392 PLANT METABOLISM 



with the release of hydrogen to some hydrogen acceptor, which can func- 

 tion in subsequent reductions. This active hydrogen may be passed from 

 one acceptor to another before it is finally used for the reduction of 

 carbon dioxide. 



Release of oxygen 



The reaction releasing oxygen is dependent upon, but apparently not 

 identical with, the primary photochemical reaction. If the photolysis 

 of water is considered as an oxidation-reduction reaction, the hydrogen 

 acceptor mentioned is reduced, and some other acceptor of the hydroxyl 

 radical remaining is oxidized. This oxidized product releases O2, and 

 it may be assumed that an enzymatic mechanism is concerned in this 

 release. 



It has been implied throughout the discussion that all the oxygen 

 formed in photosynthesis comes from water, but this fact has only been 

 accepted in recent years. Early workers suggested that it came from 

 carbon dioxide, or part from carbon dioxide and part from water. The 

 work of van Niel with the photosynthetic bacteria . suggested that, if 

 bacterial photoreduction were analogous to the photosynthesis of higher 

 plants, water should supply all the oxygen released in photosynthesis. 

 This hypothesis was verified experimentally with the stable isotopic 

 tracer O^^, when it was shown that the O^^ level of photosynthetic O2 

 corresponded exactly with the 0^^ level of HoO^*, in which photosynthe- 

 sizing algae were suspended. 



Carbon dioxide 



Normal air contains 0.03 per cent carbon dioxide, and upon this plants 

 must depend for their source of carbon. Air is taken in through the 

 stomatal openings on the leaf and is dissolved in the leaf sap. Carbonic 

 anhydrase is an enzyme in the leaves which speeds the formation of car- 

 bonic acid from the carbon dioxide and water. There is no clear experi- 

 mental answer to the question whether carbon dioxide is used in photo- 

 synthesis as the dissolved gas or as the carbonate and bicarbonate 

 ions. 



Evidently the carbon dioxide entering a leaf is bound in a loose com- 

 plex on some large molecules, a binding which may be compared roughly 

 to the binding of oxygen by hemoglobin. Many lines of evidence indicate 

 that the carbon dioxide is utilized next in a carboxylation reaction. A 

 general formulation of such a reaction is: 



RCHO + CO2 -» RCCOOH 



II 

 



