Osmosis in Plant Tissue

Linked to B1: Cell Biology

Aim

Investigate the effect of different sucrose concentrations on the mass of potato tissue.

Practical Guide

Method & analysis

Exam Questions

Past-paper style

Mark Scheme

Model answers

Independent variable

Concentration of sucrose solution (0.0–0.8 mol/dm³)

Dependent variable

Percentage change in mass of potato cylinder

Controlled variables

Length of potato cylinder, time in solution (30 min), volume of solution, temperature, same potato variety

✓Osmosis: net movement of water from high to low water potential through a partially permeable membrane

✓Dilute solution = higher water potential. Concentrated = lower water potential.

✓Potato gains mass: water entered by osmosis (external solution more dilute)

✓Potato loses mass: water left by osmosis (external solution more concentrated)

✓Isotonic point: no net water movement — water potential inside = outside

✗Not using percentage change in mass — raw mass change is unfair if cylinders start at different masses
✗Saying water moves from concentrated to dilute — say 'high to low water potential'
✗Not patting potato dry before reweighing — water on surface adds to mass

★% change = (final - initial) ÷ initial × 100
★The isotonic concentration = water potential of potato cells — where the line crosses 0% on the graph
★Use percentage change, not absolute change, to account for different starting masses