AQA GCSE Biology · Homeostasis & ResponseTriple only
Plant responses required practical
Investigate the effect of light or gravity on the growth of newly germinated seedlings.
Seedlings grown with light from one side bend towards it. This is phototropism, caused by auxin collecting on the shaded side of the shoot.
This one is only on the separate Biology GCSE, so it won’t come up in Combined Science.
Equipment
- Cress or mustard seeds
- Three Petri dishes with damp cotton wool
- A box with a slit cut in one side, and a lamp
- A dark cupboard
- Ruler and protractor
Variables
- Independent you change
- The direction of light (from one side, from above, or none)
- Dependent you measure
- The height and direction of growth of the seedlings
- Control you keep the same
- Same number and type of seeds
- Same volume of water
- Same temperature
- Same number of days
Method
- 1
Put damp cotton wool in three Petri dishes and add 10 seeds to each.
- 2
Leave them somewhere warm for a few days until they germinate.
- 3
Put one dish in a box with light coming from one side through a slit.
- 4
Put one dish in light from above, and one in a dark cupboard.
- 5
Water each dish with the same volume every day.
- 6
After 5 days, measure the height of each seedling and note the direction it is growing.
Worked example
Mean seedling height after 5 days.
| Condition | Mean height (mm) | Direction of growth | Colour |
|---|---|---|---|
| Light from one side | 32 | Bent towards the light | Green |
| Light from above | 30 | Straight up | Green |
| Dark | 45 | Straight up | Pale yellow |
Conclusion
Shoots grow towards light (positive phototropism). Light from one side makes auxin collect on the shaded side; there it makes cells elongate more, so the shoot bends towards the light. In the dark the seedlings grow taller and pale as they search for light, without making chlorophyll.
Risks
- Lamps get hotDon’t touch the bulb and keep it away from the box.
- Mould on damp cotton woolDon’t let it go mouldy; wash hands after handling.
Common mistakes
- Saying auxin moves to the light side. In shoots it collects on the shaded side.
- Saying auxin speeds up growth everywhere. In roots, more auxin slows growth, which is why roots grow down with gravity.
- Only using one seedling. Seedlings vary, so measure several and take a mean.
Better results
- Use more seeds in each dish and calculate a mean height.
- Measure the angle of bending with a protractor as well as the height.
- Measure every day to see how quickly the response happens.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Homeostasis & Response flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.