The water potential of a plant cell has two parts: the effect of the solutes and the effect of the pressure of the cell wall. They are simply added together:ψ = ψs + ψp where ψ is the water potential, ψs is the solute potential (osmotic potential, π, always zero or negative) and ψp is the pressure potential (turgor pressure, P, usually positive). Using the older symbols, ψ = π + P. All values are in kPa.Worked example 1: a cell has ψs = −800 kPa and ψp = +300 kPa. ψ = −800 + 300 = −500 kPa.Worked example 2: a cell has ψ = −600 kPa and ψs = −1000 kPa. Rearranging, ψp = ψ − ψs = −600 − (−1000) = +400 kPa.Two special cases:
• Fully turgid cell in pure water: water enters until ψp exactly balances ψs, so ψ = 0 kPa, the same as the water, and net movement stops.
• Incipient plasmolysis: the protoplast no longer presses on the wall, so ψp = 0 and ψ = ψs.Worked example 3: cell A has ψs = −700 kPa and ψp = +400 kPa, so ψ = −300 kPa. Cell B has ψs = −900 kPa and ψp = +300 kPa, so ψ = −600 kPa. Water moves from A to B, because A has the higher water potential.Always compare total water potentials, not solute potentials alone, when deciding the direction of osmosis between plant cells.
• Fully turgid cell in pure water: water enters until ψp exactly balances ψs, so ψ = 0 kPa, the same as the water, and net movement stops.
• Incipient plasmolysis: the protoplast no longer presses on the wall, so ψp = 0 and ψ = ψs.Worked example 3: cell A has ψs = −700 kPa and ψp = +400 kPa, so ψ = −300 kPa. Cell B has ψs = −900 kPa and ψp = +300 kPa, so ψ = −600 kPa. Water moves from A to B, because A has the higher water potential.Always compare total water potentials, not solute potentials alone, when deciding the direction of osmosis between plant cells.