Diffusion, Osmosis & Active Transport Explained - IGCSE / O-Level Biology (Cell Transport)
Every living cell has to move substances in and out — oxygen and glucose in, waste out, mineral ions absorbed from the soil. It does this in three ways: diffusion, osmosis and active transport. These three processes appear on every IGCSE Biology paper, from gas exchange to plant transport, so they are some of the most reliable marks you can revise. This lesson covers all three, with the exact worked example examiners use for the osmosis practical.
1. Why cells need transport — the cell membrane
Every cell is surrounded by a cell membrane that is partially permeable (also called semi-permeable or selectively permeable). It lets small molecules such as water and oxygen pass through, but blocks larger ones. This selective barrier is what makes controlled movement — especially osmosis — possible.
2. Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive — it needs no energy from the cell.
Everyday biological examples:
- Oxygen diffuses from the alveoli in the lungs into the blood; carbon dioxide diffuses the opposite way.
- Digested food molecules such as glucose diffuse from the small intestine into the bloodstream.
What affects the rate of diffusion?
| Factor | Effect on rate |
|---|---|
| Concentration gradient | Steeper gradient → faster |
| Temperature | Higher temperature → faster (particles have more energy) |
| Surface area | Larger surface area → faster |
| Distance (membrane thickness) | Shorter distance → faster |
Exchange surfaces such as the alveoli and the villi of the small intestine are adapted to maximise every one of these factors.
3. Osmosis
Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution). Like diffusion, it is passive.
A simple way to remember water potential:
- A dilute solution has lots of water → high water potential.
- A concentrated solution has little water → low water potential.
- Water moves from high to low water potential — from dilute towards concentrated.
⚠️ Common mistake: in osmosis it is the water that moves, not the dissolved solute. Never write that "the sugar moves across".
Osmosis in animal cells
Animal cells have no cell wall, so they are easily damaged:
- In a dilute solution (e.g. pure water), water enters, the cell swells and may burst (lysis).
- In a concentrated solution, water leaves and the cell shrivels (crenation).
This is why the water concentration of blood is kept steady by the body.
Osmosis in plant cells
| Solution outside cell | Water movement | State of cell |
|---|---|---|
| Dilute (high water potential) | Water enters | Turgid (firm — supports the plant) |
| Concentrated (low water potential) | Water leaves | Flaccid, then plasmolysed (membrane pulls away from the wall) |
4. The osmosis practical — and the calculation
The classic required practical uses potato cylinders. You measure each chip's mass, leave the chips in solutions of different concentration, then dry and reweigh them. Chips in water gain mass; chips in strong sugar solution lose mass.
Always express the result as a percentage change, so that chips of different starting sizes can be compared fairly:
Percentage change in mass = (change in mass ÷ starting mass) × 100
Worked example
A potato chip has a starting mass of 5.0 g. After one hour in concentrated sugar solution it has a mass of 4.4 g.
- Change in mass = 4.4 − 5.0 = −0.6 g
- Percentage change = (−0.6 ÷ 5.0) × 100 = −12%
The chip lost water by osmosis, because the sugar solution had a lower water potential than the potato cells.
5. Active transport
Active transport is the movement of particles against a concentration gradient — from a lower to a higher concentration — using energy from respiration.
Because it goes "uphill", active transport:
- requires energy released by respiration (as ATP), and
- uses carrier proteins in the cell membrane to pump particles across.
Two examples the exam loves:
- Root hair cells absorb mineral ions from the soil, even though the soil is more dilute than the cell.
- The small intestine absorbs the last of the glucose from digested food, against the gradient, so none is wasted.
6. Comparing the three processes
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Any particle | Water only | Any particle |
| Direction | Down the gradient | Down the gradient (water) | Up the gradient |
| Membrane needed? | No | Partially permeable | Membrane + carrier proteins |
| Energy needed? | No (passive) | No (passive) | Yes (from respiration) |
7. Exam tips and the common mistakes
- Osmosis moves water, not solute. Never say the sugar or salt moves.
- Only active transport uses energy. Diffusion and osmosis are passive.
- For percentage change, divide by the starting mass — and keep the + or − sign, because it tells you whether water entered or left.
- Turgid / flaccid / plasmolysed are plant-cell terms; lysis / crenation are animal-cell terms. Don't mix them.
📱 Follow us on Facebook: ISLAC – International Exam Prep
Subscribe to ISLAC for daily IGCSE lessons and quizzes!
Comments
Post a Comment