Pearson International A Level Biology Core Practical 3: Investigate temperature and alcohol effects on membrane permeability

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Estimate membrane disruption from pigment leakage while controlling tissue geometry and removing pigment released during cutting.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use equal beetroot cylinders or discs, cork borer, ruler, water baths, alcohol solutions, tubes, forceps and colorimeter. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Cut equal tissue pieces, rinse until wash water is clear and blot consistently.
2. Expose pieces for the same time to a temperature or alcohol concentration series.
3. Remove tissue, mix the solutions and measure absorbance against a suitable blank.
4. Repeat each condition, calculate means and plot response against the independent variable.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: temperature or alcohol concentration.
• Dependent variable: pigment absorbance or transmission.