What Is Diffusion?
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random molecular movement. It requires no energy — it is a passive process. At O Level, you need to know the full definition by heart. Examiners are very specific — phrases like "high to low concentration" are not enough. You must include:- "Net movement" — not just "movement"
- "Down a concentration gradient" — this is the key driving force
- "As a result of random molecular movement"
- "No energy required" — it is a passive process
What Is Osmosis?
Osmosis is a special type of diffusion — specifically the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane. Like diffusion, osmosis is also a passive process. The key features of osmosis that distinguish it from simple diffusion are:- It involves water molecules only
- It requires a selectively permeable membrane — one that allows water but not solute molecules to pass through
- It moves water from higher water potential to lower water potential (i.e. from dilute solution to concentrated solution)
Diffusion vs Osmosis: Key Differences Every Student Must Know
| Feature | Diffusion | Osmosis |
|---|---|---|
| Substances involved | Any molecules or ions | Water molecules only |
| Membrane required? | No | Yes — selectively permeable |
| Driving force | Concentration gradient | Water potential gradient |
| Energy required? | No (passive) | No (passive) |
| Direction of movement | High → low concentration | High → low water potential |
Active Transport: The Third Transport Process
Alongside diffusion and osmosis, O Level Biology also requires you to understand active transport — the movement of molecules or ions across a membrane against a concentration gradient, from lower to higher concentration. Unlike diffusion and osmosis, active transport requires energy (ATP) and uses carrier proteins in the membrane. Examples: absorption of glucose and amino acids from the small intestine into the blood, uptake of mineral ions by root hair cells. The key exam distinction: if a substance is moving against its concentration gradient, active transport is occurring. If it is moving with its concentration gradient without energy, it is either diffusion or osmosis.Osmosis in Plant Cells: Turgidity and Plasmolysis
One of the most heavily tested applications of this topic is what happens to plant cells in different solutions:In a Hypotonic Solution (More Dilute Than Cell Contents)
Water enters the cell by osmosis. The cell becomes turgid — the vacuole expands and pushes the cell membrane against the cell wall. The cell wall resists further expansion, creating turgor pressure. This is the normal, healthy state for plant cells and what keeps plant stems upright.In a Hypertonic Solution (More Concentrated Than Cell Contents)
Water leaves the cell by osmosis. The vacuole shrinks. The cell membrane pulls away from the cell wall — this is called plasmolysis. A plasmolysed cell is said to be flaccid. If this happens to too many cells in a plant, the plant wilts.In an Isotonic Solution (Same Concentration as Cell Contents)
There is no net movement of water. The cell remains the same size. This is the equilibrium state.| Solution Type | Water Movement | Cell State | Term |
|---|---|---|---|
| Hypotonic (dilute) | Into cell | Swells, firm | Turgid |
| Isotonic (equal) | No net movement | Unchanged | — |
| Hypertonic (concentrated) | Out of cell | Shrinks, membrane pulls away | Plasmolysed / Flaccid |
Key Experiments for Diffusion and Osmosis at O Level
Practical experiments appear regularly in Paper 2. You need to understand the setup, variables, and expected results for each.Osmosis Experiment: Potato Strips in Solutions
Cut potato strips to equal length and mass. Place them in solutions of different sugar concentrations. Leave for a set time, then re-measure length or mass.- Strips in distilled water → gain mass/length (water enters by osmosis)
- Strips in concentrated sugar solution → lose mass/length (water leaves by osmosis)
- Strips in isotonic solution → no change in mass/length
Diffusion Experiment: Potassium Permanganate in Water
A crystal of potassium permanganate placed in still water slowly spreads purple colour through the water — demonstrating diffusion from high to low concentration. Warm water speeds up diffusion (higher kinetic energy of molecules).How to Study Diffusion and Osmosis Effectively
Learn the Definitions Word-for-Word
The definitions of diffusion, osmosis, and active transport are some of the most directly quoted definitions in the O Level Biology paper. Write them from memory daily until every word is automatic. One missing word can cost a mark.Practise Explaining Osmosis in Context
Osmosis questions often appear as application questions — what happens to a red blood cell in salt water? What happens to a plant cell when it is dehydrated? Practise explaining these scenarios using water potential language, not just "water moves in/out".Connect to Other Topics
Diffusion and osmosis appear again in transport in plants (water moving up the xylem), nutrition (absorption in the small intestine), and excretion (in the kidney). Our guide on what is photosynthesis is a useful read alongside this one for understanding how gas exchange by diffusion connects to photosynthesis.What Most Students Get Wrong in Diffusion and Osmosis Exam Answers
The most common mistakes are: using vague language ("water moves in" instead of "water moves by osmosis down a water potential gradient"), confusing hypotonic and hypertonic, and forgetting that osmosis specifically requires a selectively permeable membrane. At IONX Labs, O Level Biology classes build the exact language precision that examiners reward. Students work through past paper questions on diffusion and osmosis systematically, writing definitions and explanations that score full marks every time.Get Help With Diffusion and Osmosis O Level Biology
Our O Level Biology classes are small, focused, and built around the Singapore syllabus. Classes are capped at 8 students, so every definition gets checked and every exam answer gets corrected.
WhatsApp to Book → Our Biology ProgrammeFurther Reading
Frequently Asked Questions
Diffusion is the net movement of any molecules or ions down a concentration gradient — no membrane is required. Osmosis is a specific type of diffusion involving water molecules only, moving down a water potential gradient through a selectively permeable membrane. Both are passive processes requiring no energy. The two most testable distinctions are: (1) osmosis involves water only, and (2) osmosis requires a selectively permeable membrane.
Water potential is a measure of the tendency of water to move from one place to another. Pure water has the highest water potential. When solutes are dissolved in water, they lower the water potential. Osmosis always moves water from higher water potential (dilute solution) to lower water potential (concentrated solution). Using "water potential" language instead of just "high to low concentration" is what earns full marks in O Level Biology exam answers on osmosis.
Water leaves the cell by osmosis, down the water potential gradient from the cell (higher water potential) to the surrounding hypertonic solution (lower water potential). The vacuole shrinks, and the cell membrane pulls away from the cell wall — this is called plasmolysis. The cell is described as flaccid. If enough cells plasmolse, the plant wilts. This process is reversible if the cell is returned to water or a less concentrated solution.
The key difference is direction and energy. Diffusion moves substances down a concentration gradient (high to low) and requires no energy — it is passive. Active transport moves substances against a concentration gradient (low to high) and requires energy in the form of ATP, plus carrier proteins in the cell membrane. Examples of active transport include absorption of glucose and amino acids from the small intestine, and uptake of mineral ions by root hair cells.
Plot the change in mass or length of the potato strips (y-axis) against the sugar concentration of the solution (x-axis). The isotonic concentration is the point on the graph where the line crosses zero — where there is no change in mass or length. At this concentration, the water potential of the solution equals the water potential of the potato cells, so there is no net movement of water by osmosis in either direction.