A Structural Model of Cholinergic Synaptic Vesicles from the Electric Organ of Torpedo marmorata Deuduced from Density Measurements at Different Osmotic Pressures

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Abstract
Density measurements made on cholinergic synaptic vesicles from the electric organs of Torpedo marmorata at different osmotic pressures are consistent with the following structural model of the vesicle. The particle behaves like a sphere 80–100 nm in diameter bounded by a semi-permeable membrane. The bulk of its soluible constituents are in true solution at physiologica osmolalities. The limiting mambrance is approximately 4–5-nm thick, suggesting that it contains large areas of phospholipid bilayer exposed to its bathing medium. The limiting membrane takes up abouit 26% (v/v) of the particle, a further 34% (v/v) of which is osmotically active water and 31% (v/v) hydrated core material at 800 mosmol/l. The byouant density of the membrane is 1.132 g · cm−3. the density of the hydrated core material is approximately 1.05 g · cm−3. The membrane is selectively permeable to small molecules when subjected to hypo-osmotic stress. It is proposed that this occurs by the formation of small transient pores in the lipid bilayer of the membrane, which are induced by stretching caused by the osmotic pressure change.