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A2 · Practice questions · Control and coordination in plants

Signals without nerves.

Six original Cambridge-style questions on plant coordination, from the snap of a Venus fly trap to planning a gibberellin experiment with barley grains.

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

Snap, stretch, sprout.

01
Explain
[5 marks]

Describe the response of a Venus fly trap to an insect landing on its lobes, and explain how closure is achieved.

  • The insect bends trigger hairs on the inner surface of the lobes. ✓
  • Ion channels open and a receptor potential is produced in cells at the base of the hair. ✓
  • If two hairs are stimulated within about 20 seconds, action potentials travel across the lobes. ✓
  • Ions and water move rapidly; cell walls loosen as protons are pumped into them and the turgor of cells in the lobes changes. ✓
  • The lobes flip from convex to concave, so the trap snaps shut in a fraction of a second. ✓
02
Explain
[5 marks]

Explain the role of auxin in elongation growth.

  • Auxin is made in the shoot tip and moves to the zone of elongation, where it binds to receptors. ✓
  • It stimulates proton pumps in the cell surface membrane to actively pump H+ into the cell wall. ✓
  • The cell wall becomes acidic (lower pH). ✓
  • This activates expansins, which loosen the bonds between cellulose microfibrils, so the wall becomes more extensible. ✓
  • Water enters by osmosis and turgor pressure stretches the cell, which elongates. ✓
03
Explain
[4 marks]

Describe the role of gibberellin in the germination of a barley grain.

  • The grain absorbs water and the embryo produces gibberellin. ✓
  • Gibberellin diffuses to the aleurone layer. ✓
  • It causes breakdown of DELLA proteins, allowing a transcription factor to switch on the gene for amylase, so amylase is made. ✓
  • Amylase hydrolyses starch in the endosperm to maltose / glucose, which the embryo uses for respiration and growth. ✓
04
Analysis
[3 marks]

Plan an experiment to show that gibberellin stimulates amylase production in barley grains.

  • Cut barley grains in half and keep the halves without embryos; soak them in a range of gibberellin concentrations (including zero as a control). ✓
  • Place the cut halves on starch agar for a fixed time, then flood the plates with iodine solution. ✓
  • Measure the area of the clear zone (no blue-black colour) around each half as a measure of amylase activity; keep temperature, time and number of grains the same. ✓
05
Data
[3 marks]

Coleoptile segments are placed in buffers. In pH 7 buffer with auxin they grow 2.4 mm in 2 hours; in pH 4.5 buffer without auxin they grow 2.2 mm; in pH 7 buffer without auxin they grow 0.4 mm. Explain what these results suggest.

  • Auxin increases growth from 0.4 mm to 2.4 mm at pH 7. ✓
  • An acidic buffer alone (pH 4.5) causes almost as much growth (2.2 mm) as auxin. ✓
  • This supports the idea that auxin works by acidifying the cell wall (proton pumping), and low pH itself loosens the wall so cells elongate. ✓
06
Recall
[2 marks]

State two ways in which coordination in plants differs from coordination in mammals.

  • Plants have no nervous system: no neurones or synapses (although some, like the Venus fly trap, produce action potentials). ✓
  • Plant responses are mostly brought about by plant hormones (such as auxin and gibberellin) changing growth, often slowly, rather than by muscles contracting. ✓

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.