title: Mechanism of acquisition of exogenous bicarbonate by internodal cells of Chara corallina.
abstract: Electrophysiological measurements on internodal cells of the alga, Chara corallina Klein ex Willd., em. R.D.W., showed that the potential across the plasmalemma was sensitive to the level of exogenous HCO 3 (-) . In alkaline solutions (pH 8) the membrane potential depolarized by 50-75 mV when exogenous HCO 3 (-) was removed from the bathing medium. In the presence of exogenous HCO 3 (-) , the membrane potential rapidly hyperpolarized when the cell was given a brief dark treatment; in the light the potential was approx.-240 mV; after the cell had been in the dark for 3-6 min the potential was -330 to -350 mV. In the absence of exogenous HCO 3 (-) the potential only hyperpolarized slowly and to a much smaller extent when cells were placed in the dark. Upon re-illuminating the cell, the potential further hyperpolarized, transiently, and then rapidly depolarized back towards the light-adapted value. (These responses were only obtained when cells were not perturbed by microelectrode insertion into the vacuole.) Analysis of membrane potential and experiments with the extracellular vibrating electrode indicated a high level of correlation between the light- and dark-induced changes in membrane potential and extracellular currents. However, when experiments were conducted in HCO 3 (-) -free media that contained 1.0 mM phosphate buffer, pH 8, it was found that the dark-induced hyperpolarization of the membrane potential and the light-dependent extracellular currents could be maintained in the absence of exogenous HCO 3 (-) . These results are interpreted in terms of two basic models by which internodal cells of C. corallina may acquire exogenous HCO 3 (-) for photosynthesis. They are consistent with HCO 3 (-) being transported across the plasmalemma via an electrically neutral HCO 3 (-) -H(+) cotransport system. The hyperpolarizing response is thought to be the consequence of the operation of an electrogenic H(+)-translocating ATPase that has a transport stoichiometry of 1 H(+) per ATP hydrolyzed.
authors.affiliation: Department of Botany, University of California, 95616, Davis, CA, USA.
authors.forename: W J
authors.initials: WJ
authors.lastname: Lucas