Inorganic Carbon Uptake during Photosynthesis

The species of inorganic carbon (CO(2) or HCO(3) (-)) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO(2) or HCO(3) (-) transport, during steady state photosynthesis, is achieved by monitoring the changes (by (14)C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO(2) or HCO(3) (-) transport) and experimental time-courses of (14)C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO(2), rather than HCO(3) (-), is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO(2) is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO(3) (-) transport, as the incorporation of (14)C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO(2) uptake alone. The contribution of HCO(3) (-) to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO(3) (-) concentration. The evidence for direct HCO(3) (-) transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of (14)C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO(2), which is partially alleviated by a high extracellular concentration of HCO(3) (-).