Required Practical: Plant Responses (Phototropism/Gravitropism) — AQA GCSE Biology (Separate)
This required practical investigates the effect of light or gravity on the growth of newly germinated seedlings.
Investigating phototropism (light)
- Grow seedlings (e.g. cress or wheat) and divide them into groups.
- Expose each group to light from different directions — e.g. all-round light, light from one side, and darkness.
- Leave for several days and measure the direction and amount of growth.
Result: seedlings grown with light from one side bend towards the light (positive phototropism); those in the dark grow tall and pale (etiolated).
Investigating gravitropism (gravity)
- Germinate seeds and pin them in different orientations on damp cotton wool.
- Some are kept still; some are placed on a slowly rotating klinostat (to cancel the one-directional effect of gravity).
- Observe the direction the roots and shoots grow.
Result: roots grow downwards (towards gravity) and shoots grow upwards (away from gravity).
Control variables
Keep temperature, water and seedling type the same; change only the direction of light (or orientation/gravity). Use several seedlings and take a mean.
Exam tips
- Light from one side → shoots bend towards it (positive phototropism).
- Roots are positively gravitropic (grow down); shoots negatively gravitropic (grow up).
- A klinostat removes the directional effect of gravity as a control.
- Use multiple seedlings to make results reliable.