The big idea: Active transport uses a pump protein and energy from ATP to force a particle against its concentration gradient.
**Passive** movement (most movement)
- Particles drift down their concentration gradient
- From where they are crowded to where they are sparse
- No energy is needed
**Active transport**
- Moves a substance the wrong way — against its gradient, from low concentration to high
- That cannot happen on its own
- A pump protein in the membrane uses energy from ATP
- Active transport
- The movement of a substance across a membrane AGAINST its concentration gradient, using energy from ATP.
- Concentration gradient
- The difference in concentration of a substance between two regions. 'Down' the gradient means high → low; 'against' it means low → high.
- Pump protein
- A membrane protein that uses ATP to move a specific particle against its gradient by changing shape.
- ATP
- The molecule cells use as their energy currency; active transport spends ATP to power the pump.
- Passive transport
- Movement DOWN a concentration gradient (e.g. diffusion, osmosis) that needs no ATP.
Two words tell you it's active: If a question says a particle moves AGAINST the gradient, or that the process uses ATP, it is active transport.
Either word on its own is the giveaway — passive transport does neither.
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Pump proteins keep the inside of a cell very different from the outside. Animal cells, for example, hold far more potassium (K⁺) inside than the fluid around them, and far less sodium (Na⁺).
What pumps do: They keep the inside of the cell high in potassium (K⁺) and low in sodium (Na⁺): the opposite of what diffusion alone would produce.
Why the gradients don't just even out: Pumps use ATP to top the gradients up faster than they leak away. This is the key data-question idea.
Leak and pump
Ions leak back
Sodium and potassium constantly leak back down their gradients through channels.
Without pumps
If nothing opposed this, the inside and outside would slowly equalise.
Pumps run continuously
It replaces what leaks away, so the steep ion gradients are maintained.
The result
A cell can keep a high-K⁺ / low-Na⁺ interior only because pumps use ATP.
Leak back → pump tops up → gradients kept
Reading an ion-concentration table: Exams often give a table showing K⁺ much higher inside the cell than outside, and Na⁺ much higher outside than inside.
What to say
- Those differences are maintained by active transport (pump proteins using ATP)
- Not by diffusion, which would erase them
- If the cell were starved of ATP, the pumps would stop and the concentrations inside and outside would gradually become equal
Passive transport
- Moves down the gradient (high → low)
- No ATP used
- Diffusion, osmosis, facilitated diffusion
- Tends to even out concentrations
Active transport
- Moves against the gradient (low → high)
- Uses ATP (a pump protein)
- Potassium uptake by animal cells; mineral uptake by roots
- Maintains steep concentration differences
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction of movement | DOWN the concentration gradient (high → low) | AGAINST the concentration gradient (low → high) |
| Energy (ATP) | No ATP used | Uses ATP (energy required) |
| Protein needed? | Simple diffusion: none; facilitated: channel/carrier | Always — a pump protein |
| Examples | Simple diffusion, osmosis, facilitated diffusion | Potassium uptake by animal cells; mineral uptake by root cells |
An easy way to remember: Active = Against + ATP. Both 'A' words go together.
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The big idea: A pump protein can move particles against their concentration gradient, because it uses energy from ATP.
The points to remember
- Pump proteins are integral proteins that move specific particles across the membrane.
- They use energy from ATP: ATP is split into ADP and phosphate.
- So they can move particles against the concentration gradient, from low to high concentration.
- The particle binds on one side; the energy changes the pump's shape, and it is released on the other.
- This is active transport. Without ATP, pumps stop, and the differences they keep slowly disappear.
Remember it as: Pump: ATP in, particle up the gradient.
Real example: root hair cells: Root hair cells take in mineral ions such as nitrate even when there are more inside the cell than in the soil. Pump proteins use ATP to do it, so roots need oxygen for respiration.
How this is tested: Active transport is tested as a 1-mark Identify, a data question on ion gradients, and a Paper 2 Describe.
| Question | What you are asked | What scores |
|---|---|---|
| 1-mark Identify | Name 'the protein that uses ATP to move particles against a concentration gradient' | A pump protein (active transport) |
| Paper 1B / data, a favourite | An ion-concentration table (K⁺ high inside, Na⁺ high outside): how does the cell maintain those gradients? | Active transport by the Na⁺/K⁺ pump, using ATP |
| Paper 2, 3-mark Describe | The three processes by which molecules cross the bilayer | Simple diffusion, facilitated diffusion and active transport, making the passive-vs-active contrast clear |
Exam-style question — describe how molecules cross the membrane
Describe three different processes by which molecules cross the phospholipid bilayer of a cell membrane. [3]
How to score all three marks
- Simple diffusion. Small, non-polar molecules pass straight through the bilayer, moving down the concentration gradient, using no ATP (passive).
- Facilitated diffusion. Ions and large polar molecules pass through a channel or carrier protein, still moving down the gradient and using no ATP (passive).
- Active transport. A pump protein moves a particle against the gradient, using ATP (active). (Award 1 mark for each correctly described process, up to 3.)
Final answer
Simple diffusion (small molecules through the bilayer, down the gradient, no ATP); facilitated diffusion (ions/large molecules through a protein, down the gradient, no ATP); active transport (a pump moves particles against the gradient, using ATP).
Why this scores full marks: Each line is a distinct process and states two scoring facts — what crosses and the gradient/ATP status.
A 3-mark 'describe' needs three separate processes, with the passive-vs-active difference made obvious — not the same idea three times.
| Process | How the particle crosses | Gradient & ATP |
|---|---|---|
| Simple diffusion | Small, non-polar molecules slip straight through the phospholipid bilayer | Down the gradient · no ATP (passive) |
| Facilitated diffusion | Ions and large polar molecules pass through a channel or carrier protein | Down the gradient · no ATP (passive) |
| Active transport | A pump protein moves the particle through, changing shape as it works | Against the gradient · uses ATP (active) |
A two-mark Explain on data:
Cells of the alga Nitella hold about 1,000 times more potassium than the pond water around them. Explain how this difference is maintained.
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