The big idea: A neuron that is not carrying a signal is not 'switched off' — it is held ready.
Across the neuron's membrane there is a small voltage called the resting potential, where the inside of the neuron is negative compared with the outside — about −70 mV.
This negative inside is set up and maintained by actively pumping ions across the membrane, which costs the cell energy (ATP). It is the starting point from which a nerve impulse can later fire.
Before any signal, the membrane sits at its resting potential of about −70 mV (the flat baseline on the left). The sodium-potassium pump uses ATP to hold the inside negative relative to the outside; only later does a stimulus trigger depolarisation.
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- Resting potential
- The voltage (potential difference) across the membrane of a neuron that is NOT conducting an impulse — the inside is about −70 mV relative to the outside.
- Membrane potential
- Any voltage across a cell membrane, caused by an uneven distribution of charged ions on the two sides.
- Sodium-potassium pump
- A membrane protein that uses ATP to move sodium ions (Na⁺) out of the neuron and potassium ions (K⁺) into it, against their concentration gradients.
- Active transport
- Movement of substances across a membrane AGAINST their concentration gradient, which requires energy from ATP.
- Concentration gradient
- A difference in the concentration of a substance between two regions — here, the difference in Na⁺ and K⁺ either side of the membrane.
Negative inside, ready to fire: The key fact to lock in: at rest, the inside of the neuron is negative (≈ −70 mV) relative to the outside.
This is not an accident — the cell spends ATP to keep it that way, so that a signal can be triggered the instant it is needed.
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The resting potential does not happen by itself. It is built and held by the sodium-potassium pump, a protein in the neuron's membrane.
Follow the cause and effect: the pump moves ions, the ions become unevenly spread, and that uneven spread of charge is the voltage.
The pump moves 3 out, 2 in: Each cycle, the sodium-potassium pump moves 3 sodium ions (Na⁺) OUT of the neuron and 2 potassium ions (K⁺) IN.
Because more positive ions leave than enter (3 out vs 2 in), the inside of the cell is left with less positive charge — it becomes negative relative to the outside.
Both ions are moved against their concentration gradients, so the pump must spend ATP to do it — this is active transport.
| Step | What the sodium-potassium pump does | Why it matters |
|---|---|---|
| Pump sodium out | Moves 3 sodium ions (Na⁺) OUT of the neuron | Removes positive charge from inside the cell |
| Pump potassium in | Moves 2 potassium ions (K⁺) IN to the neuron | Fewer positive ions enter than left, so the inside loses charge overall |
| Uses energy | Each pumping cycle uses one molecule of ATP | The ions are moved AGAINST their concentration gradients — this needs energy (active transport) |
| Potassium leaks back out | K⁺ slowly leaks back out through K⁺ channels | Leaves even fewer positive ions inside, making the inside more negative |
Then potassium leaks back out: The pump builds a high concentration of K⁺ inside the neuron. Because K⁺ is now more concentrated inside, some of it leaks back out down its concentration gradient through potassium channels.
Every K⁺ that leaks out takes a positive charge with it, making the inside even more negative.
Together, the pump and this potassium leak settle the membrane at its resting value of about −70 mV.
Where the ATP comes from: The pump cannot run for free — it needs a constant supply of ATP.
That ATP is made by cell respiration (in the neuron's mitochondria). This is why a neuron starved of oxygen or glucose quickly loses its resting potential: no respiration → no ATP → the pump stops → the gradients run down.
| Feature | Sodium-potassium pump | Potassium leak |
|---|---|---|
| Direction of ions | Na⁺ out, K⁺ in | K⁺ out |
| With or against gradient | Against the concentration gradient | Down the concentration gradient |
| Energy needed | Yes — uses ATP (active transport) | No — passive (facilitated diffusion) |
| Effect on the inside | Sets up the gradient (3 out, 2 in → net loss of +) | Makes the inside more negative still |
Sodium-potassium pump
- Moves Na⁺ OUT and K⁺ IN
- Works against the concentration gradients
- Uses ATP (active transport)
- Sets up the ion difference across the membrane
Potassium leak
- K⁺ leaks back OUT of the neuron
- Moves down its concentration gradient
- No ATP needed (passive diffusion)
- Makes the inside more negative → helps reach −70 mV
A memory hook: 3 out, 2 in — more positive ions leave than enter, so the inside goes negative.
Pump = ATP (active, against the gradient); leak = free (passive, down the gradient). Together they give −70 mV.
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How this is tested: A favourite Paper 1B short answer asks you to outline how ATP from respiration is used to establish the resting potential — your two scoring points are usually the active transport by the sodium-potassium pump (Na⁺ out, K⁺ in) and that this needs ATP because the ions move against their gradients.
On Paper 1A, a one-mark item often asks you to identify how sodium and potassium ion movements set up the resting potential — the answer is the pump moving Na⁺ out and K⁺ in, leaving the inside negative.
Because this is a membrane / ion topic, examiners also like a short data twist: a table of ion concentrations inside vs outside the axon, asking you to deduce what the pump is doing.
IB-style question — outline how ATP establishes the resting potential
Outline how ATP produced in respiration is used to establish the resting potential of a neuron. [2]
How to score both marks
- Name the active-transport step. ATP powers the sodium-potassium pump, which actively transports 3 sodium ions (Na⁺) out of the neuron and 2 potassium ions (K⁺) in per cycle.
- Link it to the negative inside. Because the ions are pumped against their concentration gradients (which needs the ATP), more positive ions leave than enter, so the inside becomes negative relative to the outside — the resting potential of about −70 mV. (Mark 1: ATP used by the pump for active transport of Na⁺/K⁺. Mark 2: ions moved against gradient, leaving the inside negative.)
Final answer
ATP from respiration powers the sodium-potassium pump, which actively transports Na⁺ out and K⁺ in against their concentration gradients; more positive ions leave than enter, so the inside becomes negative (≈ −70 mV) — the resting potential.
Why this scores full marks: The two marks are two distinct ideas: (1) ATP runs the pump that moves Na⁺ and K⁺ by active transport, and (2) moving them against the gradient leaves the inside negative.
Saying only 'ATP is used to make the resting potential' without naming the pump and the negative inside would not score both marks.
| Ion | Concentration inside the axon | Concentration outside the axon | Kept this way by |
|---|---|---|---|
| Sodium (Na⁺) | Low | High | The pump constantly moving Na⁺ out |
| Potassium (K⁺) | High | Low | The pump constantly moving K⁺ in |
| Overall charge | More NEGATIVE (≈ −70 mV) | More positive (set as 0 mV) | The pump + K⁺ leaking back out |