|MadSci Network: Cell Biology|
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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