Cambridge O Level Biology · Syllabus 5090 · Coordination and Response in Plants
Gravitropism
What is Gravitropism?
A response in which parts of a plant grow toward or away from gravity. Roots are usually positively gravitropic, growing toward gravity into the soil for anchorage and for the absorption of water and mineral ions; shoots are usually negatively gravitropic, growing away from gravity so that the shoot rises clear of the soil.
This definition is part of the Coordination and Response in Plants chapter in Cambridge O Level Biology.
Common mistakes with Gravitropism
- 12. “The root must work the same way as the shoot.” Why it is wrong: applying “more auxin on the lower side, so it elongates more” to a root predicts the root curving upward, which is the opposite of what is observed. SayThe root grows toward gravity — positive gravitropism — which anchors the plant and reaches water and mineral ions. State the observable response and its advantage, and stop there.
- 14. “Light does not need controlling in a gravitropism experiment.” Why it is wrong: if directional light is present, any curvature could be a phototropic response instead, and the experiment cannot answer its own question. SayThe seeds are kept in darkness, or in even all-round light, so that gravity is the only directional stimulus acting.
Questions students ask about Gravitropism
What is gravitropism?
Gravitropism is a response in which parts of a plant grow toward or away from gravity. Roots are usually positively gravitropic, growing toward gravity into the soil, and shoots are usually negatively gravitropic, growing away from it.
Does positive mean the response is good for the plant?
No. Positive means the organ grows toward the stimulus and negative means it grows away from it. Both kinds of response are useful: a shoot's negative gravitropism is what lifts it out of the soil.
Why does a horizontal shoot grow upward?
Gravity acts as a directional stimulus, and auxin becomes more concentrated on the lower side of the shoot. Auxin stimulates cell elongation, so the lower side elongates more and becomes longer than the upper side, curving the shoot upward. This is negative gravitropism.
Can the auxin explanation be used for roots?
Not unless the question supplies the root mechanism. The syllabus states that auxin stimulates cell elongation in shoots. For a root, describe the observable response — it grows toward gravity — classify it as positive gravitropism and give its advantage: anchorage and the uptake of water and mineral ions.
How do you investigate gravitropism?
Pin several germinating seeds horizontally against damp support inside a transparent container, mark the starting positions of the roots and shoots, and keep the container in darkness so that light cannot act as a second directional stimulus. Record the direction and angle of growth at intervals, using a slowly rotating clinostat as the control.