What it does
Osmosis is water moving through a membrane that lets water pass but holds back dissolved sugar or salt. Water moves from the side with more free water (a weaker solution) to the side with less (a stronger solution). This lab lets you put four things into sugar or salt water and see what happens:
- Potato piece and raisin: they gain or lose mass, like the practical many schools do.
- Onion peel cell: a plant cell under the microscope, where you can see plasmolysis: the inside of the cell shrinks and pulls away from the wall.
- Red blood cell: an animal cell with no wall, so in plain water it swells and bursts, and in strong salt it shrinks and crinkles.
The picture shows the beaker, an arrow for the way water moves and one cell zoomed in. It fits Class 9 Science: the fundamental unit of life (cell membrane, cell wall, osmosis, plasmolysis) and Class 11 Biology: cell, the unit of life, with water potential (ψ) at the Class 11 level. This is a teaching model with made-up round numbers, not real measurements. To go further, see diffusion calculator for how fast things spread, colligative properties calculator for osmotic pressure in chemistry and osmolarity calculator for counting dissolved particles.
How to use it
- Pick the level. Class 9 shows the words hypotonic, isotonic and hypertonic and the % change sum. Class 11 adds solute potential (ψs), pressure potential (ψp, the wall pushing back) and the cell’s water potential ψ, in megapascals (MPa).
- Pick what goes in and what is dissolved. Salt counts about double, because each unit of salt splits into two particles (Na⁺ and Cl⁻) and osmosis depends on the number of particles.
- Slide the concentration. The quick buttons jump to plain water, the strength that matches the cell (isotonic) and a strong 1 M solution.
- Slide the time or press Play from the start to watch the cell change. A potato takes about an hour, an onion cell a few minutes and a red blood cell a few seconds.
- Read the graph of % change against concentration. Press Run the practical for six pieces in six solutions with slightly messy readings, as in a real lab, and download them as a CSV file.
- Try the Check yourself questions. Each press of New questions makes fresh numbers, and Check answers shows the working.
What the words mean
- Hypotonic: the solution outside is weaker than the cell sap. Water moves in. Plant cells become firm (turgid), animal cells swell and may burst.
- Isotonic: the same strength. Water moves in and out at the same rate, so nothing changes overall. Plant cells are soft (flaccid).
- Hypertonic: the solution outside is stronger. Water moves out. Plant cells plasmolyse, animal cells shrink (crenation).
Worked example
Start with the potato in plain water, time at the end. The page shows Hypotonic: water moves in and an end change of +8.0 %: a 2.50 g piece becomes about 2.70 g, so % change = (2.70 − 2.50) ÷ 2.50 × 100 = +8.0 %. Press Same as the cell: the slider jumps to 0.30 M sugar and the change is 0.0 %. Switch to salt and press it again: 0.15 M salt is enough, because salt makes two particles. At Class 11 level, 1 M sugar gives ψs = −1 × 1 × 0.008314 × 298.15 = −2.479 MPa outside, far below the potato’s −0.74 MPa, so water rushes out and the piece loses 35 %.
Tips
- In the practical, the point where your line crosses zero estimates the strength of the cell sap. Real school results are often 0.2 to 0.4 M sugar for potato.
- Blot pieces dry before weighing in a real lab; extra water on the surface makes the gain look bigger.
- A plant cell does not burst because its wall pushes back: in the model that push is ψp, and it stops water coming in once ψ inside equals ψ outside.
FAQ
Why does the potato only gain 8 % in plain water but lose 35 % in strong sugar? The wall stops it swelling much, but nothing stops it shrinking.
Is the raisin really a plant cell? A raisin is a dried grape full of sugar, so it is many plant cells with very strong sap. It swells a lot in water and takes hours.
Are these real numbers? No. They are round teaching values. Use your own lab results for a report.
Where the numbers come from
- Water potential ψ = ψs + ψp and ψs = −iCRT (van 't Hoff), with R = 8.314462618 J/(mol·K) (CODATA 2018). Written by fibub from these standard formulas.
- The cell sap strengths, wall stiffness and soaking times are made-up round numbers chosen so the model behaves like a school practical; they are not measurements of real potatoes, raisins or cells.
- Equilibrium sizes were checked in the tests against a separate Python solution of ψ cell = ψ outside (bisection, numpy 2.5).
- Drawings are fibub's own simple shapes. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.