30 PROBLEMS IN PHOTOSYNTHESIS 



The sum of these eight reactions is 



2 H2O + ^hv -. O2 + 4 H 



Due to the action of four quanta, one molecule O2 and four enzyme-bound 

 reduction equivalents are produced from two molecules of water. According 

 to the hypothesis, four quanta are required for the production of one molecule 

 O2. The result shows agreement with Warburg's findings. There is, of 

 course, no relationship whatever between the concept of Schenck and that 

 of Warburg, the former being pure hypothesis and the latter based upon 

 experimental work of the highest precision. 



Just as Warburg compares photosynthesis and vision with respect to the 

 carotenoids (§31), Schenck (42) considers the excitation of the optical nerve 

 as a process similar to the photochemical part of photosynthesis. He assumes 

 that the reaction of a carotenoid biradical Car^ with O2, i.e., the formation 

 of the compound Car^ O., must be the first dark reaction in the process of 



vision. 



In spite of the many hypotheses, there is no definite evidence that the 

 chlorophyll molecule undergoes chemical changes during photosynthesis. 

 Using suspensions of isolated spinach chloroplasts, Witt et al. (68) found that 

 red light flashes of 10"^ sec induce changes in the absorption bands of chloro- 

 phyll; this may be considered to be the result of chemical changes in the 

 molecule. 



§ 13 Interactions Between Chlorophyll and Other Pigments 



Schenck (42) considers chlorophyll has no specific action in photosynthesis, 

 the whole process being based upon the combination of chlorophyll with 

 specific enzyme systems. Accordingly, the carotenoids should also be able 

 to combine with such specific enzyme systems and produce similar photolytic 

 arrangements. The carotenoids forming free radicals would thus replace 

 chlorophyll if necessary. Stanier el al. (27) assume that the important role 

 of the carotenoids consists in protecting the cells against photodynamic 

 damage caused by photooxidation of chlorophyll. Anderson and Fuller (1) 

 also found carotenoids in chromatophores of the purple sulfur bacteria to be 

 essential for the protection of chlorophyll against photooxidation. Green 

 carotenoid-less plants may indeed produce enough O2 to cause photodynamic 

 death. Claes (13) described carotenoid-deficient mutants of Chlorella 

 which grew only in the dark and were killed by illumination. If carotenoids, 

 in competition with chlorophyll, interfere in the transfer and change of 

 energy, it can be imagined that, in the absence of carotenoids, the formation 

 of peroxide and the consecutive production of O2 will be much more active 

 and may even be too active. Calvin (11) supports this opinion. 



Kandler and Schotz (28) observed photooxidation of chlorophyll also in 

 higher plants {Oenothera) containing great amounts of carotenoids. It is ol 

 importance to note that in Chlorella blue light is much more active than red 



