Osmosis

Osmosis is the net movement of water from an area of high water potential to an area of low water potential across a selectively permeable membrane. Osmosis is a passive process requiring no energy from ATP and is unaffected by respiratory inhibitors. The water molecules move in both directions across the membrane; the overall movement is the net movement. If the water potentials are equal on both sides of the membrane, the water moves in both directions equally so there is no net movement.

Water potential is defined as the tendency of water to leave a system by osmosis. Osmosis describes the mechanism of movement. Water potential of solutions depends on the solute concentration (solute potential) and the pressure exerted on the solution (pressure potential). Both water and solute potentials are negative values. This means that a high potential has a less negative number, for example -360 kPa is higher than -410 kPa. Pressure potential is always a positive value. All potentials have the units kPa (kilopascals). The symbol for water potential is the letter Ψ. This is often subscripted with a w for water potential, s for solute potential and p for pressure potential.

The higher the concentration of solute in a solution or cell, the lower the water potential. If glucose or ions are pumped or diffuse into a cell, the water potential of that cell gets more negative or lower.