Start every definition with net movement
Particles move randomly in all directions, even at equilibrium. Diffusion describes the net movement from a region of higher concentration to lower concentration. Net means that more particles move one way than the other; it does not mean movement occurs in only one direction.
Osmosis is the net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane. The membrane requirement and the fact that the moving substance is water must appear in a complete definition.
| Mechanism | What moves | Direction | Energy from respiration |
|---|---|---|---|
| Diffusion | Particles | Down a concentration gradient | Not required |
| Osmosis | Water | Down a water-potential gradient through a selective membrane | Not required |
| Active transport | Specific solutes | Against a concentration gradient | Required |
Use evidence to predict osmosis
Compare the solution inside and outside the cell, then follow water from higher to lower water potential. In dilute external solution, an animal cell gains water and may burst; a plant cell becomes turgid because its wall resists expansion.
In concentrated external solution, an animal cell loses water and shrivels. A plant cell becomes flaccid and may plasmolyse when the cell membrane pulls away from the wall. The cell wall itself does not prevent water loss.
Which side has higher water potential?
State the direction of net movement.
Plant wall present or animal cell only?
Turgid, flaccid, plasmolysed, swollen or shrivelled.
Rate depends on gradient and exchange design
Diffusion becomes faster with a steeper concentration gradient, larger surface area, shorter distance and higher temperature. These principles explain the thin walls and large surfaces of alveoli, villi, root hairs and capillary networks.
In experiments, rate must be measured rather than inferred vaguely. Suitable measures include change in mass per unit time, distance moved by a coloured boundary or volume exchanged over a fixed interval.
Active transport solves the uphill problem
Active transport uses energy released by respiration to move substances against their concentration gradient through membrane proteins. Root-hair cells can absorb mineral ions from dilute soil, and cells in the small intestine can maintain uptake mechanisms even when simple diffusion would not be sufficient.
A common weak answer says active transport 'uses energy' but never states why. The full explanation connects energy to carrier activity and movement from lower to higher concentration.
How to write the extended response
Begin with the observed change, state the gradient, name the movement and finish with the cellular consequence. For a potato cylinder that gains mass, for example: the external solution has a higher water potential, so water enters cells by osmosis through partially permeable membranes, increasing cell mass and turgor.
Avoid saying that water moves because it 'wants to dilute the solution.' Molecules have no intention, and the phrase does not identify the gradient or membrane.
The ideas to carry forward
- Diffusion and osmosis are net movements down gradients.
- Osmosis requires water and a partially permeable membrane.
- Active transport moves solutes against a gradient using energy from respiration.
- Strong answers connect evidence, direction, mechanism and consequence.
Answer first. Then reveal the marking logic.
01Why is 'water moves from dilute to concentrated solution' an incomplete osmosis definition?2 marks · show the biological link
Answer: It omits net movement, water potential and the requirement for a partially permeable membrane.
02What would happen to mineral-ion uptake if root-hair respiration were strongly inhibited?2 marks · show the biological link
Answer: Active transport would fall because less ATP would be available to power membrane transport proteins.
Want the next explanation when it is published?
Join the Biology channel