A study of the current-voltage relationships of electrogenetic active and passive membrane elements in riccia fluitans |
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Authors: | Hubert Felle |
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Affiliation: | Abteilung Biophysik der Pflanzen, Institut für Biologie der Universität, Auf der Morgenstelle 1, D-7400 Tübingen F.R.G. |
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Abstract: | Potassium- and proton-dependent membrane potential, conductance, and current-voltage characteristics (I–V curves) have been measured on rhizoid cells of the liverwort Riccia fluitans. The potential difference (Em) measured with microelectrodes across plasmalemma and tonoplast is depolarized to the potassium-sensitive diffusion potential (ED) in the presence of 1 mM NaCN, 1 mM NaN3, or at temperatures below 6°C. Whereas the temperature change from 25°C to 5°C decreases the membrane conductance (gm) from 0.71 to 0.43 S ? m?2, 1 mM NaCN increases gm by about 25%. The membrane displays potassium-controlled rectification which gradually disappears at temperatures below 5°C. The potassium pathway can be described by an equivalent circuit of a diode and an ohmic resistor in parallel. In the potential interval of ED ± 100 mV the measured I-V curves roughly fit the theoretical curves obtained from a modified diode equation. 86Rb+(K+)-influx is voltage sensitive: In the presence of 1 mM NaCN, 86Rb+-influx follows a hyperbolic function corresponding to a low conductance at low [K+]o and high conductance at high [K+]o. On the contrary 86Rb+-influx is linear with [K+]o when pump activity is normal. It is believed that there are two K+-transport pathways in the Riccia membrane, one of which is assigned to the low conductance (0.2 S · m?2), the other to a temperature-dependent facilitated diffusion system with a higher conductance (7.7 S · m?2). The electrogenic pump essentially acts as a current source and consumes about 39% of the cellular ATP-turnover. In the presence of 30 μM CCCP the saturation current of 0.1 A · m?2 is doubled to about 0.2 A · m?2, and the electromotive force of ?360 mV switches to ?250 mV. It is suggested that this may be due to a change in stoichiometry from one to two transported charges per ATP hydrolyzed. |
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Keywords: | Current-voltage relationship Electrogenic transport (Riccia fluitans) membrane potential (mV) diffusion potential (mV) CCCP DNP 2,4-dinitrophenol SHAM salicylhydroxamic acid |
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