Explain how the closure of stomata in a leaf is brought about when a plant experiences water stress.

OCR A-Level Biology A (H420) — 5.5 Plant and animal responses · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Plants growing in arid conditions frequently experience water stress — a state in which the rate of water loss from leaves exceeds the rate of water uptake from the roots. Under these conditions, guard cells respond to the hormone abscisic acid (ABA) to reduce further water loss.

Model answer (4 marks)

Water stress causes mesophyll cells to release abscisic acid (ABA). ABA binds to receptors on guard cells, triggering potassium ions (K⁺) to exit the guard cells through ion channels. The loss of K⁺ lowers the solute concentration inside the guard cells, raising their water potential. Water then leaves the guard cells by osmosis, reducing turgor pressure and causing the stomatal pore to close.

Examiner tips

Mention ABA release from mesophyll cells first. Show the sequence: ABA → receptor → K⁺ efflux → ↑ water potential → water loss → loss of turgor → closure.
Use the exact terms: abscisic acid, guard cells, potassium ions, ion channels, water potential, turgor pressure, stomatal pore.

Common mistakes

Confusing the direction of ion movement (e.g., saying K⁺ enters instead of exits).
Failing to link the rise in water potential to water loss by osmosis.
Using vague phrases like "water stress causes closure" without detailing the ABA pathway.

Mark scheme (4 marks)

Water stress triggers the release of abscisic acid (ABA) from mesophyll cells
ABA binds to receptors on guard cells, causing potassium ions (K⁺) to leave the guard cells via ion channels
Loss of K⁺ reduces the solute concentration inside the guard cell, raising the water potential of the guard cell
Water leaves the guard cells by osmosis, the cells lose turgor and the stomatal pore closes

Key terms in this question

water stress · stomata