The rate of diffusion is inversely proportional to the density of the substance. Both the cytoplasm and extracellular environment of a cell consist of aqueous solutions in which are dissolved a variety of solutes. Diffusion plays a very important role in passively moving solutes through these aqueous solutions and getting them where they need to go. Often, solutes and solutions need to cross plasma membranes. As we discussed in lecture, cellular membranes are selectively permeable, in that they only allow certain solutes or solutions to pass through them.
Osmosis is a kind of diffusion where water is the substance moving across the membrane.
This is especially important when a membrane is impermeable to a certain solute that is highly concentrated on one side of the membrane. If the membrane is permeable to water, then the water molecules will cross the membrane in the opposite direction the solute wants to go. This process will proceed until a dynamic equilibrium is reached and the concentration of the solute is the same on both sides and the concentration of the water is the same on both sides. That is not to say the equilibrium concentration of the solute will be the same as the water. Each substance seeks its own equilibrium without respect to the equilibria of other substances.