Diffusion in Action – The Classic Potato Osmosis Experiment

Osmosis is one of those biology topics that students often find tricky to picture. The idea of water moving across a partially permeable membrane sounds abstract — until you try it with something as simple as a potato.

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The Experiment

We cut potato chips of identical size and mass using a chipper and a scapel, then place them in solutions of different sugar concentrations. Depending on the class, these solutions are either given to the class or they must calculate the concentrations themselves. After about an hour, students measure the changes: drying the chip and measuring its change in mass, girth, and length using callipers.

• In pure water, the chips gain mass and become firm as water moves in.

• In a concentrated sugar solution, the chips lose mass and turn floppy as water moves out.

• Somewhere in between, at the isotonic point, there’s no net movement.

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The Science

Osmosis is the diffusion of water molecules from a region of high water potential to a region of low water potential, through a selectively permeable membrane.

The potato’s cell membranes act as that barrier. By recording the mass change, students see osmosis quantified.

Typical Results

Sugar concentration (mol/dm³)	Initial mass (g)	Final mass (g)	% change in mass
0.0 (pure water)	2.00	2.40	+20%
0.1	2.00	2.20	+10%
0.2	2.00	2.00	0%
0.3	2.00	1.80	–10%
0.4	2.00	1.60	–20%

When plotted, the graph of % change in mass against sugar concentration crosses the x-axis at about 0.2 mol/dm³, showing the isotonic point.

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Skills Highlight

• Fair testing – identical chips, controlled time, equal volumes.

• Graphing results – plotting percentage change against concentration reveals the isotonic point.

• Real-world links – food preservation with salt or sugar, why slugs shrivel in salt, and why plants wilt without water.

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Conclusion

The experiment shows that:

• Potato cells gain water and mass in dilute solutions (where water potential is higher outside the cells).

• Potato cells lose water and mass in concentrated solutions (where water potential is higher inside the cells).

• At the isotonic point, there is no net movement of water.

This proves osmosis is a passive process driven by water potential differences — and provides students with both visual evidence and numerical data to support the concept.

Why It Works in Teaching

The potato osmosis experiment transforms a definition into something measurable and memorable. Students don’t just learn the word “osmosis” — they watch it happen and prove it with data.