1.3.4Structure and Functions in Living Organisms: Part 2
Response to Stimuli: Plants
5 min read 6 parts Lesson 20 of 29 Unit quiz
Section 1 of 6Tropisms
Directional growth
- Directional growth A tropism is a growth response in which a plant grows towards or away from a stimulus.
- Plants cannot move from place to place, so growing in the right direction is how they respond.
Positive and negative
- Positive and negative Growth towards the stimulus is a positive tropism.
- Growth away from the stimulus is a negative tropism.
Why it matters
- Why it matters Shoots grow towards light so that leaves can absorb more light for photosynthesis.
- Roots grow downwards to anchor the plant and to reach water and mineral ions in the soil.
Section 2 of 6Light and gravity
| Stimulus | Name of response | Shoots | Roots |
|---|---|---|---|
| Light | Phototropism | Positive: grow towards light | Negative: grow away from light |
| Gravity | Geotropism (gravitropism) | Negative: grow upwards, away from gravity | Positive: grow downwards, towards gravity |
Section 3 of 6Auxin and the bending of a shoot
1Auxin made in the tip
- Auxin made in the tip Auxins are plant growth regulators, similar in role to hormones in animals.
- Most is produced in the tip of the shoot.
2Light from one side
- Light from one side Auxin moves to the shaded side of the shoot, so the shaded side holds a higher concentration.
3Shaded cells elongate
- Shaded cells elongate Auxin diffuses down to the region just behind the tip and makes cells there grow longer.
- More auxin gives faster elongation.
4Shoot bends
- Shoot bends Because the shaded side grows faster than the lit side, the shoot curves towards the light.
Section 4 of 6Key points about auxin
Auxin
Even light
- Even light When light shines evenly all round, auxin spreads equally and the shoot grows straight upwards.
Uneven light
- Uneven light One-sided light causes auxin to collect on the shaded side, producing unequal growth.
Elongation, not division
- Elongation, not division Auxin makes the existing cells longer; it does not increase the number of cells.
Growth zone
- Growth zone Only the region behind the tip responds to auxin by growing, which is why the bending happens there.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Tropism | A directional growth response of a plant to a stimulus. |
| Phototropism | Growth in response to the direction of light. |
| Geotropism | Growth in response to gravity, also called gravitropism. |
| Auxin | A plant growth regulator that controls cell elongation. |
| Cell elongation | The lengthening of existing cells, causing growth without extra cells. |
Section 6 of 6Exam technique
1
Writing that the shoot 'grows towards the light' without saying that cells on the shaded side elongate faster.
2
Saying auxin makes cells divide; the correct idea is that it makes cells elongate.
3
Placing the extra auxin on the lit side — it accumulates on the shaded side.
4
Mixing up the signsroots show positive geotropism and negative phototropism, while shoots show the opposite.