Required Practical: Osmosis (AQA GCSE Biology): Revision Notes

Aim: To investigate the effect of different concentrations of sugar solution on the mass of potato tuber cells, demonstrating the process of osmosis.

Method:

1. Preparation:

• Use a cork borer to cut several cylinders of potato tuber to ensure they are of uniform size.
• Measure and record the initial mass of each potato cylinder using a balance.

2. Set Up:

• Prepare a range of sugar solutions with different concentrations (e.g., 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M).
• Place each potato cylinder into a separate beaker containing one of the sugar solutions.
• Ensure that the potato cylinders are fully submerged in the solutions.

3. Incubation:

• Leave the potato cylinders in the solutions for a set period, typically around 24 hours, to allow osmosis to occur.

4. Final Measurement:

• After the incubation period, remove the potato cylinders from the solutions.
• Gently blot the cylinders with paper towels to remove any excess surface liquid.
• Measure and record the final mass of each potato cylinder.

5. Calculations:

• Calculate the change in mass for each potato cylinder: $\text{Change in Mass} = \text{Final Mass} - \text{Initial Mass}$

• Calculate the percentage change in mass using the formula: $\text{Percentage Change} = \left( \frac{\text{Change in Mass}}{\text{Initial Mass}} \right) \times 100$

• Record the percentage change for each sugar concentration.

6. Analysis:

• Plot the percentage change in mass against the concentration of the sugar solution on a graph.
• The point where the curve crosses the x-axis (where percentage change in mass is 0) indicates the concentration at which the external solution and the potato's internal solution have the same water potential.

Conclusion:

• This experiment demonstrates osmosis, where water moves across a selectively permeable membrane from an area of higher water potential (low solute concentration) to an area of lower water potential (high solute concentration).