Required practical · AQA, EDEXCEL, OCR
Required practical: Investigate osmosis in potatoes (or other plant tissue)
Aim
To observe the net movement of water across a partially permeable membrane by measuring mass or length changes in tissue placed in different sugar concentrations.
Method (step-by-step)
- Cut identical potato cylinders (or discs), blot dry, and record initial mass.
- Immerse sets in a range of sucrose (or salt) concentrations for a fixed time at constant temperature.
- Remove, blot gently the same way each time, and re-mass.
- Calculate percentage change in mass for each concentration; repeat for repeats and means.
- Plot % change vs concentration and estimate water potential / isotonic point from the curve.
Equipment
- Potato
- cork borer / knife
- balances
- sucrose solutions
- beakers
- water bath optional
Variables
- Independent: Solution concentration (mol dm⁻³ or %).
- Dependent: Percentage change in mass (or length).
- Control: Temperature; Time in solution; Starting dimensions of tissue; Volume of solution
Results & analysis
Hypotonic solutions gain mass; hypertonic lose mass. Near the isotonic concentration, mean change crosses zero — use repeats to justify the estimate.
Graphs & interpretation
A smooth curve with a zero-crossing is ideal; discuss anomalies from incomplete blotting or damaged epidermis.
Common mistakes (high yield)
- Inconsistent blotting (systematic error in mass).
- Different cylinder sizes between concentrations.
- Using ‘osmosis’ without linking to water potential or partially permeable membrane.
Exam-style question prompts
- Explain why mass changes in this practical.
- How would you improve the reliability of the results?
Exam tips
- Define osmosis precisely
- Always refer to concentration gradient and membrane
Related topic revision
FAQs
Exam boards differ in wording; always follow your specification and your teacher’s practical notes. Back to GCSE Biology required practicals hub.