Cell Structure (AQA GCSE Biology): Revision Notes

Practical: Investigating the Effect of Light or Gravity on the Growth of Newly Germinated Seedlings

Aim: To investigate how light (phototropism) or gravity (gravitropism) affects the growth direction of newly germinated seedlings.

Method:

1. Set-Up:

• Place moist cotton wool in the base of several Petri dishes.
• Spread the seeds evenly on the cotton wool.
• Allow the seeds to germinate in a dark place to ensure that initial growth is not influenced by light.

2. Experiment for Phototropism:

• Once germinated, place the Petri dishes in a dark box with a light source on one side.
• Ensure that the light source is constant and positioned so that light reaches the seedlings from one direction only.
• Leave the setup undisturbed for a few days.

3. Experiment for Gravitropism:

• Use a clinostat to negate the effects of gravity (optional).
• Alternatively, place the seedlings on their sides to observe the natural response to gravity.

4. Observations:

• After a few days, observe and measure the direction of seedling growth.
• Measure the length of the shoots and roots.
• Record the angle of curvature towards the light source (in phototropism) or in response to gravity (in gravitropism).

5. Results:

• Record your observations in a table, noting the direction and degree of growth.

6. Drawing:

• Draw accurate, labelled diagrams of the seedlings, showing the direction of growth.

Analysis:

• For phototropism, seedlings should bend towards the light source, demonstrating positive phototropism in the shoots.
• For gravitropism, roots should grow downwards (positive gravitropism) while shoots grow upwards (negative gravitropism).

Conclusion:

• This experiment demonstrates how external stimuli like light and gravity influence plant growth direction.

This practical helps in understanding how plants respond to environmental factors, which is crucial for their survival and growth .