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Coordination and Response: Question 10

Syllabus 14.5

Structured Extended 6 marks

A student places a young bean seedling on its side inside a dark box for two days, so that its root and shoot both lie horizontally. When the student checks the seedling, the root has curved downwards and the shoot has curved upwards.

(a) State the name given to this growth response to gravity, and state whether the root and the shoot each show a positive or a negative form of this response. [2]

(b) The plant hormone auxin accumulates on the lower side of both the horizontal root and the horizontal shoot. Explain, in terms of the effect of auxin on cell elongation, why the root curves downwards while the shoot curves upwards. [4]

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Worked solution

Part (a): Naming the response

A growth response to gravity is called gravitropism (also known as geotropism).

  • The root grows towards the direction of gravity, so it shows a positive gravitropic response.
  • The shoot grows away from the direction of gravity, so it shows a negative gravitropic response.

Part (b): Explaining the opposite curvatures

In both the horizontal root and the horizontal shoot, auxin moves towards and accumulates in a higher concentration on the lower side. However, root cells and shoot cells respond very differently to the same concentration of auxin:

  1. In the root: a high concentration of auxin inhibits cell elongation. On the lower side, where auxin has accumulated, the cells elongate less. The cells on the upper side, with less auxin, elongate more than those on the lower side. This unequal elongation, more growth on the upper side and less on the lower side, causes the root to curve downwards, towards gravity.

  2. In the shoot: a high concentration of auxin stimulates cell elongation. On the lower side, the cells with more auxin elongate more than the cells on the upper side, which have less auxin. This unequal elongation, more growth on the lower side and less on the upper side, causes the shoot to curve upwards, away from gravity.

So the same distribution of auxin, more on the lower side in both organs, produces opposite curvatures because auxin has opposite effects on cell elongation in roots compared with shoots.

Final answers

  • (a) Gravitropism (geotropism); the root shows a positive response, the shoot shows a negative response.
  • (b) Auxin accumulates on the lower side of both organs. In the root this inhibits elongation on the lower side, so it curves downwards; in the shoot this stimulates elongation on the lower side, so it curves upwards.