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Tropic responses

Paper 1Paper 2Paper 3Paper 4Paper 5Paper 6

This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4. Practical investigation skills are tested in Paper 5 and Paper 6.

What are Tropic Responses?

Tropisms are directional growth responses in plants triggered by an external stimulus. Unlike nastic movements (which are non-directional), tropic responses depend on the direction of the stimulus relative to the plant part.

There are two main types you must know:

  1. Phototropism: Response to light.
  2. Gravitropism (also called geotropism): Response to gravity.
Positive vs. Negative Tropisms:

  • Positive tropism: Growth towards the stimulus.
  • Negative tropism: Growth away from the stimulus.

This terminology is crucial for describing responses accurately in exams.

Gravitropism

Gravitropism is the growth response of a plant part to the direction of gravity.

  • Positive gravitropism: Growth towards gravity (downwards). This occurs in roots.
  • Negative gravitropism: Growth away from gravity (upwards). This occurs in shoots.
Phototropism

Phototropism is the growth response of a plant part to the direction of a light source.

  • Positive phototropism: Growth towards the light. This occurs in shoots.
  • Negative phototropism: Growth away from the light. This can occur in some roots, though it is less commonly emphasized than positive phototropism in shoots.
Investigating Tropic Responses (LO 3)
To investigate these responses, you can use seedlings placed in specific conditions. Here is how to describe the expected results for both shoots and roots under different stimuli.
Expected Response
Bends towards the light source (Positive phototropism).
Grows upwards, away from gravity (Negative gravitropism).
Generally shows little to no response to the direction of light. It continues to grow downwards due to gravity.
Bends downwards, towards gravity (Positive gravitropism).
Key Investigation Point: When investigating phototropism, ensure the experiment controls for gravity. If a plant is in a dark box, it cannot respond to light (no phototropism), but it will still respond to gravity if positioned horizontally.
⚠︎ Confusing Direction of Growth
Mistake: Students often confuse whether roots or shoots grow towards or away from stimuli.

Correction:

  • Roots: Always grow down (towards gravity). They anchor the plant and seek water/nutrients.
  • Shoots: Always grow up (away from gravity) and towards light (for photosynthesis).

Remember: Roots = Right angles down? No, just remember Roots go Down.

Describing Advantages of Tropisms
When to use: When asked to explain the advantage or benefit of a tropic response (e.g., 'Explain why phototropism is advantageous for the plant').

Why examiners accept this: Examiners look for the link between the physical movement and the physiological process it supports. Simply stating 'it helps the plant' is insufficient.

Correct phrasing example:
"The shoot grows towards the light to maximize light absorption for photosynthesis. This allows the plant to produce more glucose for energy and growth."

Key markscheme keywords: light, photosynthesis, glucose, energy.

Auxin (Chemical Control)

Auxin is a plant hormone that controls growth. For the purpose of chemical control of shoot growth, you must know four specific facts about auxin:

  1. It is made in the shoot tip.
  2. It diffuses through the plant from the shoot tip.
  3. Its distribution becomes unequal in response to light and gravity.
  4. It stimulates cell elongation (makes cells longer).
Mechanism of Phototropism in Shoots (LO 5)

Here is the step-by-step explanation of how auxin causes a shoot to bend towards light:

  1. Production: Auxin is produced in the shoot tip.
  2. Diffusion: Auxin diffuses down through the stem.
  3. Unequal Distribution: When light comes from one side, auxin moves to the shaded side of the shoot.
  4. Effect on Cells: The higher concentration of auxin on the shaded side stimulates cell elongation more than the lit side.
  5. Result: The shaded side grows longer than the lit side, causing the shoot to bend towards the light.
Mechanism of Gravitropism in Shoots (LO 5)

Here is the step-by-step explanation of how auxin causes a horizontal shoot to grow upwards:

  1. Production: Auxin is produced in the shoot tip.
  2. Diffusion: Auxin diffuses down through the stem.
  3. Unequal Distribution: Due to gravity, auxin accumulates on the lower side of the horizontal shoot.
  4. Effect on Cells: The higher concentration of auxin on the lower side stimulates cell elongation more than the upper side.
  5. Result: The lower side grows longer than the upper side, causing the shoot to bend upwards (away from gravity).
⚠︎ Misunderstanding Auxin's Role in Roots vs. Shoots
Mistake: Students often assume auxin works the same way in roots as it does in shoots.

Correction:

  • In shoots, high auxin concentration stimulates (increases) cell elongation.
  • In roots, high auxin concentration inhibits (decreases) cell elongation.

Important Syllabus Limitation: For the core syllabus requirement of explaining chemical control of shoot growth, you only need to know that auxin stimulates elongation. Do not use 'inhibition' when explaining shoot tropisms, as this is incorrect for shoots.

Explaining Chemical Control (LO 5)
When to use: When asked to 'explain the role of auxin' or 'explain how auxin controls growth'.

Why examiners accept this: Examiners require a logical chain: Source -> Movement -> Distribution -> Effect. Missing any step loses marks.

Correct phrasing example:
"Auxin is made in the shoot tip. It diffuses to the shaded side of the stem. The higher concentration of auxin stimulates cell elongation on that side, causing the shoot to bend towards the light."

Key markscheme keywords: shoot tip, diffuses, shaded side (or lower side), stimulates, cell elongation.

Past Paper Style Questions
Q:
Describe the gravitropic response of a shoot placed horizontally.
A:
The shoot grows upwards (away from gravity). This is an example of negative gravitropism.
Q:
Explain how auxin causes a plant shoot to bend towards a light source.
A:
Auxin is made in the shoot tip. It diffuses down the stem. Light causes auxin to accumulate on the shaded side. The higher concentration of auxin stimulates cell elongation on the shaded side, causing that side to grow longer and the shoot to bend towards the light.
Q:
A student investigates phototropism in roots. They place a seedling horizontally with light coming from one side. Describe the expected growth of the root.
A:
The root will grow downwards (positive gravitropism). It will show little to no response to the direction of the light.
Q:
State two variables that must be kept constant when investigating the effect of light intensity on phototropism.
A:
Any two from: Temperature, Type/Species of plant, Age of seedling, Water supply, Starting surface area.
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