H2 Biology Practical: Osmosis and Diffusion (Potato Cores & Agar Blocks)

Q: What does H2 Biology Practical: Osmosis and Diffusion (Potato Cores & Agar Blocks) cover?
A: Two cornerstone investigations for Paper 4: determine isotonic concentration via potato core mass change (osmosis) and model surface-area-to-volume effects with phenolphthalein agar blocks (diffusion). Includes serial dilutions, controlled variables, initial measurements, graphing, and evaluation pointers aligned to MMO/PDO/ACE.

TL;DR
For osmosis, cut uniform potato cores, equilibrate in a sucrose series, and plot percentage mass change vs concentration to estimate the isotonic point (where $\Delta m \approx 0$).
For diffusion, time decolourisation in phenolphthalein agar blocks of different sizes and relate time to the surface area/volume (SA/V) ratio so conclusions are quantitative.

Status: SEAB H2 Biology (9477, first exam 2026) syllabus last checked 2025-11-29 (PDF last modified 2025-11-28). Paper 4 weighting and MMO/PDO/ACE expectations unchanged; osmosis/diffusion remain staple practical contexts for data handling and evaluation.

1 Aims & assessment focus

• Osmosis: quantify how external solute concentration influences net water movement across plant tissue; estimate isotonic concentration from your graph.
• Diffusion: show how block size (and surface-area-to-volume ratio) affects time for indicator to change; discuss limits to cell size.
• Assessment hooks: MMO (accurate cutting/volumes/timing), PDO (clean tables/graphs/units), ACE (uncertainties, trends, biological explanations).

2 Apparatus & reagents

• Fresh potatoes (firm; similar cultivar), cork borer or chip cutter, scalpel, ruler, balance (0.01 g).
• Sucrose solutions for a serial series (e.g., $\pu{0.00, 0.10, 0.20, 0.30, 0.40, 0.50 mol.dm-3}$