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Diffusion vs osmosis: an HKDSE Biology explainer
Diffusion and osmosis both involve net movement, but they are not interchangeable labels. Ask three questions: what is moving, which gradient matters, and is a partially permeable membrane involved? Those questions make the distinction much easier to apply.
Diffusion: a net movement of particles
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration due to their random motion. Individual particles can move in both directions; ‘net’ describes the overall balance.
For example, oxygen can diffuse from the air in an alveolus into blood when the relevant gradient is maintained. The exact biological setting matters, but diffusion does not always require a membrane: a gas can also spread through air.
Osmosis: water, water potential and a membrane
Osmosis is the net movement of water through a partially permeable membrane from higher water potential to lower water potential. In a simple comparison of dilute solutions under the same pressure, adding solute lowers water potential.
Use water potential when deciding the direction. ‘Water moves from concentrated to dilute’ is ambiguous because it does not say whether you mean water or solute. Naming the moving substance and both sides removes that ambiguity.
Compare the same features
At equilibrium, particles or water molecules still move. There is no net movement when opposing movements balance. Avoid saying that all molecular motion stops.
| Feature | Diffusion | Osmosis |
|---|---|---|
| Moving substance | Particles, such as oxygen molecules | Water |
| Direction of net movement | Higher to lower particle concentration | Higher to lower water potential |
| Membrane | Not always required | A partially permeable membrane is required |
| Energy | Passive; no direct ATP input | Passive; no direct ATP input |
Worked example: a potato strip gains mass
Original practice example: a potato strip has an initial mass of 5.0 g. After soaking in a solution it has a mass of 5.6 g. The strip is blotted consistently before both readings. Calculate the percentage change and suggest an explanation.
Change in mass = 5.6 − 5.0 = 0.6 g. Percentage change = (0.6 ÷ 5.0) × 100 = +12%. Use the initial mass as the denominator; the positive sign shows a gain.
A possible explanation is that the surrounding solution had a higher water potential than the cell contents. Water entered the cells by osmosis through their partially permeable membranes, contributing to the increase in mass. This interpretation assumes the experiment is suitable and other sources of mass change are controlled.
What makes the experiment a fairer comparison?
For strips placed in different solutions, keep factors such as strip dimensions, soaking time and temperature consistent. Use the same blotting method so liquid left on the outside does not distort the mass comparison. Repeats help you see whether the measurements are consistent.
In a sufficiently dilute solution, a plant cell can gain water and become turgid as its wall resists expansion. Do not automatically claim it bursts like an animal cell. In a solution with lower water potential, the direction of net water movement can reverse.
Check your explanation
Try this before opening your notes: a strip changes from 4.0 g to 3.6 g. What is the percentage change, and what water-potential relationship would explain the loss?
Check the explanation
The change is −0.4 g, so the percentage change is −10%. A surrounding solution with lower water potential than the cell contents can cause net water movement out through the partially permeable membranes. The observed loss is consistent with that explanation under controlled conditions.
- Name water as the moving substance.
- State the direction using water potential.
- Identify the partially permeable membrane.
- Link the movement to the measured outcome.
In TeachAI DSE, revisit the Biology unit notes and use the checklist or quiz to test the distinction again. This article and its numbers are original teaching material, not an official HKDSE question.
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