Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane. Therefore, water moves from high to low water potential down a water potential gradient. The movement of water continues until the water potential is the same on both sides of the membrane and a dynamic equilibrium is established where no further net movement occurs.

Water potential

Water molecules possess kinetic energy and move around and collide with a partially permeable membrane. This creates pressure known as water potential (ψ) which is measured in KPa. It is a measure of the tendency of water molecules to move from one region to another. Pure water has the highest water potential at 0 KPa. The presence of dissolved solutes decreases the water potential by an amount known as the solute potential (ψs). Applied pressure increases the water potential by an amount known as the pressure potential (ψp).

Solutes lower water potential because water molecules form a hydration shell around the solutes as they dissolve. Therefore there are fewer free water molecules to collide with the partially permeable membrane and exert the water potential.

water potential ψ = solute potential ψs + pressure potential ψp

Osmosis in cells

Water moves into and out of cells by osmosis from higher to lower water potential, down a water potential gradient. Even though water is a polar molecule it is small enough to fit in between the phospholipids although this is slow. They may cross the membrane via a channel protein (aquaporin).

In hypotonic solutions, the solute concentration of the external solution is lower than the solute concentration in the cell. This results in water moving into the cell. In animal, this may result in cytolysis (cell bursting) as the plasma membrane cannot withstand the pressure. In plant, the cells will be turgid.

In isotonic solutions, the solute concentration of the external solution is the same as the solute concentration in the cell. This results in no net movement of water. In animal , the cells will be normal and healthy. In plants, the cells may become flaccid and reach a point of incipient plasmolysis.

In hypertonic solutions, the solute concentration of the external solution is higher than the solute concentration in the cell. This results in water moving out of the cell. In animals, the cells will become crenated (shriveled). In plants, the cells will become plasmolysed as the plasma membrane pulls away from the cell wall.

In hypotonic solutions, plant cells do not burst because as water enters the cell by osmosis the protoplast (cell contents inside plasma membrane) increases in volume and pushes against the cell wall. The cell wall is strong so it can withstand this internal pressure. Furthermore, The cell wall also exerts a pressure potential and prevents more water entering the plant cell. This results in turgid cells which is important in young/non-woody plants. Incipient plasmolysis is the point at which the pressure potential is 0 and occurs in isotonic solutions. If the cell loses anymore water than it will become plasmolysed.