Tropic responses

1. Overview

Plants do not have a nervous system like animals, but they must still respond to their environment to survive. They do this through tropisms—growth responses where the direction of growth is determined by the direction of an external stimulus. This allows plants to maximize their chances of photosynthesis and ensure stable anchorage in the soil.

Key Definitions

• Tropism: A growth response in which the direction of growth is determined by the direction of an external stimulus.
• Gravitropism: A response in which parts of a plant grow towards or away from gravity.
• Phototropism: A response in which parts of a plant grow towards or away from the direction of a light source.
• Positive Tropism: Growth towards a stimulus (e.g., a root growing towards gravity).
• Negative Tropism: Growth away from a stimulus (e.g., a shoot growing away from gravity).

Core Content

Phototropism in Shoots and Roots

• Shoots: Exhibit positive phototropism. They grow towards the light source to ensure leaves receive maximum sunlight for photosynthesis.
• Roots: Usually exhibit negative phototropism (though this is less pronounced). They grow away from light to ensure they stay deep within the soil.

Gravitropism in Shoots and Roots

• Shoots: Exhibit negative gravitropism. They grow upwards, away from the pull of gravity. This ensures that even if a seed is planted upside down, the shoot will eventually reach the surface to find light.
• Roots: Exhibit positive gravitropism. They grow downwards, towards the pull of gravity. This ensures the plant is firmly anchored in the soil and can reach water and mineral ions.

Investigating Tropisms

1. Phototropism Investigation: Place seedlings in a cardboard box with a small hole on one side.
   • Observation: The shoots will bend and grow towards the hole (the light source).
   • Control: A second box with no hole or light coming from directly above; the shoots will grow straight up.
2. Gravitropism Investigation: Place germinating seeds on a vertical damp cotton wool surface. Once roots appear, turn the container 90 degrees so the roots are horizontal.
   • Observation: The roots will curve downwards towards gravity.
   • Control: Use a clinostat (a device that rotates slowly). The rotation cancels out the effect of gravity because the plant is pulled equally in all directions, so the roots grow straight.

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Extended Content (Extended Only)

Chemical Control of Plant Growth

Tropic responses are not instant movements; they are the result of differential growth controlled by plant hormones called auxins.

The Role of Auxin in Shoots

Auxin controls growth through a specific step-by-step process:

1. Production: Auxin is made in the shoot tip.
2. Diffusion: Auxin diffuses from the tip downwards through the plant tissue.
3. Unequal Distribution:
   • In Phototropism: When light shines on one side of a shoot, auxin moves and accumulates on the shaded side.
   • In Gravitropism: In a horizontal shoot, auxin accumulates on the lower side due to the pull of gravity.
4. Cell Elongation: Auxin stimulates cells to absorb more water and expand. In shoots, a higher concentration of auxin causes more cell elongation.

Step-by-Step: How a Shoot Bends Towards Light

1. Light hits the shoot from one side.
2. Auxin diffuses away from the light and accumulates on the shaded side of the shoot.
3. The high concentration of auxin on the shaded side causes those cells to elongate more than the cells on the lit side.
4. Because the shaded side is growing faster/longer than the lit side, the shoot is forced to bend towards the light.

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Key Equations

• There are no mathematical equations for this topic. However, you must be able to describe the ratio of growth between the shaded and lit sides of a plant.

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Common Mistakes to Avoid

• ❌ Wrong: Saying the plant "moves" or "bends its muscles" towards the light.
• ✅ Right: Use the term "growth response" or "cell elongation."
• ❌ Wrong: Suggesting that auxin is "destroyed" by light.
• ✅ Right: Auxin redistributes or moves to the shaded side.
• ❌ Wrong: Thinking that roots and shoots respond the same way to auxin.
• ✅ Right: In shoots, auxin stimulates elongation; in roots, high concentrations of auxin actually inhibit elongation (though IGCSE focus is primarily on the shoot mechanism).

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Exam Tips

• Command Words: If a question asks you to "Describe," say what happens (e.g., "The shoot grows towards the light"). If it asks you to "Explain," you must mention auxin, its distribution, and cell elongation.
• The Tip is Key: Many exam questions involve cutting off the tip of a shoot or covering it with foil. Remember: if the tip is removed or covered, the plant will not show phototropism because the auxin source (or the light-sensing part) is gone.
• Clinostats: Always remember that a clinostat is the standard "control" for gravitropism experiments. It rotates to ensure gravity acts equally on all sides of the plant.
• Real-world Context: You might be asked why gravitropism is beneficial for a germinating seed underground. Answer: It ensures the roots find water/anchorage and the shoot reaches the surface for light before its food reserves (cotyledons) run out.