Rectification characteristics ofNitella membranes in respect to water permeability

Summary

One of the membrane characteristics of plant cells, rectification, or the direction dependence of water permeability, was investigated inCharaceae internodes using the procedures we developed (Tazawa andKiyosawa 1973) for determining the endosmotic (k _pen) and exosmotic (k _pex) water permeabilities of the membranes (plasmalemma and tonoplast) in the transcellular osmosis system. Bothk _pen andk _pex were dependent on the osmotic pressure (π _o ) of the mannitol solution, which is the driving force for the transcellular osmosis. Thus, k_pen increased andk _pex decreased with π _o . The rectification parameter, or the polarity (ρ_p), defined ask _pen/k _pex tended to unity when π _o approached zero.

InNitella flexilis the specific resistances of the membranes to endosmosis and exosmosis,k ⁻¹_pen andk ⁻¹_pex , were linearly dependent on π₀. When the cell was partitioned into two equal halves,k ⁻¹_pen =4.2×10⁴−1.1×10³π₀,k ⁻¹_pex =4.2×10⁴+2.9×10³π₀, where the specific resistances are represented in cm⁻¹ sec atm. When π _o is 0.1, 0.2, 0.3, 0.4, and 0.5 M mannitol eq., the rectification parameter is calculated as 1.3, 1.6, 1.9, 2.4, and 2.9, respectively. Essentially the same results were also obtained withChara australis.

Results were discussed on the basis of changes in the hydration of the cytoplasm. Assuming that the driving force across the protoplasmic layer can be divided into two forces; one driving water across the plasmalemma and the other driving water across the tonoplast, we deduced that the cytoplasm on the endosmosis side is hydrated, while the cytoplasm on the exosmosis side is dehydrated. Analysis showed that changes in hydration depend on the rate of flow.