PHOTOSYNTHESIS 457 



synthesis. These may be characterised as follows: (i) The 

 formation of molecular oxygen ; (2) the dark fixation of 

 CO2 ; (3) the reduction of CO2 as far as carbohydrates ; (4) the 

 synthesis of sugars from phosphotrioses ; (5) the formation 

 of ' active hydrogen ' in the shape of reduced forms of di- 

 and triphosphopyridine nucleotides ; (6) the formation of 

 high-energy bonds (ATP) (Fig. 39). 



We shall now give a very schematic exposition of the work 

 of all these aggregates, using, for the most part, the data 

 published by M. Calvin^" in his address to the Third Inter- 

 national Congress of Biochemistry held in Brussels. 



According to M. Calvin, when light falls on the laminated, 

 chlorophyll-containing aggregate, it splits off electrons. The 

 electrons and the remaining positive holes are quickly shared 

 out over the structure. 



According to A. Krasnovskii, V. Evstigneev and their col- 

 leagues, the photochemical transfer of electrons which 

 underlies the action of chlorophyll occurs by means of an 

 intermediate, reversible photoreduction of the pigment. This 

 supposition is substantially strengthened by the recent 

 observation in living, photosynthesising organisms, of rapid 

 spectral variations corresponding with those which occur 

 during the photoreduction of chlorophyll.^"^ The negative 

 charges (electrons) which are produced in one way or another 

 are used for the reduction of phosphopyridine nucleotides 

 (in system 5), while the positive charges act on water, leading 

 to its oxidation (in system 1). 



The details of the working of system (1) have not yet been 

 fully elucidated but there can be no doubt whatever that 

 the molecular oxygen given off during photosynthesis is 

 derived from water, as was asserted by A. N. Bach (Bakh)^"'^ 

 as early as 1893, and proved experimentally considerably 

 later by A. P. Vinogradov and R. V. Teis^" in their experi- 

 ments with isotopes of oxygen. The nearest thing to extra- 

 cellular photosynthesis is the reaction obtained by Hill, who 

 showed that, in the light, oxygen is split off from water in 

 the chloroplast, but only when the surrounding medium 

 contains such powerful hydrogen acceptors as quinones, 

 organic pigments and ferric salts. This is necessary in order 

 to prevent the reaction from occurring in the reverse direc- 



