D2.3 Water potential

Leave a wilting lettuce leaf in water and it crisps up; sprinkle salt on a slug and it shrivels. Both are the same physics: water moving by osmosis down a gradient. The challenge for the IB student is to describe that gradient precisely, and the tool for doing so is water potential — a single measure of the tendency of water to leave a solution. Master the convention that water moves from higher to lower water potential and you can predict what happens to any cell placed in any solution, from a red blood cell in pure water to a plant cell in concentrated salt.

Osmosis and the meaning of water potential

Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water potential to a region of lower water potential. It is a passive process — no energy (ATP) is required — driven by the random motion of water molecules.

Water potential (symbol Ψ) measures the tendency of water to move out of a system; you can think of it as how free the water molecules are to move. Two rules make it usable:

• Pure water at standard conditions has the highest water potential, defined as zero.
• Adding solute lowers the water potential, so all solutions have a negative water potential — the more concentrated the solute, the more negative the value.

Water therefore always moves towards the more negative (lower) water potential. Because a more concentrated solution has fewer free water molecules and a lower (more negative) water potential, water moves into it — which matches the everyday observation that water moves towards the saltier side.

Comparing solutions: hypertonic, hypotonic and isotonic

When comparing a cell with the solution around it, three terms describe the relationship — and these are about relative solute concentration:

• Hypertonic: the solution has a higher solute concentration (lower water potential) than the cell, so water leaves the cell.
• Hypotonic: the solution has a lower solute concentration (higher water potential) than the cell, so water enters the cell.
• Isotonic: the solution and cell have equal solute concentrations (equal water potential), so there is no net movement of water.

A common error is to forget the word net: at equilibrium water molecules still cross the membrane in both directions, but the two flows are equal, so there is no overall change. Always describe the gradient in terms of water potential, and remember the comparison is always of the solution relative to the cell.

Effects on animal and plant cells

The same gradient has dramatically different consequences depending on whether a cell has a wall:

• Animal cell in a hypotonic solution: water enters, the cell swells and, having no wall to resist the pressure, may burst (in red blood cells this is called haemolysis).
• Animal cell in a hypertonic solution: water leaves and the cell shrinks and becomes shrivelled (crenation).
• Plant cell in a hypotonic solution: water enters and the cell contents press the membrane against the rigid cell wall. The wall resists, so the cell becomes firm or turgid — the normal, healthy state that supports non-woody plants.
• Plant cell in a hypertonic solution: water leaves, the cell loses turgor (becomes flaccid), and if enough water is lost the membrane pulls away from the wall — plasmolysis.

This is why the wall matters: it prevents a plant cell from bursting in dilute surroundings, instead generating turgor pressure that holds the plant up.

Why osmosis matters: medical and practical applications

Understanding water potential has real consequences. Fluids given intravenously to patients must be isotonic with blood plasma; if pure water were injected it would be strongly hypotonic and red blood cells would take in water and burst. Likewise, tissues and organs for transplant are bathed in isotonic solutions to stop their cells gaining or losing water.

The syllabus also expects familiarity with the practical of estimating the water potential of plant tissue (for example potato) by placing samples in a range of sucrose or salt concentrations and measuring the change in mass or length. Where there is no change, the external solution’s water potential equals that of the tissue. Plotting percentage change against concentration and reading off the point of zero change gives an estimate of the tissue’s water potential — a neat link between the theory and a measurable result.