IGCSE Biology Practical Skills 5: Diffusion and Osmosis

Diffusion and osmosis are explicit Cambridge IGCSE Biology practical contexts for Papers 5 and 6. Strong investigation answers identify the moving substance, the measured proxy, the controlled gradient and the limits of each model rather than treating every membrane result as osmosis.

Choose a model that answers the question

Diffusion can be investigated through agar, liquid or a partially permeable membrane. Osmosis requires water moving through a partially permeable membrane from higher water potential to lower water potential.

Plant tissue provides living cell membranes and a measurable mass or length change. Visking tubing provides a non-living partially permeable barrier. Agar can reveal diffusion distance or proportion reached by a substance but does not model every feature of a cell.

State what the apparatus represents and what it does not. A model supports a specific inference, not proof that a living organism behaves identically.

Diffusion through agar

A coloured or indicator-containing agar block can be placed in a solution that changes its colour as diffusion occurs. Measure block dimensions before exposure, keep solution concentration, volume, temperature and time constant, then cut the block to measure penetration.

To compare block size, use cubes made from the same agar batch and expose them under identical conditions. Smaller cubes have a larger surface-area-to-volume ratio and may have a greater proportion reached in the same time.

Diffusion distance can be measured from each face, or the undiffused core can be measured. Define the rule before collecting data. A percentage or volume proportion can compare different starting sizes more fairly than raw penetration distance alone.

Use forceps and a cutting tile. If acid, alkali or indicator is involved, follow the supplied hazard information and wear eye protection.

Visking-tubing investigations

Soak and open the tubing as instructed, secure one end, add the selected solution and tie the other end. Rinse the outside thoroughly to remove material spilled during filling, check for leaks and place the tubing in a known external solution.

After a controlled time, test the liquid inside or outside using the appropriate reagent. A positive test outside supports that the tested molecule crossed the membrane. A negative result may mean the molecule did not cross, was below detection level or the method was insufficiently sensitive.

Keep tubing length or surface area, internal and external volumes, concentration, temperature and time constant when comparing conditions.

Small molecules may pass while larger molecules do not, depending on membrane pores. Do not state that a molecule is too large without evidence from the supplied results.