MadSci Network: Chemistry

Re: can materials diffuse through a membrane at the same time? How?

Date: Mon Nov 12 11:40:51 2001
Posted By: Michael Maguire, Faculty,Case Western Reserve Univ.
Area of science: Chemistry
ID: 1004403680.Ch

I'm not 100% sure what your question is?

If you are asking if more than one kind of chemical substance can diffuse 
(i.e., no transport protein is involved) through a membrane at the same 
time, the answer is yes.  Substances can diffuse through a lipid bilayer 
if they are sufficiently lipid soluble to overcome the energy barrier 
created by moving from an aqueous environment to a lipid environment.  The 
more "fat" soluble a molecule, the more likely it is to be able to diffuse 
through a membrane.  This is true of small molecules only however.  
Molecules/atoms that are charged and/or large are less likely to be able 
to diffuse through the membrane.  More than one substance can be diffusing 
across at the same time simply because the membrane around a cell has an 
extremely large surface area compared to the area/volume of a single 

If you are asking if a particular transport protein can transport more 
than one kind of substance/substrate, the answer is again yes, but NOT at 
the same time.  On each cycle of transport, one and only one set of 
substrate molecules gets transported.  Any given transport protein has 1 
or sometimes 2 primary substrates.  For example, the Na+,K+-ATPase in the 
plasma membrane of most eukarytic cells in a single cycle of transport 
moves 3 Na+ ions from the cytosol to the extracellular space and 2 K+ ions 
in the opposite direction.  Other transporters might move a glucose 
molecular into the cell along with a proton or a Na+ ion.  Still other 
transporters might move Ca2+ into or out of the cell, but nothing else.  
This does not mean however that each of these transporters cannot move 
something else if it were present.  For example if a rubidium ion (Rb+) 
were present in the extracellular space, the Na+,K+-ATPase could move Na+ 
out of and Rb+ into the cell.

If this doesn't answer the question, write out a longer question in more 
detail and I'll try to answer it.

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