Knowledge Organiser
Part of Plant Hormones · GCSE GCSE Biology revision
This topic summary covers Knowledge Organiser within Plant Hormones for GCSE Biology. Topic 11: Plant Hormones It is section 11 of 12 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 11 of 12
Practice
15 questions
Recall
20 flashcards
Knowledge Organiser
Key Terms
- Auxin: Controls cell elongation; moves to shaded side of shoot
- Phototropism: Growth response to light; shoots positive
- Gravitropism: Growth response to gravity; roots positive (down)
- Gibberellin: Promotes germination, stem elongation, fruit size
- Ethylene: Gas hormone; promotes fruit ripening
- Cell elongation: Auxin increases length of shoot cells; inhibits root cells
Must-Know Facts
- Auxin in shoots: high concentration = more elongation = grows toward light
- Auxin in roots: high concentration = less elongation (opposite to shoots)
- Light from one side → auxin moves to shaded side → unequal growth → bending
- Commercial uses: auxins (weedkillers, rooting powder), gibberellins (germination, fruit size), ethylene (fruit ripening)
- Phototropism = response to light; Gravitropism = response to gravity
- Shoots: positive phototropism, negative gravitropism; Roots: negative phototropism, positive gravitropism
Common Mistakes
- Saying auxin moves towards the light: Auxin moves AWAY from light, accumulating on the shaded side of the shoot. This causes the shaded side to elongate more, bending the shoot towards the light. The direction of auxin movement is the opposite of the direction of bending.
- Forgetting that auxin has opposite effects in roots and shoots: A high concentration of auxin promotes elongation in shoot cells but inhibits elongation in root cells. Roots are more sensitive to auxin than shoots, so the same concentration that promotes shoot growth actually slows root growth.
- Describing tropisms without explaining the mechanism: Stating "the shoot bends towards light" earns only 1 mark. For full marks always explain: auxin redistributes to the shaded side → greater cell elongation on that side → differential growth causes bending towards the light source.
- Confusing gibberellins and auxins: Auxins control cell elongation and tropic responses. Gibberellins promote seed germination, stem elongation, and fruit development. These are separate hormones with different roles — do not attribute gibberellin functions to auxin or vice versa.
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Practice Questions for Plant Hormones
When a plant shoot is lit from one side, where does auxin accumulate?
Explain how auxin causes gravitropism (geotropism) in plant roots.
Quick Recall Flashcards
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