Diffusion across membranes and osmosis in cells
BiologyCell BiologyAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Cell Biology
On the left, small molecules diffuse through membrane pores from high to low concentration until equilibrium, with particle counts on each side. On the right, a red blood cell or a plant cell sits in a hypertonic, isotonic or hypotonic solution — water osmoses in or out, causing the cell to shrink, swell, burst or plasmolyze, tracked by a volume-over-time graph.
Lesson: Transport across the cell membrane: diffusion, osmosis, hypertonic – isotonic – hypotonic solutions
What it shows
Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration. It is driven by random motion, needs no energy, continues until a dynamic equilibrium is reached, and speeds up at higher temperature. Osmosis is the diffusion of water across a partially permeable membrane toward the solution with the higher solute concentration. A red blood cell shrinks in a hypertonic solution and swells, then bursts, in a hypotonic one. A plant cell plasmolyses in a hypertonic solution but only becomes turgid in a hypotonic one, because its cellulose wall resists further expansion.
How to use
On the left, set Initial diffusing particles (left side) and Temperature, then watch the counts on each side converge. On the right, choose the Cell type and set the Solute concentration inside the cell, then change the Solute concentration outside to make the solution hypertonic, isotonic or hypotonic. Watch the cell and the V/V₀ graph, press Reset between trials, and use Change cell type to compare the two cells.
Parameters you can change
- Solute concentration outside 0–3 %
- Solute concentration inside the cell 0.3–2 %
- Cell type Red blood cell (animal cell), Plant cell (with cell wall)
- Initial diffusing particles (left side) 20–120
- Temperature 5–60 °C
Questions to explore
- Which outside concentration keeps a red blood cell's volume constant, and why?
- Why does a plant cell not burst in pure water, while a red blood cell does?
- How does raising the temperature change the time needed for diffusion to reach equilibrium?
Teaching ideas
All guides- Interactive Biology Lesson Ideas with SimulationsInteractive biology lesson ideas for grades 6–12: ten simulation activities on cells, osmosis, enzymes, photosynthesis, genetics, evolution and ecology.
- Osmosis Virtual Lab: Potato Strips in Sugar SolutionsOsmosis virtual lab: soak potato strips in 0–1.0 mol/dm³ sugar solutions, calculate the percentage change in mass and find where the mass stays the same.
- Agar Cube Diffusion Virtual Lab: Surface Area to VolumeAn agar cube diffusion virtual lab: time how fast acid clears pink agar cubes, calculate SA:V, and see why big cells and organisms need exchange surfaces.
