THE MECHANISM OF PHOTOSYNTHESIS 



333 



in a solution. He concluded that for chlorophyll a solutions, with a 

 mean distance between neighboring molecules of 80 A, transfer of energy 

 has the same probability as annihilation. The concentration for which 

 this holds is about a hundred times smaller than that in the chloroplasts. 

 This would mean that energy transfer over 10^ molecules is feasible. 

 Forster (1947, p. 177) considers the way of transport of energy as an 

 "exciton" that goes from one molecule to another like a material cor- 

 puscule in three-dimensional Brownian movement. It follows from his 

 computations that this type of energy transport would permit rather 

 large-distance transport of energy 

 in the chloroplasts. It does not, 

 however, exclude energy transport 

 through the protein. In photosyn- 

 thesis this has not yet been directly 

 demonstrated, but for photolysis of 

 carbon monoxide myoglobin, Biicher 

 and Kaspers (1946, 1947) have dem- 

 onstrated that light absorbed by 

 the protein has the same efficiency 

 as that absorbed by the pigment 

 (Fig. 5-20). 



The problem is to find out which 

 of the various possible ways of en- 

 ergy transport actually is followed. 

 Perhaps there are different ones 

 operating simultaneously, and it is 

 tempting to conclude this discus- 

 sion with the ciuotation of a view, 

 expressed by Engelmann in 1883: 

 " . . . . dass man es innerhalb 

 j edes Chromophyllkorpers mit einer 



260 



300 340 



WAVE LENGTH , 



380 



nil 



Fig. 5-20. Absorption spectrum of CO 

 myoglobin (solid line) and of its hemin 

 component (dotted line), and relative 

 photochemical activity of some wave 

 lengths in splitting off CO (crosses). 

 (From Biicher and Kaspers, 1946.) 



molekularen Mischung zu thun 

 (hat), d.h. die Farbstofftheilchen sind so innig und gleichmassig mit 

 den farblosen Stromatheilchen des Chromophyllkorpers vermengt, dass 

 allerorts zwischen beiden Molekularwirkungen stattfinden konnen." 



SUMMARY 



Photosynthesis is essentially a light-sensitized hydrogen transfer from 

 a hydrogen donor to carbon dioxide. It seems advisable to specify photo- 

 synthesis as carbon dioxide photosynthesis, since experiments both with 

 cell-free pigment-protein preparations and with intact cells have demon- 

 strated that carbon dioxide may be replaced by certain other substances. 

 The ultimate hydrogen donor for green-plant-cell photosynthesis is water, 



