Read the following and answer the questions that follow:
A biology teacher set up an experiment using freshly germinated bean seeds placed on a wire mesh over a water-filled flask. The setup was placed inside an open-sided box facing a window. After three days, the shoots were observed to bend towards the light and the roots bent away from the light.
(a) Name the type of tropism shown by (i) the shoot and (ii) the root in response to light. Are these responses the same or opposite in direction? (1 mark)
(b) The teacher then placed identical fresh seedlings in a dark box with a small hole on one side. After three days, the shoots bent towards the hole. Name the hormone responsible for this bending, state where it is produced, and explain the mechanism by which it causes the shoot to curve. (2 marks)
(c) Name one other type of tropism in plants and give one specific example from the plant kingdom. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:02 · grounding rag
Model Answer
(a)
(i) Shoot — positive phototropism (bends towards light).
(ii) Root — negative phototropism (bends away from light).
These responses are opposite in direction.
(b)
The hormone responsible is auxin, produced at the shoot tip.
When light falls from one side, auxin diffuses to the shady side of the shoot. This causes cells on the shady side to elongate more than cells on the light side, making the shoot curve/bend towards the light (the hole in the box).
(c)
Geotropism — roots grow downwards (towards gravity) and shoots grow upwards (away from gravity) in response to the pull of the earth.
(Alternate: Chemotropism — growth of pollen tubes towards ovules.)
Source: Chapter 6, Section 6.2.2 — Movement Due to Growth
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Explanation
- (a) Examiners want both the correct term AND the direction qualifier (positive/negative). Stating they are "opposite" is essential for the 1 mark.
- (b) Three points are needed for 2 marks: name of hormone → where produced → mechanism (differential elongation). Don't just name auxin; explain the unequal distribution and resulting unequal cell growth.
- (c) Any valid tropism with a correct example scores the mark. Geotropism is the safest choice as it is explicitly in the textbook; chemotropism (pollen tube → ovule) is also directly mentioned and acceptable.