|MadSci Network: Biochemistry|
Your second idea is the more correct, though the first contributes. The primary reason that alcohol (and most any organic solvent) destroys a cell membrane is that it is either solubilizing the phospholipid and cholesterol or other lipids, or, with some solvents, causing a phase change (reorganization of the lipids into another structure) that makes the membrane very, very leaky (effectively it pokes holes in the membrane rather than just dissolving them). A third reason is that alcohol desiccates cells by drawing the water out. In fact, 70% alcohol is better for the latter than 95%-100%. The latter actually almost seals the membrane and not as much water exchanges.
Alcohols and organic solvents will also denature some, though not all, proteins. This probably happens when a cell is hit with alcohol but it is not the major factor in generating cell death or leakiness.
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