Required Practical: Plant Growth (AQA GCSE Biology): Revision Notes

Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings.

Method:

Investigating the Effect of Light:

1. Prepare Petri Dishes:

• Place cotton wool or philtre paper in each Petri dish.
• Moisten the cotton wool or philtre paper with water.
• Place several seeds on top of the cotton wool or philtre paper in each Petri dish.

2. Experimental Setup:

• Divide the Petri dishes into two groups:
• Group 1: Place these dishes in a well-lit area (e.g., under a lamp).
• Group 2: Place these dishes in a dark box or lightproof enclosure.

3. Control Variables:

• Ensure both groups are kept at the same temperature and have the same amount of water.

4. Observation Period:

• Observe the seedlings daily for a week, noting the direction and length of growth.

5. Recording Data:

• Measure the length of the seedlings using a ruler.
• Note the direction of growth relative to the light source.

Investigating the Effect of Gravity:

1. Prepare Petri Dishes:

• Follow the same preparation steps as the light experiment.

2. Experimental Setup:

• Place the Petri dishes in a vertical position so that the seedlings grow horizontally.
• Secure the dishes in a stand or holder to keep them stable.

3. Observation Period:

• Observe the seedlings daily for a week, noting the direction of root and shoot growth.

4. Recording Data:

• Measure the length of the roots and shoots.
• Note the direction of growth relative to gravity.

Results

• Light Experiment:
• Seedlings in light should grow towards the light source (positive phototropism).
• Seedlings in darkness may grow taller and more spindly as they attempt to reach light.
• Gravity Experiment:
• Roots should grow downwards (positive gravitropism).
• Shoots should grow upwards, away from gravity (negative gravitropism).

Data Analysis:

• Phototropism:
• Discuss how light direction affects the growth of shoots, causing them to grow towards the light.
• Explain the role of auxins in phototropism, where they accumulate on the shaded side, promoting cell elongation and bending the shoot towards the light.
• Gravitropism:
• Discuss how gravity affects the growth of roots and shoots.
• Explain that auxins accumulate on the lower side of the root and shoot, but they inhibit growth in roots, causing them to bend downwards, and promote growth in shoots, causing them to bend upwards.

Conclusion:

• Summarise how light and gravity influence plant growth.
• Relate the practical results to the broader concepts of plant hormones and their roles in phototropism and gravitropism.

Evaluation:

• Consider potential sources of error in the experiment, such as uneven light distribution or inconsistent moisture levels.
• Suggest improvements, such as using more precise methods to measure growth or repeating the experiment multiple times for accuracy.