Osmosis is the diffusion of water through a selectively permeable membrane. It is a type of passive transport because no additional energy must be added for this process to occur. If there is a greater concentration of water molecules on one side of the membrane(high solvent, low solute), and fewer molecules on the other side of the membrane(low solvent, high solute), a concentration gradient is established. The side where there is a greater concentration of solvent has the probability that water molecules will more frequently collide with other water molecules, than on the side where there are fewer molecules. Because the membrane is permeable to water molecules, they can pass through its tiny pore spaces. Eventually, the molecules will diffuse from high to low concentration until both sides of the membrane have equal numbers of water molecules and osmosis will no longer occur. This is known as an equilibrium.