The big idea: Water always moves by osmosis from a higher water potential to a lower water potential. The overall water potential of a cell is set by two things added together.
Solute potential (Ψs) — the pull of the dissolved solutes inside the cell. The more solute, the lower (more negative) the water potential.
Pressure potential (Ψp) — the outward push of the cell contents against the cell wall. This push raises the water potential.
Put together: water potential = solute potential + pressure potential (Ψ = Ψs + Ψp). The wall is what makes this matter — it lets a pressure potential build up that an animal cell cannot.
- Water potential (Ψ)
- A measure of the tendency of water to leave a cell or solution by osmosis. Water moves from a higher (less negative) to a lower (more negative) water potential.
- Solute potential (Ψs)
- The part of the water potential caused by dissolved solutes. Adding solutes lowers (makes more negative) the water potential; it is always zero or negative.
- Pressure potential (Ψp)
- The part of the water potential caused by physical pressure — in a plant cell, the outward push of the contents against the cell wall. It usually raises the water potential.
- Cell wall
- The rigid outer layer of a plant cell. It resists expansion, so as water enters it lets a pressure (turgor) build up instead of letting the cell burst.
- Turgor
- The firmness of a plant cell when it is full of water and pushing against its wall — the result of a high pressure potential.
Why the wall changes everything: An animal cell has no wall, so when water rushes in it just keeps swelling and can burst.
A plant cell's wall resists, so water entry instead builds up a pressure potential — the cell becomes firm (turgid) rather than bursting.
That extra pressure term is exactly why a walled cell needs two potentials to describe it, not one.
The water potential of a cell is just its solute potential plus its pressure potential (Ψ = Ψs + Ψp). To predict which way water moves, work out the water potential of the cell and of the solution around it, then let water flow from the higher value to the lower value.
The two parts pull in opposite directions: solutes lower the water potential, while pressure raises it. The balance between them decides what happens.
Solute potential — solutes pull water in: Dissolved solutes lower the water potential (make it more negative).
The more concentrated the cell contents, the more negative the solute potential, and the stronger the pull drawing water into the cell.
A solute potential is always zero (pure water) or negative — it can never be positive.
Pressure potential — the wall pushes back: As water enters, the cell contents press outward on the cell wall, and the wall pushes back. This pressure potential raises the water potential (makes it less negative).
In a fully turgid cell the pressure potential is high and positive; in a flaccid cell it falls to about zero.
This is the term an animal cell does not have — only a walled cell can build it up.
Solute potential (Ψs)
- Caused by dissolved solutes
- Lowers the water potential (more negative)
- More solute → stronger pull on water
- Always zero or negative
Pressure potential (Ψp)
- Caused by the wall pushing back
- Raises the water potential (less negative)
- High in a turgid cell, near zero when flaccid
- The term a walled cell has and an animal cell lacks
What happens in each solution: Hypotonic (dilute) outside: the solution's water potential is higher than the cell's, so water enters. The cell swells, pressure potential rises, and it becomes turgid — the wall stops it bursting.
Hypertonic (concentrated) outside: the solution's water potential is lower than the cell's, so water leaves. The cell goes flaccid, and with more water loss the membrane pulls away from the wall — the cell is plasmolysed.
| Solution outside | Net water movement | Effect on the walled cell |
|---|---|---|
| Hypotonic (more dilute outside) | Water enters the cell | Cell swells, pushes on the wall, pressure potential rises — the cell becomes turgid (firm). It does NOT burst — the wall resists. |
| Isotonic (equal) | No net movement | The cell is at equilibrium — neither turgid nor plasmolysed (sometimes called incipient plasmolysis at the point of change). |
| Hypertonic (more concentrated outside) | Water leaves the cell | The cell loses water and goes flaccid; with more loss the membrane pulls away from the wall — the cell is plasmolysed. |
A memory hook: Solutes Suck water in; the wall Pushes back. As a turgid cell takes in water, its rising pressure potential raises the water potential until it equals the outside — and net flow stops.
Lose too much water and the pressure potential drops to zero: the cell goes flaccid, then plasmolysed.
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How this is tested: On Paper 2/3 an Outline question asks how solute potential and pressure potential combine to give a walled cell's water potential — give both parts and the rule that water moves from a higher to a lower water potential.
A longer Explain question asks for the effect on a plant cell in hypotonic vs hypertonic solutions: water entry → turgor; water loss → flaccid → plasmolysis.
Membrane and cell topics are Paper-1B data favourites — you may be given water-potential values for a cell and a solution and asked to deduce the direction of net water movement.
IB-style question — outline how the two potentials combine
Outline how solute potential and pressure potential combine to give the water potential of a plant cell, and how this determines the direction of water movement. [4]
How to score all four marks
- State the relationship. A plant cell's water potential = solute potential + pressure potential (Ψ = Ψs + Ψp).
- Solute potential. Dissolved solutes lower the water potential (make it more negative), pulling water in.
- Pressure potential. The cell contents push on the wall, and this pressure potential raises the water potential (makes it less negative).
- The rule for direction. Water moves by osmosis from the higher water potential to the lower one, so the combined value of cell and solution decides whether water enters or leaves. (Award 1 mark per distinct point, max 4.)
Final answer
Water potential = solute potential + pressure potential. Solutes lower it; the wall's pressure potential raises it; water then moves from the higher water potential to the lower one.
✓ Why this scores full marks: It names both contributions, states the direction each pushes the water potential (solutes down, pressure up), and gives the rule that water flows from high to low water potential.
A 4-mark 'outline' needs four separate scoring points — not the same idea reworded.
| Contribution | What it is | What it does to water potential |
|---|---|---|
| Solute potential (Ψs) | The effect of dissolved solutes inside the cell | Always lowers (makes more negative) the water potential — more solutes means a stronger pull on water |
| Pressure potential (Ψp) | The outward push of the cell contents against the cell wall | Usually raises (makes less negative) the water potential as the cell fills and the wall pushes back |
| Water potential (Ψ) | The combined result: Ψ = Ψs + Ψp | Sets the overall tendency of water to enter or leave — water moves from higher Ψ to lower Ψ |