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The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

The function of water channels in Chara: The temperature dependence of water and solute flows provides evidence for composite membrane transport and for a slippage of small organic solutes across water channels

Planta (Heidelberg) 202(3): 324-335

Using the cell pressure probe, the effects of temperature on hydraulic conductivity (Lp; osmotic water permeability), solute permeability (permeability coefficient, P-s), and reflection coefficients (sigma-s) were measured on internodes of Chara corallina, Klein ex Willd., em R.D.W. For the first time, complete sets of transport coefficients were obtained in the range between 10 and 35 degree C which provided evidence about pathways of water and solutes as they move across the plasma membrane (water channel and bilayer arrays). Test solutes used to check for the selectivity of water channels were monohydric alcohols of different molecular size and shape (ethanol, n-propanol, iso-propanol, and tert-butanol) and heavy water (HDO). Within the limits of accuracy, Q-10 values for Lp and for the diffusive water permeability (P-d) were identical (Q-10 for Lp = 1.29 +- 0.17 (+- SD; n = 15 cells) and Q-10 for P-d = 1.25 +- 0.16 (n = 5 cells)). The Q-10 values were equivalent to activation energies of E-a = 16.8 +- 6.4 and 16.6 +- 10.0 kJ cntdot mol-1, respectively, which is similar to that of selfdiffusion or of viscous flow of water. The Q-10 values and activation energies for P-s of the alcohols were significant- ly larger (ethanol: Q-10 = 1.68 +- 0.16, E-a = 37.1 +- 5.9 kJ cntdot mol-1; n-propanol: Q-10 = 1.75 +- 0.40, E-a = 43.1 +- 15.3 kJ cntdot mol-1; iso-propanol: Q-10 = 2.12 +- 0.42, E-a = 52.2 +- 14.6 kJ cntdot mol-1; tert-butanol: Q-10 = 2.13 +- 0.56, E-a = 51.6 +- 17.1 kJ cntdot mol-1; +-SD; n = 5 to 6 cells). Effects of temperature on reflection coefficients were most pronounced. With increasing temperature, sigma-s values of the alcohols decreased and those of HDO increased. The data indicate that water and solutes use different pathways when crossing the membrane. Ordinary and isotopic water use water channels and the other test solutes use the bilayer array (composite transport model of membrane). Changes in sigma-s values with temperature were found to be a sensitive measure for the open/closed state of water channels. The decrease of sigma-s with temperature was theoretically predicted from the temperature dependence of P-s and Lp. Differences between predicted and measured values of sigma-s allowed estimation of the bypass flow (slippage) of solutes through water channels which did not completely exclude test solutes. The permeability of channels depended on the structure and size of test solutes. It is concluded that water channels are much less selective than is usually thought. Since water channels represent single-file or no-pass pores, solutes drag along considerable amounts of water as they diffuse across channels. This results in low overall values of sigma-s. The sigma-s of HDO was extremely low. Its response to temperature was opposite to that for the sigma-s of the alcohols. This suggested a stronger effect of temperature on the hydraulic (osmotic) than on the diffusive water flow across individual water channels, i.e. a differential sensitivity of different mechanisms to temperature.

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Accession: 009579950

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DOI: 10.1007/s004250050134

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