Diffusion occurs until molecules reach an equilibrium. During the time that the particles are diffusing into the cell, there is a point in which there are as many particles getting into the cell, as there are getting out of it.
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Diffusion is the pathway of only small nonpolar molecules that freely move through the membrane.
Diffusion is a slow passive transport that does not need the energy to occur but requires short distances and pronounced concentration gradients to be efficient.
Since it depends on the concentration gradient, molecules keep passing through the membrane until concentrations on both sides get equal.
When intracellular and extracellular concentrations are equilibrating, there is a moment in which molecules diffuse in both directions.
At this point, there are as many particles diffusing into the cell as there are getting out of the cell.
Diffusion continues to occur until there are equal concentrations of molecules on both sides of the membrane.
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