Required Practical: Osmosis in Plant Tissue — AQA GCSE Biology
This required practical investigates the effect of sugar (or salt) solution concentration on plant tissue by measuring osmosis.
Osmosis recap
Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (high water concentration) to a concentrated solution (low water concentration).
Method
- Cut equal-sized pieces of potato (e.g. cylinders using a cork borer), blot dry and measure the mass of each.
- Place each piece in a different concentration of sugar solution (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³).
- Leave for a set time (e.g. 30 minutes), then remove, blot dry and re-measure the mass.
- Calculate the percentage change in mass for each concentration.
Results
- In dilute solutions, water moves into the tissue by osmosis → mass increases.
- In concentrated solutions, water moves out of the tissue → mass decreases.
- Where the mass does not change, the solution concentration equals the concentration inside the cells.
Improving accuracy
- Use percentage change (not just change) to allow for different starting masses.
- Control variables: temperature, time, size of pieces, type of potato.
- Repeat and calculate a mean.
Exam tips
- Always calculate percentage change in mass = (change ÷ start) × 100.
- Explain results using water moving in/out by osmosis.
- The point of no mass change gives the internal cell concentration.
- Blotting dry and controlling variables improve reliability.