Cells in solutions under the microscope sim

What the sim shows

A compound light microscope on the bench, with seven dropping bottles (sucrose in mol/dm³) and a tray of two slides: a strip of red onion epidermis, and red blood cells in 0.9 % salt solution. Put a cell in a solution and the result depends on which side has the lower water potential. Osmosis is the net movement of water molecules from a dilute solution (higher water potential) to a more concentrated solution (lower water potential) through a partially permeable membrane. Animal cell, which has only a membrane: in a dilute solution, water enters, the cell swells and may burst (haemolysis in a red blood cell); in a concentrated solution, water leaves and the cell shrinks and crenates. Plant cell, which also has a rigid cellulose wall: in a dilute solution, water enters, the vacuole swells and pushes the cytoplasm against the wall. The wall resists, pressure builds, and the cell becomes turgid. It does not burst. In a concentrated solution, water leaves, the cell becomes flaccid, and if enough leaves the cytoplasm pulls away from the wall. That is plasmolysis. Plasmolysed cells are killed if they stay plasmolysed for a long period of time. Model: what matters is the concentration of dissolved particles; NaCl splits into two ions, so 0.9 % NaCl (0.154 mol/dm³) has 0.31 mol/dm³ of particles, the same as the red cell contents. Red cell volume follows V = 0.4 + 0.6 × 0.31 ÷ c (Boyle–van 't Hoff; 40 % of a red cell does not take part), and the cell bursts at 1.64 × its normal volume, a sphere with the same membrane area (135 µm²). The onion cell contents match 0.38 mol/dm³ sucrose (illustrative); below that, the stretched wall pushes back and the cell stops swelling. The close-up shows 60 µm across for red cells and 300 µm for onion, drawn flat as seen down the microscope; cells reach their new size in one step (in real life it takes a few minutes).

• Each dropper: drag it to the edge of the cover slip to irrigate (one drop = one step)
• The close-up: part of the field of view at ×400, with a scale bar
• Red onion sap is purple (anthocyanin); red blood cells are red (haemoglobin)

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
• Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
• Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.

Free guide: 12 practicals as sims, and how to run one in a lesson →

Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.