O-Level Biology Osmosis Evidence Lab

TL;DR
Osmosis questions dominate the water-relations strand of the 6093 syllabus and almost always appear as paired setups (dialysis tubing plus plant tissue).
Nail your mass/length measurements, present percentage change tables with sig. figs, and give ACE commentary that links the gradient direction to cell turgor or plasmolysis.
Keep a bank of fair-test controls and improvement statements so you can answer every modification prompt.

1 | Link back to syllabus language

• Section 2 of the syllabus expects candidates to explain osmosis in terms of water potential and semi-permeable membranes; Paper 3 enforces this with quantitative tasks (SEAB 2026 syllabus, PDF).
• Typical objectives examined:
  • Planning (P): selecting appropriate solute concentrations, controlling temperature, ensuring equal surface area or volume.
  • MMO: accurate weighing to 0.01 g, measuring lengths with a mm ruler, timing immersion periods.
  • PDO: calculating percentage mass/length change, plotting graphs with labelled axes and units.
  • ACE: comparing gradients, interpreting curves (e.g. identifying isotonic point), evaluating experimental design.

2 | Parallel setups to rehearse

A. Visking tubing sugar-osmosis test

• Fill dialysis tubing with 10 cm³ of 0.5 mol dm⁻³ sucrose solution, tie securely with cotton string.
• Suspend in a beaker containing 200 cm³ of distilled water; leave for 20 minutes.
• Use a syringe to collect the tubing contents at the end and record final volume/mass.
• Optional modification: reverse the gradient (distilled water inside, sucrose outside) to observe water inflow.

B. Potato strip or disc experiment

• Cut potato cylinders using a cork borer; trim to 5.0 cm using a sharp scalpel on a tile.
• Blot dry and take initial mass on a 2 decimal place balance (SEAB’s apparatus list includes balances accurate to 0.01 g).
• Immerse in labelled beakers containing 0.0, 0.2, 0.4, 0.6, 0.8 mol dm⁻³ sucrose for 20 minutes.
• Remove, blot to remove surface solution, record final mass and, if required, final length.

Use this consolidated data table template for PDO security: