The Potato Osmosis Experiment in IGCSE Biology

Two pieces of potato start at 5.0 g each. After soaking, one weighs 5.6 g and the other weighs 4.5 g. Both spent the same amount of time in a beaker. Why did their masses change in opposite directions?

The answer is osmosis. Water moves across the potato cells’ partially permeable membranes. Whether the potato gains or loses mass depends on the net direction of that movement. Cambridge IGCSE Biology syllabus, Section 3.2

How does the potato osmosis experiment work?

A student measures the starting mass of three similarly sized potato pieces. They place one piece in distilled water, one in a dilute sucrose solution and one in a more concentrated sucrose solution. After the same soaking time, they remove each piece, gently blot away surface liquid and measure its final mass.

The independent variable is the sucrose concentration. The dependent variable is the percentage change in potato mass. To make the comparison fair, the student keeps the potato source, piece dimensions, solution volume, temperature, soaking time and blotting method the same. Repeating each concentration with fresh pieces lets them calculate a mean.

Worked example — calculate the percentage change in mass

Here is an illustrative set of results.

Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100

For the potato in the more concentrated solution:

1. Find the change in mass. 4.5 − 5.0 = −0.5 g.

2. Divide by the initial mass. −0.5 ÷ 5.0 = −0.10.

3. Multiply by 100. −0.10 × 100 = −10%.

The minus sign tells you that the potato lost mass. Using percentage change helps you compare pieces even when they start at slightly different masses.

How do you explain the results in an exam?

Use three pieces of information in your answer: which side had the higher water potential, which way water moved and how the mass changed.

In distilled water, the surrounding liquid had a higher water potential than the potato cells. Water moved into the cells by osmosis through their partially permeable membranes, so the potato gained mass.

In the more concentrated sucrose solution, the potato cells had a higher water potential than the surrounding liquid. Water moved out of the cells by osmosis, so the potato lost mass.

A quick check before moving on: if the mass went down, your explanation should say that the net movement of water was out of the cells.

What does zero percentage change tell you?

Plot sucrose concentration on the horizontal axis and percentage change in mass on the vertical axis. If the graph crosses the horizontal axis, read the concentration at 0% change.

At that concentration, the potato tissue and the surrounding solution have approximately the same water potential. Water molecules still move across cell membranes in both directions, with no net movement of water.

Try it yourself

A potato piece starts at 4.0 g and finishes at 3.6 g.

1. What is the percentage change in mass?

2. Which way did water move overall?

3. Why should the student blot the potato before weighing it?

Answers — 1. −10%, because (3.6 − 4.0) ÷ 4.0 × 100 = −10%. 2. Water moved out of the potato cells overall. 3. Blotting removes surface liquid that would otherwise add to the measured mass.

For a broader revision plan, read TheTuitionE’s IGCSE exam preparation guide for UAE parents.

If your child can calculate the percentage change but finds it harder to explain the result, bring a recent osmosis question to a free TheTuitionE demo class.