AQA GCSE (9-1) Biology: Plant Hormones

Key Takeaways for Plant Hormones

1. Plant Hormones Overview

• Definition: Chemicals produced by plants to regulate growth and responses to stimuli.
• Key Hormones:
  • Auxins (e.g., IAA): Control cell elongation, phototropism, and gravitropism.
  • Gibberellins: Promote stem elongation, seed germination, and flowering.
  • Ethene: Triggers fruit ripening, leaf fall, and flower opening.

2. Tropisms

• Definition: Directional growth responses to stimuli.
  • Positive Phototropism: Shoots grow towards light (e.g., sunflower stems bending towards sunlight).
  • Positive Gravitropism: Roots grow downwards in response to gravity (e.g., carrot taproots).
  • Hydrotropism: Roots grow towards water (overpowers gravitropism; e.g., cress roots in a Petri dish with water droplets).
• Tip: Remember “Phototropism = Plants reach for Photons.”

3. Auxins and Their Role

• Function:
  • Cause cell elongation (cells on the shaded side elongate more, bending the stem towards light).
  • Produced in shoot/root tips and diffuse to other regions.
• Key Experiment (Darwin’s Seedlings):
  • Seedlings with tips removed did not bend towards light, proving auxins are made in the tip.
  • Mica/gelatine barriers showed auxins diffuse down the dark side (impermeable mica blocks bending; gelatine allows diffusion).

4. Experiments on Plant Hormones

• Light Direction Experiment (Yoghurt Pots):
  • Pot A (No Light): Shoot grows straight (no light stimulus).
  • Pot B (Light from Top): Shoot grows straight upwards.
  • Pot C (Light from Side): Shoot bends towards the light.
• Auxin Concentration Experiment:
  • Data Table:IAA (mg/dm³)Growth Angle (°)0023488131219
  • Math Tip: Plotting a graph of concentration vs. angle allows prediction. For 6 mg/dm³, interpolate between 4 and 8 mg/dm³:
    Angle≈8+(6−4)(8−4)×(13−8)=10.5∘Angle≈8+(8−4)(6−4)​×(13−8)=10.5∘

5. Other Plant Hormones

• Ethene:
  • Uses: Ripens fruit (e.g., bananas), causes “one bad apple spoils the bunch” effect by diffusing ethene.
  • Experiment: Starch test measures ripeness (less starch = riper; iodine turns blue-black with starch).
• Gibberellins:
  • Uses: Break seed dormancy, increase fruit size (e.g., seedless grapes).

6. Applications of Plant Hormones

• Selective Weedkillers: Auxins cause broadleaved weeds to overgrow and die (e.g., lawn treatments).
• Rooting Powder: Contains auxins to stimulate root growth in cuttings (e.g., geranium propagation).
• Tissue Culture: Auxins/gibberellins grow clones from plant cells.
• Fruit Ripening: Ethene sprayed on shipped fruit to ripen before sale.

7. Practical Tips

• Controlled Variables:
  • Keep seedlings in the dark after auxin application to ensure light doesn’t interfere.
  • Use multiple seedlings for reliability (e.g., Joe’s experiment with 5 cuttings).
• Graph Skills:
  • Label axes (e.g., Concentration of IAA (mg/dm³) vs. Angle of Growth (°)).
  • Use a line of best fit for predictions.

8. Key Terms & Definitions

• Cell Elongation: Lengthening of plant cells due to auxins.
• Reproducible Results: Same results when others repeat experiments (e.g., starch tests for ripeness).
• Propagation: Asexual reproduction using cuttings (creates clones).

Exam Tips

• 6-Marker Strategy: For experiments, describe method, control variables, and expected results (e.g., “Explain how auxins cause phototropism”).
• Diagrams: Draw auxin distribution in shoots (higher on shaded side → bending).
• Memory Aid: “Auxins Affect Angles; Ethene Enhances Edibility.”

50 GCSE Questions on Plant Hormones

Section A: Definitions & Key Concepts

1. Define positive phototropism.
2. What is positive gravitropism?
3. Explain the term hydrotropism.
4. Define cell elongation.
5. What is a tropism?

Section B: Plant Hormones

6. Name three plant hormones.
7. What is the primary role of auxins?
8. Describe two functions of gibberellins.
9. How does ethene affect fruit ripening?
10. Why does “one bad apple spoil the bunch”?

Section C: Tropisms & Experiments

11. Explain why plant shoots grow towards light.
12. Why do roots grow downwards?
13. Describe Darwin’s experiment with grass seedlings.
14. What happens if the tip of a shoot is removed? Why?
15. How did inserting a mica barrier into a shoot affect its growth?

Section D: Auxin Experiments

16. A seedling’s shoot bends towards light. Explain how auxins cause this.
17. In an experiment, a shoot tip is covered with clingfilm. Predict the result.
18. Using Table 13.1, calculate the mean growth angle for seedlings treated with 8 mg/dm³ IAA.
19. Plot a graph for Table 13.1. Predict the angle at 6 mg/dm³ IAA.
20. Why were seedlings kept in the dark after applying IAA?

Section E: Applications of Hormones

21. How do selective weedkillers work?
22. What is rooting powder, and how is it used?
23. Explain how tissue culture uses plant hormones.
24. Why are bananas sprayed with ethene before sale?
25. How do gibberellins help in agriculture?

