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IGCSE · Practice questions · Tropic responses

Bending by growing.

Six original Cambridge-style questions on tropic responses: definitions, investigations with controls, data on shoot curvature and (Extended) how auxin makes a shoot bend.

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

Direction, growth, auxin.

01
Recall
[2 marks]

Define gravitropism and phototropism.

  • Gravitropism: a response in which parts of a plant grow towards or away from gravity. ✓
  • Phototropism: a response in which parts of a plant grow towards or away from the direction of the light source. ✓
02
Analysis
[4 marks]

Describe how you would investigate the response of shoots to light coming from one direction. Include a control.

  • Grow several young seedlings (same type and age) in pots. ✓
  • Put one group in a light-proof box with a slit in one side, so light enters from one direction only. ✓
  • Control: an identical group lit evenly from above (or rotated on a clinostat); keep temperature and water the same. ✓
  • After 2 to 3 days, record the direction of growth or measure the angle of curvature of the shoots. ✓
03
Data
[4 marks]

Shoots were lit from one side for different times and the angle of bending was measured.

Time / hours02468
Mean angle of bending / °06183035

(a) Describe the results. (b) Calculate the mean rate of bending between 2 and 6 hours.

  • (a) There is no bending at first; bending begins slowly, increases fastest between 2 and 6 hours, then the rate slows between 6 and 8 hours. ✓
  • The angle increases from 0 to 35° over 8 hours. ✓
  • (b) Change in angle = 30 − 6 = 24° over 4 hours. ✓
  • Rate = 24 ÷ 4 = 6° per hour. ✓
04
Explain
[3 marks]

A seedling is laid horizontally in a dark cupboard for two days. Describe and explain the direction of growth of its shoot and root.

  • The shoot curves and grows upwards, away from gravity: negative gravitropism. ✓
  • The root curves and grows downwards, towards gravity: positive gravitropism. ✓
  • It is kept in the dark, so the responses are to gravity, not light; growing down helps the root anchor the plant and absorb water, and growing up helps the shoot reach light. ✓
05
Explain
[5 marks]

Extended. Explain how auxin causes a shoot to grow towards light coming from one side.

  • Auxin is made in the shoot tip. ✓
  • It diffuses down the shoot from the tip. ✓
  • Light from one side causes auxin to be unequally distributed: more collects on the shaded side. ✓
  • Auxin stimulates cell elongation, so cells on the shaded side elongate more than those on the lit side. ✓
  • The shaded side grows longer, so the shoot bends towards the light. This is chemical control of plant growth. ✓
06
Analysis
[3 marks]

Extended. The tip of a shoot is cut off and the shoot is lit from one side. It does not bend. In a second experiment, a block of agar that had been in contact with a cut shoot tip is placed on the left half of a cut stump, and the shoot is kept in the dark. It bends to the right. Explain these results.

  • Removing the tip removes the source of auxin, so there is no auxin to stimulate unequal growth and the shoot does not bend. ✓
  • Auxin from the tip diffused into the agar block, and from the block it diffuses down the left side of the shoot only. ✓
  • Auxin stimulates cell elongation, so cells on the left side elongate more and the shoot bends to the right, even in the dark: growth is controlled chemically. ✓

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.