114 PHOTOSYNTHESIS 



at a rate comparable with that of the maximum rate of photo- 

 synthesis shown in the Hving plant. 



The 'chloroplast reaction' (or 'Hill reaction') is a term used 

 to describe the production of oxygen by isolated chloroplasts 

 or by preparations from them. While this process does not 

 depend directly on carbon dioxide it shares two of the char- 

 acteristic features of photosynthesis: the conversion of light 

 energy into chemical energy, and the appearance of oxygen 

 in molecular form. The oxygen produced is stoichiometric- 

 ally equivalent to the reduction of an added substance. The 

 nature of the added substance, or hydrogen acceptor, will 

 determine the amount of chemical energy accumulated as 

 a result of illumination. The in vitro preparation may be re- 

 garded as a photocatalyst, if the term is used in a special 

 sense, meaning here that the over-all reaction would not 

 tend to take place spontaneously in the dark. 



HYDROGEN ACCEPTORS FOR THE CHLOROPLAST REACTION 



Chloroplasts suspended in an aqueous extract of acetone- 

 treated leaves evolve oxygen in the light at a maximum 

 pressure of the order of i mm. Hg. pressure. If instead of 

 acetone extract potassium ferric oxalate is added to the 

 illuminated chloroplast preparation as an external hydrogen 

 acceptor, oxygen is evolved up to a pressure of 4 mm. Hg. 

 Practically complete reduction of the ferric compound to 

 ferrous takes place and the amount of iron reduced is in 

 stoichiometric equivalence to the oxygen produced. The 

 photochemical reaction may be represented by the equation: 



4K3Fe(C204)3+2H20+4K+ - 4K4Fe(C204)3+4H++02 

 AF= + i8,7oo cals. per hydrogen atom transferred at pH 7 



If the re-oxidation of the ferrous oxalate is prevented by 

 adding potassium ferricyanide, oxygen is produced even in 

 the presence of 21% (i.e. in air). The chloroplasts of some 

 plants, e.g. Beta vulgaris and Spinacia oleracea, will react 

 directly with ferricyanide and the reaction is then repre- 

 ;sented by the equation: — 



4K3Fe(CN)6+2H20+4K+ - 4K4Fe(CN)6+4H++02 

 AF= +8,500 cals. per hydrogen atom transferred at pH 7 



