MadSci Network: Cell Biology

Re: What exactly happens after cell diffusion reaches equilibrium?

Date: Mon Dec 4 16:11:11 2000
Posted By: Michael Maguire, Faculty,Case Western Reserve Univ.
Area of science: Cell Biology
ID: 974581958.Cb

I am assuming that your question means what happens after a substance 
(water, glucose, Na+, etc.) diffuses across the cell's plasma membrane 
until it reaches equilibrium.

Quite simply, nothing happens that wasn't happening before.  Nothing 
stops, nothing starts, the same molecules are moving back and forth across 
the membrane through the same pore/channel/transporter.

What has been happening during the period of approach to equilibrium is 
that the RATEs of movement from out to in and from in to out have been 
changing, one slowing and one speeding up.

Example:  Lots of glucose outside the cell, very little glucose inside;  
the membrane is slightly permeable to glucose. Since the glucose 
concentration outside is higher than that inside, glucose will diffuse 
through the transporter into the cell's interior.  However, glucose inside 
can also diffuse out.  Because the glucose concentration gradient is "out" 
to "in" however, the amount of glucose diffusing out to in per unit time 
(RATE) is much greater than the RATE of diffusion from in to out.  This 
means that, with time, the amount of glucose inside rises.  At some point, 
the concentration of glucose inside and the concentration of glucose 
outside are identical.  AT THIS POINT, you are at equilibrium.  However, 
equilibrium does NOT mean that no glucose movement across the membrane is 
taking place.  Pretty much the same amount of movement is taking place, it 
is just that the RATE of glucose going in equals the RATE of glucose going 
out;  therefore, there is no net change in total glucose concentration 
inside or outside.

Never assume that equilibrium means that things "stop", it just means that 
the rate in one direction equals the rate in the other direction.

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