THE MECHANISM OF PHOTOSYNTHESIS 321 



the continuous supply of reducing power by the action of the Hght and 

 the "tendency to eUminate oxygen and oxidizing products" cause a shift 

 of the redox potential, which promotes another type of metabolism also 

 in parts of the system not directly affected by light. In this respect 

 direct observations on the connection between photosynthesis and redox 

 potentials (see later) are of interest. 



All results obtained so far by using carbon isotopes in the study of 

 photosynthesis fit very well into the general concept developed in the 

 preceding sections. The chain of photosynthesis appears to consist in a 

 transfer of hydrogen atoms (or eventually of electrons, or of "reducing 

 power") from an ultimate hydrogen donor (water in the case of green- 

 plant cells) to an ultimate acceptor (carbon dioxide). One of the links 

 in this chain, obviously the basic or most important one — whether it is 

 one single step or a more complex mechanism — requires light energy in 

 order to work. 



The present writer and his collaborators concluded: "We may visu- 

 alize that, in the dark, the various links of the process are already being 

 prepared with the aid of energy available in the structural elements of 

 the cell. The role of the light energy is to make a chain of these links" 

 (Dorrestein et al, 1942, p. 367). 



4-4. EXPERIMENTS ON ISOLATED CHLOROPLASTS 



The study of isolated chloroplasts, which is discussed in Chap. 6, is 

 only briefly considered here in its bearing on the general mechanism of 

 photosynthesis. Isolated systems such as chloroplasts and grana have 

 so far been found incapable of photochemical carbon dioxide reduction, 

 but with a variety of hydrogen acceptors other than carbon dioxide, a 

 photochemical evolution of oxygen may occur (Holt and French, 1949). 

 Only the very simple reaction discovered by Warburg and Llittgens 

 (1944), 



O OH 



+ H2O 



+ 0, 



O OH 



will be mentioned here. 



At first the view was held that the process occurring in the illuminated 

 chloroplast preparations was a direct photochemical splitting of water 

 attached to the photosynthetic apparatus. Some observations, however, 

 such as light-saturation phenomena (Holt and French, 1949), which indi- 

 cate the participation of at least one dark reaction, afford evidence that a 

 more complicated mechanism is in play. Warburg and Liittgens (1944), 



