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Coordination and Response - Tropic responses in plants

Grade 11IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Tropism is a growth response in which the direction of growth is determined by the direction of an external stimulus. Positive tropism is growth toward the stimulus, while negative tropism is growth away from it.

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Phototropism is the growth response of a plant to light. Shoots show positive phototropism to maximize light absorption for photosynthesis (6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2).

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Gravitropism (or geotropism) is the growth response to gravity. Roots show positive gravitropism to anchor the plant and reach H2OH_2O and minerals deep in the soil, while shoots show negative gravitropism.

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Auxin is a plant hormone produced in the shoot and root tips. It regulates growth by stimulating cell elongation in shoots and inhibiting cell elongation in roots.

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In phototropism, auxin produced in the tip diffuses downwards. Under unidirectional light, auxin accumulates on the shaded side of the shoot, leading to a higher concentration [Auxin]shaded>[Auxin]lit[Auxin]_{shaded} > [Auxin]_{lit}.

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In gravitropism, if a plant is placed horizontally, gravity causes auxin to accumulate on the lower side of both the shoot and the root. In the shoot, this stimulates growth on the lower side (bending up); in the root, it inhibits growth on the lower side (bending down).

📐Formulae

Growth Rate∝[Auxin] (in shoots)\text{Growth Rate} \propto [Auxin] \text{ (in shoots)}

Growth Rate∝1[Auxin] (in roots)\text{Growth Rate} \propto \frac{1}{[Auxin]} \text{ (in roots)}

ΔL=Lfinal−Linitial\Delta L = L_{final} - L_{initial}

💡Examples

Problem 1:

A coleoptile (young shoot) is exposed to light from the right side. Explain the distribution of auxin and the resulting movement of the shoot.

Solution:

Auxin is synthesized in the tip and moves to the shaded side (the left side). The concentration [Auxin]left>[Auxin]right[Auxin]_{left} > [Auxin]_{right}. This causes the cells on the left side to elongate more than the cells on the right, resulting in the shoot bending toward the light.

Explanation:

Auxins are sensitive to light and migrate away from it. In shoots, higher auxin levels trigger the release of H+H^+ ions into cell walls, increasing elasticity and allowing for greater cell expansion (elongation).

Problem 2:

A seedling is placed horizontally in a dark room. Describe the response of the root and the shoot after 4848 hours.

Solution:

The shoot will exhibit negative gravitropism and bend upwards, while the root will exhibit positive gravitropism and bend downwards. Gravity causes auxin to settle on the lower side of both organs.

Explanation:

In the shoot, the high [Auxin][Auxin] on the lower side stimulates elongation, pushing the tip up. In the root, the high [Auxin][Auxin] on the lower side inhibits elongation, so the upper side grows faster, pushing the root tip down.

Tropic responses in plants - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology