Tropic ResponsesCambridge IGCSE Biology: Revision notes
Section 1
What Is a Tropic Response?
A tropism is a growth response of a plant to a directional stimulus. Tropisms are named after the stimulus that causes them.
- Gravitropism (geotropism): a response in which parts of a plant grow towards or away from gravity
- Phototropism: a response in which parts of a plant grow towards or away from the direction of a light source
A response is described as positive if the plant part grows towards the stimulus, and negative if it grows away from it.
Do not confuse gravitropism with phototropism — always state which stimulus (gravity or light) is causing the named response.
Section 2
How Do Shoots and Roots Respond?
Shoots and roots respond in opposite directions to the same two stimuli:
| Organ | Response to light | Response to gravity |
|---|---|---|
| Shoot | Positively phototropic (grows towards light) | Negatively gravitropic (grows away from gravity, upwards) |
| Root | Negatively phototropic (grows away from light) | Positively gravitropic (grows towards gravity, downwards) |
These responses can be investigated experimentally, for example by growing seedlings in a box with a single light source entering from one side, or by fixing germinating seeds at different angles and observing the direction of shoot and root growth over several days.
A seedling placed on its side will bend its shoot tip upwards and its root tip downwards within a day or two, showing negative and positive gravitropism respectively.
Section 3
Why Are Tropic Responses Useful to a Plant?
Phototropism and gravitropism in shoots are examples of the chemical control of plant growth — they are not caused by muscle movement (plants have none) but by a growth-regulating chemical that changes the rate of cell elongation on different sides of the shoot.
Growing shoots towards light maximises light capture for photosynthesis, while growing roots towards gravity (and away from light) anchors the plant and directs roots towards water and mineral ions in the soil.
Section 4
How Does Auxin Produce a Tropic Response?
Auxin is the plant hormone responsible for tropic responses in shoots:
- Auxin is made in the shoot tip
- Auxin diffuses backwards through the plant from the tip
- Auxin becomes unequally distributed across the shoot in response to light or gravity
- Auxin stimulates cell elongation on the side of the shoot where it accumulates
- Unequal elongation on the two sides causes the shoot to bend
In phototropism, auxin accumulates on the shaded side of the shoot, so that side elongates more and the shoot bends towards the light. In gravitropism, auxin accumulates on the lower side of a horizontal shoot, causing more elongation there and upward bending.
Remember the sequence: made in the tip → diffuses down → unequal distribution → unequal elongation → bending. Marks are often given for each stage.
Must Know
- Gravitropism = growth response to gravity; phototropism = growth response to light
- Shoots: positively phototropic, negatively gravitropic
- Roots: negatively phototropic, positively gravitropic
- Tropic responses in shoots are controlled by the chemical auxin, not nerves or muscles
- Auxin is made in the shoot tip and diffuses down, becoming unequally distributed in response to light or gravity
- Auxin stimulates cell elongation on the side where it accumulates, causing the shoot to bend
That's the notes covered.
Carry on to the next subtopic.