Osmosis Experiments

Investigating osmosis using dialysis tubing

Dialysis tubing can be used as a model partially permeable membrane, and this is specifically included in the Cambridge syllabus as a required practical context. A typical investigation follows this sequence:

1. Put a solution inside a piece of dialysis tubing.
2. Seal the tubing.
3. Measure its initial mass, or another suitable starting measurement.
4. Place it in a second solution.
5. Leave it for a controlled period of time.
6. Remove it and carefully remove excess liquid from its outside.
7. Measure it again.
8. Compare the initial and final measurements.

If the tubing gains mass in a suitably designed osmosis investigation, there has been net movement of water into the tubing. If it loses mass, there has been net movement of water out of the tubing. That interpretation depends on the investigation being designed well: dialysis tubing can, in principle, allow some small solutes through as well as water, so the solutions and the membrane should be chosen so that movement of other substances does not make the mass change misleading. Read as a straightforward osmosis result, a mass change is only safe to interpret as water movement when the experiment has been set up with that in mind.

At Supplement level, you should be able to explain a mass gain or loss by identifying which side of the membrane — inside or outside the tubing — had the higher water potential, then applying the same higher → lower reasoning used throughout osmosis. The tubing itself is only a model; the biological principle it demonstrates is identical to water movement across a real cell membrane.

Investigating osmosis in plant tissue

Plant tissue can be placed into solutions of different concentrations to investigate how the surrounding solution affects water movement into or out of its cells. A clear method uses equally sized pieces of tissue throughout:

1. Prepare several solutions of different concentrations.
2. Cut pieces of plant tissue to the same dimensions.
3. Measure the initial mass of each piece.
4. Place the pieces into the different solutions for the same length of time.
5. Keep other important variables — such as temperature and the size of the tissue pieces — constant.
6. Remove the tissue.
7. Gently remove excess solution from its surface in the same way for every sample.
8. Measure the final mass.
9. Compare the change in mass between the different concentrations.

If a piece of tissue gains mass, water has moved into its cells overall. If it loses mass, water has moved out of its cells overall. At Supplement level, that translates directly into a statement about water potential:

• a mass gain indicates that the surrounding solution had a higher water potential than the tissue;
• a mass loss indicates that the surrounding solution had a lower water potential than the tissue.

If there is no overall change in mass, there was no net movement of water — the water potentials of the tissue and the surrounding solution were approximately equal.

When the tissue pieces do not all start at exactly the same mass, the fairest way to compare results across different concentrations is to work in percentage change rather than raw mass, so results from pieces of slightly different starting sizes can still be compared directly:

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

This calculation itself is not a named Chapter 3 syllabus point — it comes from Cambridge’s wider mathematical requirements around percentages and percentage change, which apply across the whole course. What Chapter 3 specifically requires is the plant-tissue investigation itself, comparing tissue placed in solutions of different concentrations; percentage change is simply the tool you reach for when a data question needs samples of different starting masses compared fairly.

What the mass changes actually mean for the structure of the cells themselves — turgid, flaccid and plasmolysed — is covered on osmosis in plant cells.