Explain how auxin causes a plant shoot to bend towards light. In your answer, describe (i) where auxin is pro…

Question

Explain how auxin causes a plant shoot to bend towards light. In your answer, describe (i) where auxin is produced, (ii) how it redistributes in response to light, and (iii) the cellular mechanism by which bending occurs.

Accepted Answer

## Model Answer (i) **Production:** Auxin is synthesised at the **shoot tip**. (ii) **Redistribution:** When light falls from one side, auxin diffuses towards the **shady side** of the shoot, making its concentration higher there. (iii) **Cellular mechanism:** The higher concentration of auxin causes cells on the shady side to **grow longer** than cells on the light side. This unequal elongation makes the shoot **bend towards the light**. *Source: Chapter 6, Section 6.2.2 – Movement Due to Growth* --- ## Explanation The passage gives all three points explicitly. Examiners look for three distinct, labelled steps: (1) site of synthesis = shoot tip, (2) lateral diffusion to shady side, (3) differential cell elongation causing bending. Use the exact textbook terms — "auxin," "shoot tip," "shady side," "grow longer" — to score full marks. Avoid vague phrases like "moves to one side" without specifying which side.

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