14.5 Tropic responses
- Syllabus
- 0610–2026–2027
- Topic
- 14.5
- Level
- —
Learning objectives
14.5.1Gravitropism as a response in which• Describe gravitropism as a response in which parts of a plant grow towards or away from gravity14.5.2Phototropism as a response in which• Describe phototropism as a response in which parts of a plant grow towards or away from the direction of the light source14.5.3Gravitropism and phototropism• Investigate and describe gravitropism and phototropism in shoots and roots14.5.4Phototropism and gravitropism• Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth14.5.5Role of auxin in controlling shoot• Explain the role of auxin in controlling shoot growth, limited to: (a) auxin is made in the shoot tip (b) auxin diffuses through the plant from the shoot tip (c) auxin is unequally distributed in response to light and gravity (d) auxin stimulates cell elongation
Define gravitropism
Gravitropism is a growth response in which part of a plant grows towards or away from gravity.
| Response | Direction | Typical example |
|---|---|---|
| positive gravitropism | towards gravity | roots grow downwards |
| negative gravitropism | away from gravity | shoots grow upwards |
Gravitropism is directional growth in response to gravity, not simply any downward or upward movement.
Define phototropism
Phototropism is a growth response in which part of a plant grows towards or away from the direction of a light source.
| Response | Direction | Typical example |
|---|---|---|
| positive phototropism | towards light | shoots grow towards a one-sided light source |
| negative phototropism | away from light | roots may grow away from light |
Phototropism is directional growth caused by the direction of light; photosynthesis uses light but is not a tropic response.
Investigate tropic responses in roots and shoots
A tropism investigation changes one directional stimulus and records the direction of root and shoot growth.
| Organ | Gravity response | Light response |
|---|---|---|
| root | grows towards gravity | commonly grows away from light |
| shoot | grows away from gravity | grows towards light |
| Design feature | Purpose |
|---|---|
| germinating seedlings of similar age | comparable starting material |
| one-sided light or darkness | isolate light direction |
| horizontal placement or clinostat control | test or cancel a constant gravity direction |
| keep water, temperature and time constant | fair comparison |
| record starting and final root/shoot directions | describe the growth response |
A clinostat rotates seedlings so gravity does not act continuously on one side; it is a control for directional gravity, not a source of light.
Explain tropisms as chemical control of shoot growth
Phototropism and gravitropism of shoots are chemical growth responses controlled by auxin.
directional stimulus → unequal auxin distribution → unequal cell elongation on opposite sides → shoot curves
In a shoot, the side with more auxin elongates more, so the unequal growth produces curvature towards light or away from gravity.
The shoot bends because its two sides elongate at different rates, not because mature cells move from one side to the other.
Explain how auxin controls shoot growth
Auxin is made in the shoot tip and controls curvature by changing cell elongation below the tip.
| Order | Event |
|---|---|
| 1 | auxin is made in the shoot tip |
| 2 | auxin diffuses through the plant away from the tip |
| 3 | light or gravity causes auxin to become unequally distributed |
| 4 | auxin stimulates cell elongation in the shoot |
| 5 | cells on the side with more auxin elongate more, so the shoot bends |
With light from one side, more auxin accumulates on the shaded side; greater elongation there bends the shoot towards the light.
Auxin is produced in the shoot tip but acts on elongating cells below it; it stimulates cell elongation rather than cell movement.