The “diffusion through a membrane” equations: diffusion is driven by the difference in concentrations on the two sides of the membrane. It is also affected by the permeability coefficient and the area of the membrane:
The permeability coefficient is determined by the equation, P = KD/Δx.
A continuous equation constantly checks its direction so its graph has no breaks. A discrete equation checks its direction at each Δt. A discrete curve may contain breaks.
In a discrete model, rate of flow across membrane is calculated at each Δt through a process of iteration (repeating the last step by substituting the values calculated from the last step) until equilibrium (when concentrations on both sides of the membrane are the same) is reached. Iteration of a discrete curve yields a graph that is very similar to a continuous curve.
You should now be able to apply the diffusion equations to determine which way a substance moves across a membrane which is selectively permeable.
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