Responses in Plants, Paper 2 essay guide

Essay themes this chapter supports

• Explain the mechanism of phototropism from stimulus to response: Define phototropism as a directional growth response to light, and state that shoots are positively phototropic. → Describe the role of auxin: produced at the shoot tip, moves laterally to the shaded side when light comes from one direction. → Explain the effect on growth: auxin promotes elongation, so shaded-side cells elongate more than lit-side cells. → Link the unequal growth to the visible bending of the shoot towards the light, and its importance for capturing light for photosynthesis.
• Compare tropisms with nastic movements: Define both terms and give one example of each (phototropism; leaf-folding of Mimosa pudica). → Contrast direction: a tropism's direction depends on the stimulus; a nastic movement's does not. → Contrast mechanism and speed: unequal growth over hours or days versus a rapid turgor change within seconds. → Explain why the fast nastic response suits defence while slow tropic growth suits long-term positioning for light, water and anchorage.
• Discuss geotropism and the statolith hypothesis, including hydrotropism and thigmotropism: State that roots are positively geotropic and shoots are negatively geotropic, and define geotropism. → Describe the statolith hypothesis: statoliths settle towards the lower side of root cap cells under gravity, triggering redistribution of auxin. → Explain the opposite effect of auxin on root cells (inhibits elongation) compared with shoot cells (promotes elongation) to account for the opposite curving of roots and shoots. → Briefly describe hydrotropism overriding geotropism in dry soil, and thigmotropism in a climbing tendril, as related but less-examined tropisms.
• Discuss agricultural and horticultural applications of phytohormones: State that synthetic auxins reproduce the effects of the natural hormone at controlled concentrations. → Describe rooting promotion in cuttings and seedless fruit production, linking each to the mechanism of auxin action. → Describe ripening control for transport and, at a much higher concentration, use as a selective weedkiller. → Explain why dosage must be followed precisely, since too little has no effect and too much can damage or kill the crop.

How to structure your essay

1. Open with one or two sentences that define the topic and set the scope.
2. Devote one paragraph to each key concept, in a logical order.
3. Use precise biological terms and, where relevant, a labelled diagram.
4. Give an example or state where the process happens in the body or plant.
5. Close by linking the ideas back to the question.

Planning grid

Key terms to include

• tropism
• phototropism
• geotropism
• hydrotropism
• thigmotropism
• auxin
• phytohormone
• statolith
• root cap
• nastic movement
• turgor pressure
• positive tropism
• negative tropism
• seedless fruit
• selective weedkiller

A model essay outline

1. Types of responses, Tropisms are directional growth responses of a plant organ to a stimulus that comes from one particular direction: phototropism responds to light, geotropism to gravity, hydrotropism to water, and thigmotropism to touch, and each is named after the stimulus that causes it.
2. Positive and negative tropism, Growth towards a stimulus is called a positive tropism and growth away from it a negative tropism; for example, shoots are positively phototropic and negatively geotropic, while roots are negatively phototropic and positively geotropic, so each organ grows in the direction that best helps the whole plant survive.
3. Phytohormones, Phytohormones are chemical messengers made in one part of a plant that travel to another part, usually in very small amounts, to control growth and responses; auxin is the phytohormone most closely linked to tropisms, while other phytohormones control processes such as fruit ripening, dormancy and ageing.
4. Auxin and phototropism, When light shines from one direction, auxin produced at the shoot tip moves towards and accumulates on the shaded side rather than the lit side, making the cells on that shaded side elongate more than those on the lit side, so the shoot bends towards the light source.
5. Application of phytohormones, Synthetic phytohormones based on auxin are used commercially to promote root growth in stem cuttings before they are planted, to produce seedless fruit by triggering ovary development without fertilisation, to control the timing of fruit ripening for transport and sale, and, at a much higher concentration, as selective weedkillers. Because these hormones act at very low concentrations, farmers must follow recommended dosages carefully, since too much can damage or kill the crop instead of helping it.

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Source:SRC-DSKP-EN