Question Constellation Sign up for free AQA · GCSE · Biology · Higher
Experimental control apparatus purpose
A student investigated the effect of gravity on the growth of bean seedlings. Figure 6 shows both sets of apparatus: apparatus A is stationary and apparatus B rotates slowly.
The student left both sets of apparatus in a dark cupboard for 24 hours.
Apparatus B is a control. Explain why apparatus B is needed.
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Marking points
- 1 Explains that B provides a comparison with A.
- 2 Explains how rotation removes the one-sided gravity stimulus, showing gravity caused A's result.
Full-mark answer
Apparatus B can be compared with A. Because B rotates, gravity acts on the roots from all directions rather than one fixed direction, so a difference between A and B shows that gravity caused the response in A.
Why this answer loses marks
Apparatus B makes gravity disappear.
Gravity still acts; rotation averages its direction, so the setup provides a comparison with the stationary roots.