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Diffusion vs osmosis

Diffusion is a transport phenomenon where there is a net movement of molecules from an area in which they are highly concentrated to an area in which they are less concentrated.

In a beaker with a semipermeable membrane, a high concentration of orange salt molecules is on the left and a lower concentration of orange salt molecules on the right at the start of diffusion. The water level is equal on both sides. Once molecules have diffused through the water, the concentration of molecules is equal on both sides.

Figure 1: Diffusion from an unequal distribution of solutes (left panel) to an equal distribution of solutes (right panel). There is a net movement of solutes from an area of high concentration (right of membrane) to an area of low concentration (left of membrane) until equilibrium is reached.

Osmosis is the net movement of water molecules across a semi-permeable membrane. The water moves from high water potential (a high concentration of water molecules, and therefore a low concentration of dissolved molecules) to low water potential (a high concentration of dissolved molecules).

In a beaker with a semipermeable membrane, a high concentration of orange salt molecules is on the left and a lower concentration of orange salt molecules on the right prior to osmosis. The water level is equal on both sides. Water molecules move across the semipermeable membrane until the concentration of molecules is the same on both sides of the membrane. The water level is now higher on the left side of the membrane.

Figure 2: Osmosis from an unequal distribution of solutes (top) to an equal distribution of solutes (bottom). There is a net movement of water molecules from an area of high water potential (right) to an area of low water potential (left) until equilibrium is reached.