ISSN:
1432-1424
Keywords:
Chara
;
bicarbonate transport
;
extracellular pH
;
recessed-tip pH microelectrodes
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
Notes:
Summary Experiments were undertaken on the fresh water algaChara corallina to determine the form of inorganic carbon (CO2 or HCO 3 − ) 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 modelling this situation (N.A. Walker, F.A. Smith & I.R. Cathers, 1980,J. Membrane Biol. 57∶51–58, J.M. Ferrier, 1980,Plant Physiol. 66∶1198–1199), and these were tested by placing recessed-tip pH microelectrodes in the unstirred layer surrounding cells in stagnant solution (bulk pH 8.2, buffered only with 1mm HCO 3 − ). Even as close as 2 μm from the cell wall, the pH was typically 7.2 to 7.6 in the acid band center — over 1 pH unitgreater than that suggested by the models for CO2 entry at the necessary rate for C-fixation. Further evidence for the entry of HCO 3 − , 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 5mm phosphate or 5mm HEPES, raised the pH at the cell surface in the acid regions from around 7.2 to 7.8 or higher. 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-related14C-influx. However,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, of14C-fixation. The similarity of our radial pH profiles near the wall with that predicted by the model (Walker et al., 1980) assuming H+−HCO 3 − cotransport, together with the effects of buffer, and the results of increased solution flow rate, lead to the conclusion that cotransport of HCO 3 − with H+ is the likely method of entry of inorganic carbon. Longitudinal pH profiles of theChara cell were obtained at a distance of 25 μm 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01870541
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