|MadSci Network: Cell Biology|
I'm not sure of the context of your question. In general, there are two ways that a small molecule like ethanol (or glycerol or a sugar) can get across a membrane. There can either be a transport protein that mediates passage across the membrane or the substance can diffuse across. Ethanol can do both. It is not a charged molecule and it's small, so it can diffuse across the membrane. That means it is dissolved in the water outside the cell, then dissolves in the lipid of the membrane when it contacts the membrane through random motion. Once in the membrane it will diffuse through the membrane by random motion and eventually dissolve itself back into the water. It's random which side of the membrane it exits. So if it enter from outside, it doesn't necessarily end up inside, it's roughly 50:50 whether it ends inside or outside. Some but not all cells have a protein that can transport ethanol. It would have a binding site for ethanol. Once bound, the ethanol will cause the protein/transporter to change shape so that when the ethanol dissociates from the protein, it ends up inside the cell. A transporter has, usually, more direction than just random diffusion. It will preferentially move the substrate (ethanol) one direction, not necessarily 100% of the time but most of the time.
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