Revise Water potential for IB Biology (D2.3)

Water potential, written with the Greek letter psi ($\Psi$), is a measure of the tendency of water to move out of a system by osmosis. It captures, in one number, how 'free' the water is to leave. The reference point is fixed by convention: pure water at atmospheric pressure has a water potential of exactly 0. We measure $\Psi$ in units of pressure — this lesson uses kilopascals (kPa). Because dissolving solutes lowers water's tendency to move, and cells are full of solutes, the water potential of a living cell or a real solution is almost always a NEGATIVE number. The whole predictive power of the idea comes from one rule that never changes: water moves from higher (less negative) water potential to lower (more negative) water potential.

Two independent factors set a cell's water potential, and the equation simply adds them together:

\Psi = \Psi_s + \Psi_p

The solute potential $\Psi_s$ (sometimes called osmotic potential) is the contribution of dissolved solutes. Because solutes can only lower water's tendency to move relative to pure water, $\Psi_s$ is always negative — the more concentrated the solution, the more negative it becomes. The pressure potential $\Psi_p$ is the contribution of physical pressure on the water. In an open beaker of solution there is no extra pressure, so $\Psi_p = 0$ and the solution's $\Psi$ is simply its $\Psi_s$. Inside a plant cell, water entering makes the contents press outward on the rigid cell wall; the wall pushes back and generates a positive pressure potential (turgor). Under tension, as in the xylem of a transpiring plant, $\Psi_p$ can even be negative.

Osmosis is the net movement of water across a partially permeable membrane, and water potential tells us exactly which way it runs. Rather than argue about 'where the water is more concentrated', we compare two $\Psi$ values and let water move from the higher to the lower. The flow continues until the two water potentials are equal, at which point there is no net movement — the system is at equilibrium (though water molecules still cross the membrane in both directions at equal rates). Because $\Psi$ folds pressure in alongside solutes, this single rule works even where a solute-only account fails, such as a turgid plant cell that has stopped taking in water despite sitting in dilute surroundings.

Place a plant cell in a HYPOTONIC solution — one with a higher (less negative) water potential than the cell — and water enters by osmosis. The vacuole swells and pushes the membrane against the cellulose cell wall, which resists and generates a positive pressure potential. As $\Psi_p$ rises, the cell's overall $\Psi$ becomes less negative until it matches the surroundings and net inflow stops. The cell is now TURGID, firm and supportive; the wall prevents it from bursting. In an ISOTONIC solution there is no net movement and, with the membrane no longer pressing hard on the wall, the cell is FLACCID and the tissue limp. Place the cell in a strongly HYPERTONIC solution — lower (more negative) water potential than the cell — and water leaves. So much water can be lost that the membrane peels away from the cell wall: this is PLASMOLYSIS, and severe plasmolysis kills the cell.

Animal cells face the same water-potential gradients but lack a cell wall, so nothing generates a rising pressure potential to halt the flow. In a HYPOTONIC solution (higher $\Psi$ outside) water floods in and the cell swells and may burst — lysis, or haemolysis for a red blood cell. In a HYPERTONIC solution (lower $\Psi$ outside) water leaves and the cell shrinks and shrivels (crenation in a red blood cell). Only in an ISOTONIC solution, where inside and outside water potentials are equal, is there no net movement and the cell keeps its normal shape. This is exactly why the water potential of blood and tissue fluid is kept tightly regulated, and why cells for transfusion or culture are suspended in isotonic solutions.

To predict where water will go between any two regions — two cells, a cell and a solution, or two solutions — work each one's total water potential from $\Psi = \Psi_s + \Psi_p$, then compare. Water moves from the higher (less negative) $\Psi$ to the lower (more negative) $\Psi$. State the direction explicitly as 'from X to Y' and justify it by naming which value is higher. The two worked examples below show this in action, including the calculation-and-direction combination that D2.3 exam questions love.

D2.3 questions pair a short calculation with a stated, justified direction, and our marking engine awards them analytically — each distinct valid point is a separate mark. Method marks (M) credit correct reasoning or a correct set-up, answer marks (A) credit a correct value or conclusion, and error-carried-forward (ECF) means a wrong number early on does not cost you the marks that follow, provided your method is written down. Study how each mark below is tied to a specific, named step rather than to loose phrasing.

Water potential is the engine behind water transport across the whole plant: a gradient of steadily falling $\Psi$ runs from moist soil, through the root and up the xylem under tension, to the leaf air spaces and out into a dry atmosphere with an extremely negative water potential. The same signed-number reasoning underlies osmoregulation in animals, the design of intravenous and rehydration fluids, and why salting food or over-fertilising soil draws water out of cells. Master the habit — work each region's $\Psi = \Psi_s + \Psi_p$, compare, and let water fall to the more negative value — and you can predict water movement in any system you meet.