Tropisms — Plant Responses to Stimuli
🌱 Tropisms — Plant Responses to Stimuli
Plants respond to stimuli but more slowly than animals:
| Tropism | Response to | Example |
|---|---|---|
| Phototropism | Light | Shoots grow towards light (positive) |
| Geotropism/Gravitropism | Gravity | Roots grow down (positive); shoots grow up (negative) |
Tap any label to zoom into that step of the auxin mechanism.
- 1Shoot tip — Where auxin is produced. Light is detected here too.
- 2Auxin accumulates on the shaded side — Auxin moves AWAY from light and piles up on the dark side of the stem.
- 3Elongation zone — Cells on the shaded side stretch longer — this is what makes the shoot bend.
- 4Positive phototropism — The outcome — the whole shoot bends toward the light.
- 5Root tip — opposite rule — In roots, high auxin INHIBITS growth. Roots bend DOWNWARD with gravity.
Tap any label to zoom into that step of the auxin mechanism.
Biology glossary
- What is phototropism?
- Phototropism is the growth response of a plant to light. Shoots show positive phototropism — they grow towards the light source.
- What is gravitropism?
- Gravitropism is the growth response of a plant to gravity. Roots show positive gravitropism (grow downward, towards gravity). Shoots show negative gravitropism (grow upward, away from gravity).
Plants respond to stimuli but more slowly than animals:
Earn the mark scheme marks
Memory Aids
"Auxin Avoids the light" — auxin distribution mnemonic: In a shoot lit from one side, auxin moves away from the light (to the shaded side). Higher auxin on the shaded side means more growth on that side, so the shoot bends towards the light. Remember: auxin avoids the light, but the shoot follows the auxin and ends up bending towards light.
Shoot vs Root auxin response — "S+ R-" (Shoots positive, Roots negative):
- Shoots: high auxin = more elongation (+)
- Roots: high auxin = less elongation (-)
- Same hormone, opposite responses because roots are more sensitive
Three plant hormones and their uses — "AGE":
- Auxin: weed killers, rooting powder (tropisms)
- Gibberellin: seed germination, fruit size, brewing (maltose from barley)
- Ethylene: fruit ripening (bananas, transported green)
Now try it yourself
Quiz · Question 1 of 16
When a plant shoot is lit from one side, where does auxin accumulate?
Tap an answer to check it
This topic in real past papers
Every real exam question we've found on plant hormones, with a full worked answer.