Aqueous Interfaces in Plant Cells

S. K. Asunmaa

Abstract


A subunit, 90 to 100 Å in diameter, consisting of lipid and protein components, is proposed for semipermeable plant cell membranes. The hydrocarbon chains of the lipid molecules are oriented toward the periphery, the hydrophilic molecular segments toward the center of a lipid annulus where the hydrophilic region stabilizes a 22-Å sheath of ordered water, too rich in hydrogen bonds to dissolve electrolytes. The ordered water leaves a core of ordinary water for ion transport. The subunit thus acts as an ion-gate, because of synchronized movement of the lipids for opening and closing the pore, powered by surface adsorbed proteins. The subunit model would consist of 27 galactosyl diglyceride units (mono- or di- or both) in chloroplast membranes from which Wehrmeyer (1967) isolated a 80,000-g fraction that consists of 90-Å diameter subunits.

Membrane permeability data and the Small-Bourgès (1965) phase diagram for the ternary system cholesterol-lecithin-water (in agreement with X-ray and NMR data) are discussed as supporting evidence for the ion-gate model, after a short review of the model design for the cholesterol-lecithin complexes, which in the subunit configuration may adsorb proteins such as the circular polypeptide alamethicin. The role of plant proteins requires further analysis. Electron micrographs of preparations of plant cell membranes, animal cell plasma membranes and proteolipid membranes in vitro, as published by various research groups, are presented as morphological support for the ion-gate model.


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