solutions. When solu- 

 tions of chlorophyll or chlorophyllides are strongly illumin- 

 ated in an organic solvent in the absence of oxygen Porrett 

 and Rabinowitch (1937) showed that the absorption coeffi- 

 cient is reversibly lowered by a small amount. The effect is 

 far too small to be observed as a fading of the colour. Oxygen 



