Mastering Resting Potential, Action Potential and Propagation: Common OCR A Level Biology Questions Answered
Prior Knowledge to Recap
Before diving into action potentials, make sure you're confident with these key concepts:
• Electrochemical gradients – understanding that ions move due to both concentration differences AND electrical charge differences across membranes is fundamental to grasping the resting potential
• Active transport mechanisms – the sodium-potassium pump uses ATP to move ions against their concentration gradients, which is essential for establishing the resting potential
• Channel proteins and their properties – knowing the difference between always-open channels, voltage-gated channels, and ligand-gated channels helps you understand different phases of the action potential
• Membrane permeability – appreciating that the lipid bilayer is impermeable to ions (they must use channels) and that different ions have different permeabilities explains why the resting potential exists
• Positive and negative ions – being clear about which ions carry positive charges (Na⁺, K⁺) and understanding how their movement affects the charge across the membrane
Links to GCSE Content
This topic builds directly on your GCSE Biology knowledge:
• Electrical impulses in nerves – at GCSE you learned that nerves carry electrical signals; now you'll understand the precise mechanism of how these electrical signals are generated and transmitted
• Movement of substances – GCSE covered diffusion and active transport; the action potential uses both of these processes as ions move through channels (diffusion) while the sodium-potassium pump works constantly (active transport)
• The nervous system – you learned that electrical impulses travel along neurones; A level reveals exactly how the membrane potential changes from -70mV to +30mV and back again, and how this "wave" propagates along the axon
Common Question Types and How to Answer Them
Let me walk you through five frequently asked questions from past OCR papers that specifically target resting potential, action potential, and propagation.
Question 1: Understanding What Happens During an Action Potential
Answer: B ✓
How to work through this systematically:
The key to these questions is understanding what's happening at each stage of the action potential. Let me break down the graph positions:
Position 1 (Resting potential at -70mV):
The membrane is at rest
Na⁺/K⁺ pump: Operating (it ALWAYS operates - this is crucial!)
Na⁺ channels: Closed
K⁺ channels: Some open (the membrane is slightly permeable to K⁺ at rest)
Position 2 (Rising phase - Depolarisation):
Membrane potential is becoming less negative/more positive
Na⁺/K⁺ pump: Still operating (never stops!)
Na⁺ channels: OPEN ← This is the cause of depolarisation
K⁺ channels: Closed (haven't opened yet)
Position 3 (Peak at +30mV):
Maximum depolarisation reached
Na⁺/K⁺ pump: Still operating
Na⁺ channels: Closing/inactivating
K⁺ channels: Just opening
Position 4 (Falling phase - Repolarisation):
Membrane potential returning to negative
Na⁺/K⁺ pump: Still operating
Na⁺ channels: Closed
K⁺ channels: OPEN ← This is the cause of repolarisation
Therefore, B is correct because at position 2 (depolarisation):
✓ Na⁺/K⁺-pump IS operating (yes)
✓ Voltage-gated Na⁺ channels ARE open (yes)
✓ Voltage-gated K⁺ channels are NOT open yet (no)
CRITICAL MISCONCEPTION TO AVOID:
Many students think the sodium-potassium pump only works during certain phases or switches on to restore the resting potential. This is wrong!
The markscheme confirms that the pump operates continuously throughout ALL phases of the action potential. It's constantly pumping 3Na⁺ out and 2K⁺ in, using ATP, maintaining the concentration gradients that make the action potential possible.
Memory aid for what causes what:
Na⁺ channels open → sodium RUSHES IN → depolarisation (UP)
K⁺ channels open → potassium RUSHES OUT → repolarisation (DOWN)
Na⁺/K⁺ pump → ALWAYS RUNNING → maintains gradients
Question 2: Identifying Phases of the Action Potential
Answer: B ✓
How to identify each phase correctly:
You need to know the precise definitions of each term:
Depolarisation:
Membrane potential becomes less negative (moving towards 0mV and beyond to positive values)
Caused by Na⁺ channels opening → Na⁺ rushing IN
On the graph: the rising/upward phase
Position 2 shows depolarisation ✓
Repolarisation:
Membrane potential returns to resting potential (becoming more negative again)
Caused by K⁺ channels opening → K⁺ rushing OUT
On the graph: the falling/downward phase
Position 4 shows repolarisation
Hyperpolarisation:
Membrane potential becomes MORE negative than resting potential (goes below -70mV)
Caused by K⁺ channels staying open slightly too long
On the graph: the dip below the resting potential line
Position 5 shows hyperpolarisation ✓
Return to resting potential:
Membrane potential returns to exactly -70mV
Sodium-potassium pump activity maintains this
Position 6 shows return to resting potential
Therefore B is correct:
✓ Depolarisation at position 2 (going UP)
✓ Hyperpolarisation at position 5 (dipping BELOW -70mV)
Why the others are wrong:
A: Position 4 is repolarisation (not depolarisation), position 6 is resting potential (not hyperpolarisation)
C: Position 6 is resting potential, not repolarisation (repolarisation is the falling phase at position 4)
D: Position 6 is resting potential (not hyperpolarisation) - hyperpolarisation is the dip at position 5
Exam technique: Draw a line at -70mV on the graph. Anything:
Going above this = depolarisation
Coming back down to it = repolarisation
Going below it = hyperpolarisation
At -70mV and stable = resting potential
Top tip: Learn this sequence by heart: Resting → Depolarisation → Repolarisation → Hyperpolarisation → Return to Resting
Question 3: Understanding Repolarisation at the Molecular Level
Answer: A ✓
Let's work through the logic:
What is repolarisation?
