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Articles

Inorganic Carbon Uptake during Photosynthesis ¹

I. A Theoretical Analysis Using the Isotopic Disequilibrium Technique

Department of Biology, York University, Downsview, Ontario, Canada M3J 1P3

Equations have been developed which quantitatively predict the theoretical time-course of photosynthetic ¹⁴C incorporation when CO₂ or HCO₃^– serves as the sole source of exogenous inorganic carbon taken up for fixation by cells during steady state photosynthesis. Comparison between the shape of theoretical (CO₂ or HCO₃^–) and experimentally derived time-courses of ¹⁴C incorporation permits the identification of the major species of inorganic carbon which crosses the plasmalemma of photosynthetic cells and facilitates the detection of any combined contribution of CO₂ and HCO₃^– transport to the supply of intracellular inorganic carbon. The ability to discriminate between CO₂ or HCO₃^– uptake relies upon monitoring changes in the intracellular specific activity (by ¹⁴C fixation) which occur when the inorganic carbon, present in the suspending medium, is in a state of isotopic disequilibrium (JT Lehman 1978 J Phycol 14: 33-42). The presence of intracellular carbonic anhydrase or some other catalyst of the CO₂-HCO₃^– interconversion reaction is required for quantitatively accurate predictions. Analysis of equations describing the rate of ¹⁴C incorporation provides two methods by which any contribution of HCO₃^– ions to net photosynthetic carbon uptake can be estimated.