O-Level and SEC G3 Biology K325
B2: Movement of Substances
Distinguish diffusion, osmosis, and active transport using gradients, membranes, energy, and context.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Movement of Substances is assessed through direction, membrane conditions, gradients, and energy use. Distinguish diffusion, osmosis, and active transport first, then predict the effect on a cell or organism from the conditions given.
Diffusion and exchange surfaces
Diffusion is the net movement of particles from higher concentration to lower concentration because particles move randomly. A steeper concentration gradient, higher temperature, larger surface area, and shorter diffusion distance generally increase the rate. Diffusion does not require energy released by the cell.
Exchange surfaces such as alveoli, villi, and root hairs are effective because they provide a large area and a short pathway. Blood flow, ventilation, or continuous uptake helps maintain a concentration gradient. Link each adaptation to the rate of net movement.
Osmosis and cell outcomes
Osmosis is the net movement of water through a partially permeable membrane from higher water potential to lower water potential. It concerns water, not the solute. A dilute solution has higher water potential than a more concentrated solution when the other conditions are comparable.
A plant cell gaining water becomes turgid because its wall resists expansion. Losing water makes it flaccid, and severe loss can pull the cell membrane from the wall. An animal cell can burst in a sufficiently dilute solution or shrink in a concentrated solution because it lacks a cell wall.
Active transport and osmosis investigations
Active transport moves a substance against its concentration gradient through membrane proteins using energy released by respiration. It helps root hair cells take up mineral ions and allows the small intestine or kidney tubules to absorb useful substances when diffusion alone is insufficient.
In a potato osmosis investigation, keep cylinder dimensions, potato source, solution volume, time, and temperature controlled. Blot specimens consistently before measuring mass. Use percentage change to compare pieces with different starting masses, and repeat each concentration.
Formulae and relationships
| Relationship | Use |
|---|---|
| Compare osmosis results with different starting values. |
Worked examples
Example 1: A potato cylinder changes from 5.00 g to 4.35 g. Calculate its percentage mass change and infer the direction of net water movement.
- Find the change: .
- Divide by and multiply by .
- Use the negative mass change as evidence that water moved out of the potato tissue overall.
Answer: The mass change is ; net water movement was out of the tissue.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Define diffusion and osmosis through net movement and the relevant gradient.
- Reserve water potential and partially permeable membrane language for osmosis.
- Identify when active transport requires energy to move substances against a gradient.
Official outcome coverage
K325 B2: 3 mapped outcomes, references B2(a), B2(b), B2(c). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Plan an osmosis investigation with equal specimen dimensions, controlled time and temperature, surface drying, repeats, and percentage change.
Exam traps and retrieval check
Avoid these traps
- Defining osmosis as movement of a solution or solute instead of water.
- Calling every movement across a membrane active transport without checking gradient and energy use.
- Comparing raw mass changes when specimens had different starting masses.
Check from memory
What membrane condition is required for osmosis?
A partially permeable membrane.
Which process can move ions against a concentration gradient?
Active transport using energy released by respiration.
Why blot potato cylinders before weighing?
To remove surface solution so it does not distort the mass measurement.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.