MadSci Network: Cell Biology

Re: How does a Daphnia absorb Ethanol

Date: Thu Mar 25 12:56:33 2004
Posted By: Michael Maguire, Professor
Area of science: Cell Biology
ID: 1079290442.Cb

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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