Section F: Data Analysis

26. Joe tested two rooting powders (Table 13.2). Calculate the mean for Rapid-Root.
27. Does Rapid-Root’s claim of “75% more roots” hold? Show calculations.
28. Explain why iodine solution is used to test fruit ripeness.
29. If Apple A turns blue-black with iodine, what does this indicate?
30. What makes experimental results reproducible?

Section G: Practical Investigations

31. Describe how to investigate light’s effect on seedling growth.
32. Why use four Petri dishes in the radish seedling experiment?
33. What is the purpose of a control in an experiment?
34. How would you test if roots grow towards water?
35. Why use multiple seedlings in experiments?

Section H: Advanced Questions

36. Compare agriculture and horticulture.
37. Why might a root grow upwards in a Petri dish with water droplets?
38. Explain how a shoot bends if auxins diffuse down the dark side.
39. If a shoot tip is cut off and replaced with gelatine, what happens?
40. Why does a shoot with a side-cut mica barrier not bend?

Section I: Maths & Graphs

41. Calculate the percentage increase in growth angle when IAA increases from 4 mg/dm³ to 12 mg/dm³.
42. Using Table 13.1, estimate the angle for 10 mg/dm³ IAA.
43. If a seedling grows 15 mm in 4 days, calculate its daily growth rate.
44. Plot a line graph for Joe’s data (Table 13.2).
45. If 20% of apples are over-ripe (Level 10), how many in a batch of 250?

Section J: Synoptic Questions

46. Explain how plant hormones improve crop yields.
47. Why might overusing selective weedkillers reduce biodiversity?
48. How does phototropism increase photosynthesis?
49. Evaluate the importance of Darwin’s seedling experiments.
50. Why are plant hormones cheaper than manual labour in agriculture?

Detailed Answers

Section A

1. Positive phototropism: Growth of plant shoots towards light (e.g., sunflower stems).
2. Positive gravitropism: Growth of roots downwards in response to gravity (e.g., carrot roots).
3. Hydrotropism: Roots grow towards water (e.g., cress roots in a damp Petri dish).
4. Cell elongation: Lengthening of plant cells due to hormones like auxins.
5. Tropism: Directional growth response to stimuli (e.g., light, gravity).

Section B

6. Auxins, gibberellins, ethene.
7. Auxins control cell elongation, phototropism, and gravitropism.
8. Gibberellins: Promote stem elongation, seed germination, and flowering.
9. Ethene triggers enzymes to break down cell walls, ripening fruit.
10. Rotting apples release ethene, which diffuses and ripens nearby fruit.

Section C

11. Auxins concentrate on the shaded side, elongating cells → shoot bends towards light.
12. Roots sense gravity; auxins inhibit cell elongation on the lower side, causing downward curvature.
13. Darwin covered seedling tips; those with uncovered tips bent towards light, proving tips produce auxins.
14. Shoot doesn’t bend; auxins are produced in the tip.
15. Mica blocked auxin diffusion on the dark side → no bending.

Section D

16. Auxins on the shaded side cause cells to elongate → shoot curves towards light.
17. Shoot bends normally; clingfilm is permeable to auxins.
18. Mean angle = 13°.
19. Prediction:
    Angle≈8+(6−4)(8−4)×(13−8)=10.5∘Angle≈8+(8−4)(6−4)​×(13−8)=10.5∘
20. To ensure light doesn’t interfere with auxin distribution.

Section E

21. Synthetic auxins cause weeds to grow uncontrollably → die.
22. Rooting powder contains auxins to stimulate root growth in cuttings.
23. Hormones in growth medium trigger cell division → cloned plants.
24. Ethene ripens bananas during transport.
25. Gibberellins break seed dormancy and increase fruit size.

Section F

26. Rapid-Root mean:
    11+6+8+7+85=8511+6+8+7+8​=8
27. Claim check:
    Rapid-Root mean = 8; Ready Root = 5.
    8−55×100=60% (Claim false)58−5​×100=60% (Claim false)
28. Iodine turns blue-black with starch; less starch = riper fruit.
29. High starch → unripe (Level 1).
30. Same results when repeated by others using different methods.

Section G

31. Place seedlings under pots with varied light access (top/side/no light) → measure growth direction.
32. To test different light conditions (control, top light, side light, no light).
33. To compare results and isolate variables (e.g., light vs. no light).
34. Grow seeds near water source; observe root direction.
35. To reduce anomalies and improve reliability.

Section H

36. Agriculture: Food/fuel crops. Horticulture: Ornamental/edible plants.
37. Hydrotropism overpowers gravitropism.
38. Cells on dark side elongate more → curvature towards light.
39. Shoot bends; gelatine allows auxin diffusion.
40. Mica blocks auxin movement on the dark side → no elongation.

Section I

41. Percentage increase:
    19−88×100=137.5%819−8​×100=137.5%
42. Estimate: ~16° (extrapolate trend).
43. Daily rate:
    15 mm4 days=3.75 mm/day4 days15 mm​=3.75 mm/day
44. Plot mean roots (Water=1, Ready Root=5, Rapid-Root=8).
45. Over-ripe apples:
    250×0.2=50250×0.2=50

Section J

46. Hormones increase growth rates, fruit size, and germination success.
47. Weedkillers reduce plant diversity → fewer habitats for insects/birds.
48. More light → more photosynthesis → more glucose for growth.
49. Darwin showed auxins are produced in tips, foundational to plant biology.
50. Hormones automate processes (e.g., rooting, ripening), reducing labour costs.