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A2 · Practice questions · Stomata: homeostasis in plants

Turgid open, flaccid shut.

Six original Cambridge-style questions on stomata and guard cells, from the opening mechanism to the role of abscisic acid in a drought.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Ions, osmosis, shape.

01
Explain
[2 marks]

Explain why the regulation of stomatal aperture is described as a balance.

  • Stomata must open for carbon dioxide to diffuse into the leaf for photosynthesis. ✓
  • But open stomata also allow water vapour to diffuse out (transpiration), so they close when the risk of water loss is high. ✓
02
Explain
[5 marks]

Describe the mechanism by which a stoma opens in the light.

  • ATP (from light-dependent reactions / mitochondria) powers proton pumps that move H+ out of the guard cells. ✓
  • The inside of the cell becomes more negative, so K+ channels open and K+ ions diffuse in (malate is also produced). ✓
  • The water potential of the guard cells falls below that of the neighbouring cells. ✓
  • Water enters by osmosis and the guard cells become turgid. ✓
  • The inner wall is thicker / less elastic than the outer wall, so the cells curve apart and the pore opens. ✓
03
Recall
[3 marks]

Describe the structure of guard cells, giving three features.

  • Two kidney- (sausage-) shaped cells surrounding a pore. ✓
  • Cell wall next to the pore (inner wall) is thicker than the outer wall; cellulose microfibrils are arranged radially. ✓
  • They contain chloroplasts (and many mitochondria), unlike other epidermal cells; no plasmodesmata connect them to neighbouring cells. ✓
04
Explain
[4 marks]

Explain the role of abscisic acid in the closure of stomata when a plant is short of water.

  • Water stress causes roots and leaves to produce abscisic acid (ABA). ✓
  • ABA binds to receptors on the cell surface membrane of guard cells. ✓
  • This causes calcium ions to enter the cytoplasm, acting as a second messenger, which opens channels so that K+ (and anions) leave the guard cells. ✓
  • The water potential of the guard cells rises, water leaves by osmosis, the cells become flaccid and the stoma closes, reducing water loss. ✓
05
Data
[3 marks]

The stomatal aperture of a plant is measured over 24 hours. It is 2 μm at 03:00, starts to rise at 05:00 before sunrise at 06:00, reaches 10 μm at 11:00, falls to 7 μm at 14:00 on a hot, dry afternoon and is 2 μm again at 22:00. Explain these observations.

  • Opening starts before sunrise, showing a daily (circadian) rhythm controlled by an internal clock. ✓
  • The aperture is largest in the morning light (fivefold increase from 2 to 10 μm), allowing CO2 uptake for photosynthesis. ✓
  • The partial closure at 14:00 reduces water loss when transpiration would be high (possibly ABA / water stress); closure at night as no CO2 is needed in the dark. ✓
06
Analysis
[3 marks]

A student treats leaf epidermis with abscisic acid and a substance that prevents calcium ions entering the guard cell cytoplasm. Predict and explain the effect on the stomata compared with abscisic acid alone.

  • Prediction: the stomata stay open (or close much less) than with ABA alone. ✓
  • Ca2+ is the second messenger that links ABA binding to the opening of ion channels. ✓
  • Without a rise in Ca2+, K+ and anions do not leave, the water potential does not rise and water does not leave by osmosis. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.