What is the primary function of a clinostat in a plant growth practical?

A clinostat rotates the seedlings slowly to ensure that gravity acts equally on all sides of the plant. This cancels out the directional effect of gravity, allowing it to serve as a control for gravitropism experiments.

How does the effect of auxin differ between the cells in a plant shoot and the cells in a plant root?

In shoots, a high concentration of auxin stimulates cell elongation, causing faster growth on that side. In roots, a high concentration of auxin inhibits cell elongation, causing slower growth on that side.

Why must gravitropism experiments be conducted in a light-proof box?

Light is a stimulus that triggers phototropism. By using a light-proof box, light is removed as a confounding variable, ensuring that any observed directional growth is solely a response to gravity.

Define 'Positive Gravitropism' and identify which part of the plant exhibits it.

Positive gravitropism is growth toward the direction of gravity (downwards). This response is exhibited by plant roots to ensure they grow into the soil to find water and minerals.

What happens to the growth of a seedling placed on a rotating clinostat?

The seedling will grow straight out in the direction it was originally pointed. Because the rotation prevents auxin from accumulating on any one side, the cells elongate at an equal rate all around the stem or root.

In a horizontal shoot, why does the tip bend upwards?

Gravity causes auxin to accumulate on the lower side of the shoot. Since auxin stimulates elongation in shoots, the lower side grows faster than the upper side, forcing the shoot to curve upwards.

What is the difference between phototropism and gravitropism?

Phototropism is a growth response to the direction of light, whereas gravitropism is a growth response to the direction of gravity. Both are controlled by the redistribution of auxin.

Why is damp cotton wool used instead of just placing seeds in a dry petri dish?

Damp cotton wool provides the water necessary for seed germination. Water is a required condition for the metabolic processes that allow the seedling to begin growing and responding to stimuli.

What is a common mistake when explaining why roots grow down?

A common mistake is stating that auxin stimulates root growth. In fact, auxin inhibits elongation in roots; the side with less auxin grows faster, which is what causes the downward bend.

Where is auxin primarily produced in a germinating seedling?

Auxin is produced in the actively growing tips (meristems) of the shoots and the roots. From there, it diffuses to the regions of cell elongation behind the tips.

Required Practical: Plant Growth | AQA GCSE Biology

Homeostasis & Response

Required Practical: Plant Growth

Summary

This practical investigation explores how external stimuli, specifically gravity and light, influence the growth patterns of germinating seedlings through the distribution of the plant hormone auxin. By isolating variables using specialized equipment like clinostats and light-proof boxes, researchers can observe the distinct directional growth responses known as tropisms in both roots and shoots.

1. Definition & Core Concepts

Tropisms are growth responses in plants where the direction of growth is determined by the direction of an external stimulus. Phototropism refers to growth in response to light, while gravitropism (or geotropism) refers to growth in response to gravity.

Auxins are a class of plant hormones produced in the tips of shoots and roots that coordinate these growth responses. They diffuse from the tips to the regions of cell elongation, where they regulate how much cells grow in length.

The practical specifically focuses on observing positive responses (growth toward a stimulus) and negative responses (growth away from a stimulus) in newly germinated seedlings.

2. Underlying Principles

The fundamental mechanism of plant growth response is the unequal distribution of auxin. When a stimulus like gravity or one-sided light is present, auxin accumulates on one side of the plant organ, causing that side to grow at a different rate than the other.

In shoots, a higher concentration of auxin promotes cell elongation. If a shoot is horizontal, auxin settles on the lower side due to gravity, causing the lower side to grow faster and the shoot to bend upwards (negative gravitropism).

In roots, a higher concentration of auxin inhibits cell elongation. When a root is horizontal, auxin accumulates on the lower side, slowing its growth relative to the upper side, which causes the root to bend downwards (positive gravitropism).

Diagram showing the divergent growth of shoots and roots from a horizontal seed due to gravity.

3. Methods & Techniques

To investigate gravitropism, seedlings are typically placed in petri dishes with damp cotton wool. The seeds are oriented in different directions (e.g., root facing up, down, or sideways) to ensure the observed response is due to gravity rather than initial orientation.

A clinostat is a critical piece of apparatus that slowly rotates the petri dish. This rotation constantly changes the direction of the gravitational pull relative to the plant cells, effectively cancelling out the effect of gravity and serving as a control.

The experiment must be conducted in a light-proof box. This is essential to ensure that any observed growth is a response to gravity alone and not a phototropic response to ambient light.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Required Practical: Plant Growth

Summary

1. Definition & Core Concepts

The practical specifically focuses on observing positive responses (growth toward a stimulus) and negative responses (growth away from a stimulus) in newly germinated seedlings.

2. Underlying Principles

Diagram showing the divergent growth of shoots and roots from a horizontal seed due to gravity.

3. Methods & Techniques

The experiment must be conducted in a light-proof box. This is essential to ensure that any observed growth is a response to gravity alone and not a phototropic response to ambient light.

4. Key Distinctions

It is vital to distinguish between the effects of auxin in different plant organs. While auxin is the primary messenger in both, its physiological effect is opposite in shoots compared to roots.

Feature	Shoot Response	Root Response
Stimulus: Light	Positive Phototropism (towards)	Negative Phototropism (away)
Stimulus: Gravity	Negative Gravitropism (away/up)	Positive Gravitropism (towards/down)
Auxin Effect	Stimulates cell elongation	Inhibits cell elongation

The stationary setup shows the natural gravitropic response, whereas the clinostat setup results in straight growth because the gravitational stimulus is applied equally to all sides of the plant over time.

5. Exam Strategy & Tips

When describing the clinostat, always state that it 'cancels out the effect of gravity' or 'ensures gravity acts equally on all sides.' Avoid saying it 'removes gravity,' as gravity is still present.

Identify the independent variable (usually the presence of a directional stimulus like light or gravity) and the dependent variable (the direction or angle of growth of the roots and shoots).

Always mention control variables such as temperature, water availability (damp cotton wool), and the use of a light-proof box to isolate gravity as the only factor affecting growth.

6. Common Pitfalls & Misconceptions

A common error is assuming auxin always makes plants grow faster. Students must remember that in roots, high auxin concentrations actually slow down growth, which is what allows the root to bend downwards.

Another misconception is that the entire plant bends. In reality, only the region behind the tip (the zone of elongation) contributes to the bending through differential growth rates.

Students often forget that light is a confounding variable in gravity experiments. If the box is not light-proof, the seedlings may grow toward light leaks rather than responding purely to gravity.