Diffusion, osmosis and active transport are three ways of moving substances across a membrane. Tell them apart with three questions: what moves, which way, and whether energy is used.
This lesson opens movement across membranes. The next lesson, predicting cells in different solutions, applies osmosis.
How do the three compare?
Learn the table, then use it on new situations.
| Feature | Diffusion | Osmosis | Active transport |
|---|---|---|---|
| What moves | Any particles, such as oxygen | Water only | Ions or molecules, such as glucose or mineral ions |
| Direction | High to low concentration | High to low water concentration | Low to high concentration |
| Energy needed | No | No | Yes, from respiration |
| Membrane needed | Not always | Partially permeable membrane | Carrier proteins in the membrane |
The first two are passive, because they need no energy from the cell. Active transport is the only one that works against a gradient.
Worked example: classify four situations
Situation A: Oxygen moves from the air in the alveolus into the blood.
Oxygen is at a higher concentration in the air than in the blood, and it moves down the gradient without energy. This is diffusion.
Situation B: Water moves into a root hair cell from the soil.
The soil water has a higher water concentration than the cell sap, and water crosses the partially permeable membrane. This is osmosis.
Situation C: Mineral ions move into a root hair cell even though their concentration inside the cell is already higher than in the soil.
The ions move from low to high concentration, which is against the gradient. The cell uses energy from respiration and carrier proteins. This is active transport.
Situation D: A drop of ink spreads through still water.
Ink particles spread from the region of high concentration to low, with no membrane and no energy. This is diffusion.
Use the same pattern each time: state the concentration on each side, say which way the substance moves, then name the process.
Why is active transport needed?
A cell sometimes needs to take in a substance even when its concentration inside is already higher. Without active transport, the cell could not build up that substance.
Look for clues in the question: the phrase “against the concentration gradient”, or a note that the cell uses energy. If movement stops when a respiration inhibitor is added, the process is active transport.
The mistake that costs marks
The most common slip is to use osmosis for any movement of a substance. Osmosis names only the movement of water.
| Student wrote | Problem | Better version |
|---|---|---|
| “Glucose moves by osmosis” | Osmosis is only for water | “Glucose moves by diffusion, or by active transport if against the gradient” |
| “Active transport goes from high to low” | Reversed | “Active transport goes from low to high concentration” |
| “Osmosis needs energy” | It is passive | “Osmosis does not need energy” |
| “Diffusion stops when concentrations are equal” | Particles still move | “There is no net movement when concentrations are equal” |
Avoid writing “water moves from a strong solution to a weak one”, because “strong” is unclear. Say “from high water concentration to low water concentration”.
Check yourself
A student places a sugar solution inside a bag made of partially permeable membrane and places the bag in pure water. The bag gains mass over 30 minutes. Name the process and explain the direction.
Answer
The process is osmosis. Pure water has a higher water concentration than the sugar solution, so water moves into the bag across the membrane.
This increases the mass of the bag. No energy is needed, because water is moving down its concentration gradient.
What to study next
Next, apply osmosis to living cells in predicting cells in different solutions. The biological data interpretation trainer has tables to practise on.
If you want a teacher to describe new situations and have you classify each one, see online one-to-one Biology tuition or the one-hour trial class (from RM50).