Explain water movement and responses in plant cells
Water potential predicts movement across cell membranes; plant cell walls resist expansion and help cells become turgid.
Section 4.2 · Movement into and out of cells
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain membrane transport and investigation reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Relate phospholipid and protein structure to their roles, and distinguish a concentration gradient from the direction of net movement.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What resists expansion as water enters a plant cell?
What you will learn
- Explain water movement between plant cells and solutions using water potential
- Relate water uptake and loss to turgid, flaccid and plasmolysed states
Water moves down the water-potential gradient
Across a partially permeable membrane, net water movement is from higher water potential to lower water potential. A solution with a more negative water potential than the cell tends to draw water out; a solution with a less negative water potential tends to drive water in.
The syllabus does not require solute potential and pressure potential. Use the supplied overall water-potential values rather than decomposing them into components.
A cell wall changes the outcome
When water enters a plant cell, the vacuole expands and the cell contents press against the cell wall. The wall resists further expansion, producing turgor; a turgid cell is firm.
Water loss reduces turgor. A cell can become flaccid; with substantial water loss, the cell surface membrane may pull away from the wall (plasmolysis). The wall maintains an outer shape but does not prevent water movement.
Pause and explain
What term describes a firm plant cell with high turgor?
Worked example
A plant cell at ψ = −300 kPa is placed in a solution at ψ = −100 kPa. Predict water movement and the likely pressure effect.
Show the worked solution
- The solution has higher water potential because −100 is less negative.
- Net water enters the cell, increasing turgor against the wall.
Answer Net water movement is from the solution (−100 kPa) into the cell (−300 kPa). The vacuole expands and the contents press against the cell wall, so turgor increases and the cell becomes more turgid.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A plant cell is placed in a solution with lower water potential than the cell. State the net movement and one likely cell-state change.
Give me a hint
Compare water potentials before naming the direction.
Compare my reasoning
- Water moves from the cell’s higher potential toward the lower-potential solution.
- Water loss reduces turgor; the cell may become flaccid.
Net water moves out of the cell into the lower-water-potential solution. Loss of water reduces turgor, so the cell may become flaccid; with greater loss, plasmolysis can occur.
Look for these in your work
- Direction correct
- Plant-specific consequence stated
Explain how a plant cell becomes turgid when placed in a dilute solution.
Give me a hint
Include water potential, vacuole and wall resistance.
Compare my reasoning
- Water enters down the water-potential gradient.
- The contents push against the wall, which resists expansion.
If the dilute solution has higher water potential than the cell, water enters by osmosis. The vacuole expands and cell contents press against the wall; wall resistance creates turgor and the cell becomes firm.
Look for these in your work
- Water-potential direction correct
- Wall and turgor link included
A plant cell loses water but retains its cell wall. Explain why its shape and turgor do not change in the same way as an animal cell.
Give me a hint
Contrast wall support with loss of internal pressure.
Compare my reasoning
- Water loss reduces internal volume and pressure.
- The wall remains as an outer support while the contents can pull away.
Water loss reduces the volume and pressure of the cell contents. The cell wall remains as an outer supporting structure, so the cell becomes flaccid and may become plasmolysed: its membrane can pull away from the cell wall; it does not simply collapse into a crenated shape.
Look for these in your work
- Turgor effect explained
- Wall-specific outcome distinguished
Common mistakes
- Reversing the direction for negative water potentials
- Saying the cell wall prevents osmosis
- Using animal-cell lysis language for a walled plant cell
What can you explain now?
What resists expansion as water enters a plant cell?
Compare with the explanation
The cell wall
The wall resists the pressure from the cell contents.
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Make it stick
How does water potential predict plant-cell water movement, and how do cell walls affect the result?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.