1892 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



active carbonic anhydrase also may be involved. It can further be asked 

 whether the bicarbonate ions that had penetrated into the cell (or had 

 been produced in the cell sap by the hydration of CO2 molecules) can 

 participate in photosynthesis through a reaction independent of (and, 

 perhaps, competitive with) the reaction bj^ which carbon dioxide molecules 

 enter it, e. g. : 



(37D.1) CO2 + A = ACO2 and 



(37D.2) HCO3- + A = ACO2 + OH- 



The alternative is for the intercellular bicarbonate to participate in 

 photosynthesis only by supporting the carbon dioxide level in the cell sap. 

 The bicarbonate ions entering the cell would then be added to the internal 

 bicarbonate reserve (c/. chapter 8, section B.2), and contribute to photo- 

 synthesis only via the relatively sluggish dehydration reaction, which, in 

 strong light, may fail to keep pace with photosynthesis. (The efficiency 

 of this "internal CO2 buffering" must depend on the content of the sap in 

 carbonic anhydrase; cf. Vol. I, p. 380. We mentioned above Osterlind's 

 suggestion that carbonic anhydrase in plants undergoes "activation" in 

 hght.) 



(b) Carbon Dioxide Curves 



The demonstration of the participation of HCOs" ions in carbon trans- 

 fer into photosynthesizing cells of some species (or strains) of aquatic 

 plants brings a new element of uncertainty into plotting and interpretation 

 of "carbon dioxide curves" of photosynthesis, P = /[CO2] {cf. table 27.1 

 and figs. 27. 2A to 27.4). In sections 5 and 9g of chapter 27, it w^as pointed 

 out that the shape of many of the CO2 curves found in the literature must 

 have been determined by the siipply of carbon dioxide to the cells (through 

 diffusion and convection), so that the intrinsic relationship between the 

 rate of photosynthesis and the carbon dioxide concentration in situ (i. e., at 

 the chloroplast surface) has been more or less completely masked. It was 

 suggested that even in experiments in which carbonate buffering, intense 

 stirring, and use of small plant objects (unicellular algae) have assured the 

 absence of a CO2 concentration gradient outside the cells, it was still uncer- 

 tain whether the observed residual dependence of P on fC02] could not have 

 been caused by slow diffusion of carbon dioxide through the cell wall, and 

 from this wall to the chloroplast. Consequently, even calculations based on 

 the shape of carbon dioxide curves obtained under ideal external supply con- 

 dition can give only an upper limit for the dissociation constant of the 

 hypothetical carbon dioxide-acceptor compoimd in photosynthesis (cf. p. 

 934). 



