Biology · 3.2 Osmosis · Paper 5/6 practical
Osmosis. Find the balance.
Identical potato cylinders are left in sucrose solutions of different concentration. In dilute solutions they gain mass (water in); in concentrated ones they lose mass (water out). Plot percentage change against concentration — where the line crosses zero is the concentration of the cell sap.
0610 Topic 3.2 — Osmosis
% change in mass
Paper 5/6 — Practical
0 min
mass 5.00 g
Variables
0.0
0.0 = distilled water (hypotonic) … 1.0 = strong sugar solution (hypertonic). Repeat across the range to find where the mass doesn't change.
Live readouts
Initial mass
5.00 g
Current mass
5.00 g
% change
0.0%
Water moves
—
% change = (final − initial) ÷ initial × 100. Using % change makes cylinders of slightly different starting mass comparable.
Results table
Record the % change at several concentrations.
% change vs concentration — zero crossing = cell sap
📋 Method (Cambridge practical procedure)
- Cut potato cylinders of equal length with a cork borer; blot dry and record each initial mass.
- Put one in each sucrose concentration (e.g. 0.0–1.0 mol/dm³) for the same time (~1 hour).
- Remove, blot gently to remove surface liquid, and re-weigh each cylinder.
- Calculate percentage change in mass = (final − initial) ÷ initial × 100.
- Plot % change against concentration. Where the line crosses 0% the external solution matches the cell sap concentration (no net osmosis).
⚠ Control variables & precautions
- Same potato, same size cylinders, same time, same temperature — only the concentration changes.
- Blot each cylinder the same way; surface water adds error to the mass.
- Use % change (not raw mass) so different starting masses can be compared fairly.
- A cylinder gains mass and becomes firm (turgid) in water; loses mass and becomes floppy (flaccid) in strong solution.
🎯 Syllabus reference (0610)
- 3.2 Osmosis — investigate osmosis using a model (Visking tubing) and living tissue (potato); explain water movement across a partially permeable membrane down a water-potential gradient; describe turgid and flaccid cells.