Osmosis is the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.The cell membrane is permeable to water molecules. Although water is a polar molecule, it is small enough to pass through the phospholipid bilayer.Water moves across the phospholipid bilayer in both directions.The water potential Ψ is the pressure created by water molecules. Under standard conditions of temperature and pressure (25°C, 100kPa), pure water has a water potential of zero.The addition of a solute to pure water lowers the water potential i.e. makes it more negative.The solute particles are not able to diffuse across the selectively permeable membrane. The water molecules interact with the solute reducing the ability of the water molecules to diffuse across the membrane.As a result more water molecules diffuse from the side with less solute and higher water potential to the side with lower water potential.