AQA GCSE · Question 04.3 · Homeostasis and Response
Apparatus B is a control. Explain why apparatus B is needed.
How to approach this question
1. Define the purpose of a control in a scientific experiment.
2. Explain what is happening in apparatus B (the clinostat). How does rotation affect the stimulus of gravity?
3. Link this to the results seen in apparatus A.
Full Answer
In a scientific investigation, a control is used as a baseline for comparison. It is set up to be identical to the main experiment in every way except for the one variable being tested (the independent variable).
In this case, the independent variable is the directional stimulus of gravity.
- In **Apparatus A (Stationary)**, gravity constantly pulls downwards.
- In **Apparatus B (Rotating)**, the device (a clinostat) slowly rotates. This means that over time, the stimulus of gravity is applied equally to all sides of the root. Gravity is still present, but it is not acting in one constant direction relative to the root.
Therefore, apparatus B is needed to show what happens when there is no *directional* gravitational stimulus. By comparing the growth of the root in A (with directional gravity) to the growth in B (without directional gravity), the student can conclude that the downward growth in A is specifically a response to the direction of gravity.
Common mistakes
✗ Simply stating "it's a control" without explaining its purpose.
✗ Saying it "removes gravity" - gravity is still present, but its directional effect is cancelled out.
✗ Confusing it with the need for a fair test, without explaining the specific role of this control.
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