SEC G3 Combined Science Biology component K327/K328
B2: Movement of Substances
Explain diffusion and osmosis, then interpret tissue investigations without adding active transport.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Diffusion and osmosis are net movements driven by concentration differences, but osmosis specifically concerns water crossing a partially permeable membrane.
Diffusion in organisms
Diffusion is the net movement of particles from higher concentration to lower concentration. Random movement occurs in both directions, but the overall movement follows the concentration gradient.
Diffusion moves oxygen and carbon dioxide across alveoli and leaf surfaces. It also moves digested nutrient molecules from the small intestine into blood when there is a suitable concentration gradient.
Osmosis and tissues
Osmosis is the net movement of water through a partially permeable membrane from a solution with higher water potential to one with lower water potential.
Plant tissue gains water and becomes turgid in a dilute solution but loses water and becomes flaccid, then plasmolysed, in a concentrated solution. Animal cells lack a supporting cell wall, so they can swell and burst in a sufficiently dilute solution or shrink in a concentrated solution.
Investigating osmosis
Prepare equal-sized tissue samples, record initial mass or length, immerse them in solutions of known concentration for equal times, remove surface liquid consistently, and measure the final value.
Calculate change or percentage change and plot it against concentration. Control tissue source, temperature, time, and solution volume.
Formulae and relationships
| Relationship | Use |
|---|---|
| Compare osmosis results for samples of different starting sizes. |
Worked examples
Example 1: A potato cylinder loses mass in concentrated sugar solution. Explain.
- The outside solution has lower water potential than the cell contents.
- Water crosses cell membranes out of the tissue by osmosis.
- Loss of water reduces the cylinder's mass.
Answer: Water leaves the potato cells by osmosis, so mass decreases.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Explain Diffusion in organisms with the named terms, evidence, and causal links kept distinct.
- Use Osmosis and tissues to interpret the evidence given and justify each conclusion.
- Apply Investigating osmosis to a new example, then check the conclusion against the information given.
Official outcome coverage
K327 B2: 2 mapped outcomes, references B2(a), B2(b). Check the official K327 syllabus.
K328 B2: 2 mapped outcomes, references B2(a), B2(b). Check the official K328 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Investigate diffusion or osmosis with plant tissue or a partially permeable membrane. Define the changed variable, control size, time and temperature, blot samples consistently before mass measurement, repeat trials, and explain anomalous results.
Exam traps and retrieval check
Avoid these traps
- Defining osmosis without a membrane.
- Saying sugar molecules move by osmosis.
- Comparing final masses without considering different starting masses.
Check from memory
What drives net diffusion?
A concentration difference.
What substance moves in osmosis?
Water.
Why blot tissue before weighing?
To remove surface liquid consistently.
Official Combined Science scope
This shared Combined Biology owner serves both K327 and K328. Diffusion, osmosis, their biological roles, and tissue investigations.