Surface Area to Volume Ratio – Agar Cube Diffusion
BiologyCell BiologyAges 15–16
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Sign in to playVirtual lab: drop pink agar blocks (made with dilute sodium hydroxide and phenolphthalein), cubes of side 0.5–3 cm or flat and long cuboids, into hydrochloric acid and watch the colour clear from the outside in as the acid diffuses. Time how long each block takes to turn colourless, read the percentage of its volume reached by the acid after a fixed time, and calculate surface area, volume, SA:V and the distance to the centre. Graphs show why cells stay small and why large organisms need exchange surfaces and transport systems.
Lesson: Surface area to volume ratio, cell size and diffusion (agar cube practical)
What it shows
Cells exchange substances by diffusion across their surface, but they use them throughout their volume. In this classic practical, agar made pink with an alkali and phenolphthalein is cut into blocks and placed in hydrochloric acid. The acid diffuses in from every face, and the pink colour clears from the outside in. Small cubes have a large surface area to volume ratio and clear quickly, while the centre of a large cube is far from the surface and clears much later. Comparing cubes, flat sheets and long blocks of the same volume links the results to cell size, villi, alveoli and transport systems.
How to use
Choose the Agar blocks set and the Hydrochloric acid concentration, then press Add acid and start. Watch the pink cores shrink and read the table: the time to turn colourless and the % of volume reached at the Fixed time. Tick Cut blocks in half to see inside. Change the Graph to compare SA:V and time against side, or % reached against SA:V. With Cut your own blocks, set the Shape and sizes, then press Cut and add.
Parameters you can change
- Agar blocks Cubes 0.5, 1, 2, 3 cm, Same volume (8 cm³), different shapes, Cut your own blocks
- Hydrochloric acid concentration 0.1 mol/dm³, 0.5 mol/dm³, 1 mol/dm³
- Cube side (Cut your own blocks) 0.5–3 cm
- Fixed time for the % reading 1–30 min
- Graph % reached against time, SA:V and time against side, % reached against SA:V
Questions to explore
- Why does doubling the side of a cube make it take about four times as long to turn completely colourless?
- The flat sheet and the 2 cm cube have the same volume. Which has the larger SA:V, and how does that change the results?
- How do villi in the small intestine and alveoli in the lungs solve the problem shown by the large agar cube?
