Amine transport at the plasmalemma of Riccia fluitans |
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Authors: | Hubert Felle |
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Institution: | Abteilung Biophysik der Pflanzen, Institut f:ur Biologie I der Universität Tübingen, Auf der Morgenstelle 1, 7400 Tübingen F.R.G. |
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Abstract: | Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH4Cl and 5–100 μM CH3NH3Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m?2. Uptake of 14C]methylamine is stimulated by increasing K+]o and inhibited by increasing Na+]o or H+]o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent Km values of 2 ± 1 μM ammonia and 25–50 μM methylamine; Km values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH4+ and CH3NH3+ into the cytoplasm. |
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Keywords: | Amine transport Current-voltage characteristics Depolarization Ammonia Methylamine (Riccia fluitans) |
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