ROLE OF BICARBONATE IONS IN PHOTOSYNTHESIS 



199 



be reduced each second in one liter of plant cells (Vol. II, Chapter 28), 

 i. e. twenty times more than can be hydrated during the same period. 

 Calculations of this type were first carried out by Burr (1936), who 

 obtained, for a number of plants, ratios between the rates of uncatalyzed 

 hydration and photosynthesis ranging from 1 : 73 to 1 : 2440. He 

 concluded that hydration of carbon dioxide cannot represent a necessary 

 step in photosynthesis. 



It may be argued that the conversion of CO2 into HCOa" ions in 

 plant cells is catalyzed, if not by carbonic anhydrase, at least by milder 

 catalysts such as phosphates (c/. page 176). Experiments carried out 

 by Burr (1936) failed to reveal any catalj^tic effects of mashed leaves 

 on the hydration velocity of carbon dioxide; and the same result was 

 obtained by Mommaerts (1940). Neish (1939), on the other hand, 

 gave some figures for the carbonic anhydrase activity of leaf matter as a 

 whole, and of separated chloroplast matter (Table 14. VIII). According 

 to his measurements, 1 ml. of a suspension containing 1 mg. of dry leaf 

 material can liberate, at 22° C, about 1 mm.^ carbon dioxide per second 

 from 1 ml. of a 0.2 M NaHCOs solution mixed with an equal volume of 

 a phosphate buffer (pH 6.8). It appears impossible to calculate from 

 this rate of catalytic dehydration the maximum rate of hydratioi\ of 

 carbon dioxide by the same amount of leaves, and thus to decide whether 

 Neish's results contradict directly the conclusions of Burr and Mom- 

 maerts. 



12 

 10 



8 



a 6 



4 



2 



m 



hll 



I 



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II 



ii 



ii 



m^^ 



12345678 123456 



Time, hours 



Fig. 20.— Hourly yield of photosynthesis (P) of Hydrilla (left) and Cabomba (right) 

 in carbonate solution (shaded) and after transfer into distilled water (white) (after 

 Gessner 1937). 



The carbon dioxide-bicarbonate-carbonate equihbrium in the cells, 

 even if it does not lie in the direct path of photosynthesis, may play a 

 part in this process by providing carbon dioxide reserves within the cell 

 which help to even out short-time variations in the external supply of 



