Osmosis is the diffusion of a solvent across a selectively permeable membrane. Osmotic pressure is a measure of the tendency of water (or solvent) to move across the selective permeable membrane. Iso-osmotic solutions have the same concentration of solute particles. Hyperosmotic solutions have higher concentration, and hypo-osmotic solutions have lower concentration of solute particles than a reference solution. When cells are kept in an iso-osmoticc solution, they will neither swell nor shrink. However, when the cells are kept in a hyper-osmotic solution they will shrink, whereas in a hypo-osmotic solution they will swell or may lyse.