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.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Define the term tropism. [2]

(b) State two stimuli that can cause a tropic response in plants. [2]

(c) Give one example of a negative tropism. [1]

Worked Solution:

(a)

1. Tropism is defined as a directional growth response... $\boxed{}$ ...of a plant...
2. ...in response to an external stimulus. $\boxed{}$

How to earn full marks:

• You must mention "directional growth" to get the first mark.
• Include the phrase "external stimulus" for the second mark.

(b)

1. Light is a stimulus that can cause a tropic response. $\boxed{}$
2. Gravity is another stimulus that can cause a tropic response. $\boxed{}$

How to earn full marks:

• Award 1 mark per correctly stated stimulus.
• Accept: water, chemicals, touch.
• Do not accept: temperature.

(c)

1. Negative gravitropism of a shoot is an example. $\boxed{}$

How to earn full marks:

• Award 1 mark for a correct example.
• Accept: negative phototropism of a root.
• Do not accept: positive tropism examples.

Common Pitfall: Students often confuse tropisms with taxes. Remember that tropisms are growth responses, while taxes are movement responses. Also, be sure to specify whether the tropism is positive or negative and which part of the plant is exhibiting the response.

Exam-Style Question 2 — Extended Response [8 marks]

Question:

An investigation was carried out to study the effect of light on the growth of seedlings. Ten seedlings were placed in a box with a small hole on one side, allowing light to enter. Another ten seedlings were placed in a box with no hole. Both boxes were kept in the same temperature and humidity conditions. After one week, the direction of growth of the shoots was observed.

(a) State one variable that must be kept constant in this investigation, other than temperature and humidity. [1]

(b) Describe the expected results for the seedlings in both boxes. [2]

(c) Explain why the seedlings in the box with the hole grew towards the light. [5]

Worked Solution:

(a)

1. The type of seedling must be kept constant. $\boxed{}$

How to earn full marks:

• Award 1 mark for a correct variable.
• Accept: age of seedlings, volume of soil/water, species of seedling, initial height of seedlings.
• Do not accept: light.

(b)

1. Seedlings in the box with the hole will grow towards the hole. $\boxed{}$
2. Seedlings in the box with no hole will grow upwards (vertically). $\boxed{}$

How to earn full marks:

• Award 1 mark for describing the growth of seedlings in the box with the hole.
• Award 1 mark for describing the growth of seedlings in the box with no hole.

(c)

1. Light causes auxin to redistribute... $\boxed{}$
2. ...to the shaded side of the shoot. $\boxed{}$
3. A higher concentration of auxin on the shaded side... $\boxed{}$
4. ...stimulates cell elongation on that side. $\boxed{}$
5. This unequal growth causes the shoot to bend towards the light. $\boxed{}$

How to earn full marks:

• Award 1 mark for mentioning auxin redistribution.
• Award 1 mark for stating that auxin moves to the shaded side.
• Award 1 mark for stating that a higher concentration of auxin is on the shaded side.
• Award 1 mark for stating that auxin stimulates cell elongation.
• Award 1 mark for explaining that unequal growth causes the shoot to bend.

Common Pitfall: When explaining phototropism, be sure to link the redistribution of auxin to the shaded side of the shoot. Many students incorrectly state that auxin moves to the side exposed to light. Also, remember that auxin stimulates cell elongation in shoots, but it can inhibit cell elongation in roots.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

(a) Define the term gravitropism. [2]

(b) Describe an experiment to investigate gravitropism in roots. [4]

Worked Solution:

(a)

1. Gravitropism is the growth response... $\boxed{}$
2. ...of a plant to the stimulus of gravity. $\boxed{}$

How to earn full marks:

• Both points are needed for full marks.

(b)

1. Place germinating seeds on damp paper in a petri dish. $\boxed{}$
2. Orient the petri dish vertically. $\boxed{}$
3. Observe the direction of root growth over several days. $\boxed{}$
4. Roots will grow downwards, demonstrating positive gravitropism. $\boxed{}$

How to earn full marks:

• Award 1 mark for describing seed placement in a petri dish.
• Award 1 mark for describing the vertical orientation of the petri dish.
• Award 1 mark for mentioning observation over time.
• Award 1 mark for stating that roots grow downwards.

Common Pitfall: When describing experiments, be specific about the setup and how you will measure or observe the response. Simply saying "observe the roots" isn't enough; you need to mention the direction of growth and the time frame. Also, remember to control other variables like light exposure and moisture levels.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A student investigates the effect of different concentrations of auxin on the growth of plant stem sections. The student cuts several stem sections of equal length and places them in solutions containing different concentrations of auxin. After 24 hours, the length of each stem section is measured. The results are shown in the table below.

Auxin Concentration (ppm)	Average Length Increase (mm)
0	1
10	5
20	8
30	10
40	9
50	7

(a) Describe the trend shown by the data in the table. [2]

(b) Explain how auxin stimulates cell elongation in the stem sections. [4]

(c) Suggest why the length increase decreased at auxin concentrations above 30 ppm. [3]

Worked Solution:

(a)

1. As the auxin concentration increases from 0 to 30 ppm, the average length increase also increases. $\boxed{}$
2. However, as the auxin concentration increases from 30 to 50 ppm, the average length increase decreases. $\boxed{}$

How to earn full marks:

• Award 1 mark for describing the increase in length up to 30 ppm.
• Award 1 mark for describing the decrease in length above 30 ppm.

(b)

1. Auxin causes increased plasticity (flexibility) of the cell walls. $\boxed{}$
2. This allows cells to stretch more easily. $\boxed{}$
3. Auxin also stimulates the active transport of hydrogen ions ($H^+$) into the cell wall. $\boxed{}$
4. The increased acidity loosens the bonds in the cell wall, allowing it to expand. $\boxed{}$

How to earn full marks:

• Award 1 mark for stating that auxin increases cell wall plasticity.
• Award 1 mark for stating that this allows cells to stretch more easily.
• Award 1 mark for mentioning active transport of hydrogen ions.
• Award 1 mark for stating that increased acidity loosens cell wall bonds.

(c)

1. High concentrations of auxin can inhibit cell elongation. $\boxed{}$
2. This may be due to the production of ethene (ethylene) at high auxin concentrations. $\boxed{}$
3. Ethene inhibits cell elongation. $\boxed{}$

How to earn full marks:

• Award 1 mark for stating that high auxin concentrations can inhibit cell elongation.
• Award 1 mark for suggesting ethene (ethylene) production.
• Award 1 mark for stating that ethene inhibits cell elongation.

Common Pitfall: When describing trends in data, be sure to include specific values from the table to support your description. Don't just say "it increases and then decreases"; mention the auxin concentrations at which these changes occur. Also, remember that the effect of auxin can vary depending on its concentration and the plant tissue involved.