Hexose transport and membrane depolarization in Riccia fluitans

In the aquatic liverwort Riccia fluitans, the uptake of (14)C-labeled 3-O-methyl glucose (3-OMG) and membrane depolarization (ΔΨ m ) caused by different hexoses has been studied as a function of time and concentration of hexose, K(+) and H(+), respectively. The rate of uptake of the non-metabolized 3-OMG shows two components: (A)A pH-dependent saturable uptake with a km value around 0.1 mM which saturates at 2.1 and 7.2 μmol G DW (-1) h(-1) at pH 6.8 and 5.0, respectively; and (B) a pH-insensitive uptake component which increases linearly with the external 3-OMG concentration and does not saturate ≦4 mM. Hexoses rapidly depolarize the plasmalemma of the thallus cell and increase its electrical conductance. The maximal ΔΨ m was 60±2 mV, the concentrations (mM) for half-maximal ΔΨ m were 0.24 glucose, 0.32 galactose, 0.37 2-deoxy glucose, 0.38 3-OMG, 0.57 mannose, and 34 fructose. In terms of a hexose carrier model and an equivalent circuit for the hexose-induced depolarized state of the membrane, it is proposed that a hexose carrier operates either electrogenically in its protonated, pH-and voltage-sensitive state, or by transmembrane diffusion of its uncharged state.