Bi carbonate transport in chara corallina evidence for co transport of bi carbonate with protons

Lucas, W.J.; Keifer, D.W.; Sanders, D.

Journal of Membrane Biology 73(3): 263-274


ISSN/ISBN: 0022-2631
DOI: 10.1007/bf01870541
Accession: 004822406

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Experiments were undertaken on the freshwater alga C. corallina to determine the form of inorganic C (CO2 or HCO3-) which enters the cell during photosynthesis at alkaline pH. Recent proposals have centered on the possibility that proton efflux in alkaline solution is able to generate, in the immediate vicinity of the cell, a sufficiently low pH to raise the partial pressure of CO2 and, hence, facilitate its passive permeation into the cell. Predictions have been made by modeling this situation, and these models were tested by placing recessed-tip pH microelectrodes in the unstirred layer surrounding cells in stagnant solution (bulk pH 8.2, buffered only with 1 mM HCO3-). Even as close as 2 .mu.m from the cell wall, the pH was typically 7.2-7.6 in the acid band center, over 1 pH unit greater than that suggested by the models for CO2 entry at the necessary rate for C-fixation. Further evidence for the entry of HCO3-, rather than CO2, at high solution pH was obtained from experiments in which the radial pH gradient in the unstirred layer was reduced. Buffer solutions containing 5 mM phosphate or 5 mM HEPES raised the pH at the cell surface in the acid regions from .apprx. 7.2 to .gtoreq. 7.8. This pH increase (reduction in acid gradient) would have greatly reduced the CO2 level at the cell surface and should, therefore, have greatly reduced the CO2-related 14C-influx. 14C-fixation was reduced by only 31% (phosphate) or 15% (HEPES), compared with buffer-free controls. Reduction of the unstirred layer thickness by fast solution flow resulted in a stimulation, and not a reduction, of 14C-fixation. The similarity of our radial pH profiles near the wall with that predicted by the model assuming H+-HCO3- cotransport, together with the effects of buffer and the results of increased solution flow rate, lead to the conclusion that cotransport of HCO3- with H+ is the likely method of entry of inorganic C. Longitudinal pH profiles of the Chara cell were obtained at a distance of 25 .mu.m from the wall. These revealed much sharper delineation of the acid and alkaline bands than has previously been possible with miniature pH electrodes. Profiles of local electric field, obtained with a vibrating probe, were in excellent agreement with the high resolution pH profiles. This supports the hypothesis that membrane proton transport has a role (direct) in the generation of the extracellular currents.

Bi carbonate transport in chara corallina evidence for co transport of bi carbonate with protons