Edexcel IGCSE Biology · Spec 2.17
Osmosis Practical
Required practical: investigating osmosis in potato tissue and calculating the percentage change in mass.
Biology revision video
Osmosis Practical
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Potato cylinders, and reading the crossing point
Osmosis moves water, so if water moves into a piece of potato it gains mass, and if water moves out it loses mass. Weighing the potato before and after is therefore a way of measuring osmosis without seeing it.
The method
Cut several cylinders of potato of the same length and diameter, using a cork borer so they match. Blot each one dry and record its starting mass.
Put one cylinder into each of a range of sugar or salt solutions, from pure water up to a concentrated solution, and leave them for the same length of time, usually twenty to thirty minutes.
Remove each cylinder, blot it dry in exactly the same way as before, and record its final mass.
Blotting consistently matters more than it sounds. Surface water left on one cylinder and not another is a mass difference that has nothing to do with osmosis.
Why percentage change, not change
Calculate the change in mass, divide by the starting mass, and multiply by 100.
The cylinders are never exactly identical, so a 0.4 g gain means more for a small cylinder than a large one. Working in percentages removes that difference and makes the results comparable, and saying why is often a mark in itself.
Reading the graph
Plot percentage change in mass against concentration. The line slopes downwards, positive at low concentrations and negative at high ones.
Where it crosses zero, there was no net movement of water. At that point the solution has the same water potential as the potato cells, so water moved in and out at equal rates. Reading off that concentration tells you the concentration inside the cells, which is the real purpose of the experiment.
Explaining the two ends
In dilute solution, the solution has a higher water potential than the cell contents, so water moves into the cells by osmosis. They swell and become turgid, and the cylinder gains mass. The cell wall stops them bursting.
In concentrated solution, the solution has a lower water potential, so water moves out. The cells become flaccid, and at the extreme the membrane pulls away from the wall, which is plasmolysis. The cylinder loses mass and feels floppy.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for an osmosis question credits three ideas: that water moved in, that it moved from a high water potential to a low water potential, and that it continued until the pressure prevented more water entering.
Its guidance accepts several phrasings of the middle point, including down a water potential gradient, from high to low concentration of water, and from dilute to concentrated. What it will not accept appears on a different question, where it says plainly: reject salt moves by osmosis. Osmosis moves water, and only water.
The same mark scheme also instructs the examiner to ignore references to equal concentrations of water inside and outside the bag. Reaching equilibrium is not what stopped the water; pressure was.
A principal examiner report adds a preference worth adopting. It notes that candidates often refer to concentrations of salt or of water, and that in most situations it is better to refer to movement from a higher water potential to a lower water potential. The phrase avoids the confusion of a concentrated solution having a low concentration of water.
Making it fair
Keep the cylinders the same size, the volume of solution the same, the time the same, and the temperature the same. Use potato from the same potato, since different potatoes have different starting concentrations.
Repeat each concentration and take a mean, and use a balance reading to two decimal places, because the mass changes are small.
Spec 2.17
What you need to know
- Investigate osmosis using potato tissue
- Describe the method and the measurements
- Explain the results
Active recall
Quick check
Answer each question before opening the answer.
What happens to potato mass in a dilute solution and in a concentrated solution?
In dilute solution water moves in, so mass increases. In concentrated solution water moves out, so mass decreases.
What does the point of zero mass change tell you?
The external solution concentration equals the concentration inside the potato cells (isotonic).
Biology revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.