The membrane potential is returning from positive (+30mV) back towards negative (-70mV)
The inside of the cell is becoming more negative again
On a graph, this is the downward/falling phase
What needs to happen to cause this?
For the inside to become more negative, we need positive ions to leave the cell.
Step 1: What happens to sodium channels?
During depolarisation, Na⁺ channels were open (letting Na⁺ rush in)
During repolarisation, Na⁺ channels must be CLOSED
This stops positive sodium ions from entering
Eliminates options B and C
Step 2: What happens to potassium channels?
K⁺ channels OPEN during repolarisation
K⁺ ions rush OUT of the cell (down their concentration gradient)
This removes positive charge from inside the cell
This makes the inside more negative
Confirms options A or D
Step 3: Is membrane potential increasing or decreasing?
This is where you need to be careful with terminology!
"Membrane potential" refers to the voltage value
At the peak: +30mV (a high value)
During repolarisation: moving from +30mV back to -70mV
The value is going from +30 → 0 → -70
The numerical value is DECREASING (getting smaller/more negative)
Therefore A is correct:
✓ Sodium channels: closed
✓ Potassium channels: open
✓ Membrane potential: decreasing (from +30mV towards -70mV)
Common mistake: Students often think "decreasing" means "becoming more negative" so they choose the wrong answer. Think about the actual numbers: +30 to -70 is a decrease in value (even though it's becoming more negative).
Memory aid:
Depolarisation = sodium channels open, potential goes UP
Repolarisation = potassium channels open, potential goes DOWN (decreasing)
Question 4: How TTX Affects Action Potentials (Extended Response)
Model Answer using markscheme points:
"Sodium ions/Na ions/Na⁺ cannot enter (the neurone)" ✓
"No/prevents depolarisation of membrane" ✓
"(Membrane) remains at resting potential" ✓
"Prevents action potential being generated" ✓
"Impulse not conducted (along axon)" ✓
"(So) no release of neurotransmitter" ✓
(Award 4 marks maximum from these points)
Markscheme guidance - What to write:
DO NOT ALLOW "cannot enter membrane" - they enter the neurone/cell, not the membrane
ALLOW "sodium ions/Na ions/Na⁺ stay outside"
ALLOW "action potential" for "impulse"
Markscheme guidance - Award 3 max if: The explanation refers to what would normally happen in a neurone instead of what happens in the presence of TTX
How to structure your answer:
Think about the sequence of events that's being blocked:
Na⁺ channels can't open (given in question) ↓
Na⁺ can't enter ✓ ↓
No depolarisation ✓ ↓
Membrane stays at resting potential ✓ ↓
No action potential generated ✓ ↓
No impulse conduction ✓ ↓
No neurotransmitter release ✓
Examiner insight from markscheme:
"Higher ability candidates were able to demonstrate understanding of the transmission of nerve impulses and the consequences of voltage-gated sodium ion channels being unable to open. Responses from lower ability candidates often lacked detail such as not stating that it is the axon membrane that is not depolarised. Some responses also showed confusion regarding the concepts."
What makes a great 4-mark answer:
"TTX prevents voltage-gated sodium channels from opening, so sodium ions cannot enter the neurone. This prevents depolarisation of the axon membrane, which remains at resting potential of -70mV. Therefore, no action potential is generated and the impulse cannot be conducted along the axon."
This gets 4 marks because it:
States Na⁺ can't enter ✓
States no depolarisation ✓
States membrane remains at resting potential ✓
States no action potential generated ✓
Common mistakes to avoid:
Don't say "cannot enter membrane" - say "cannot enter neurone/cell"
Don't just describe what happens normally - explain what happens with TTX
Don't forget to specify it's the axon membrane that doesn't depolarise
Don't confuse the sequence - Na⁺ must enter before depolarisation can occur
Question 5: Identifying Key Events on an Action Potential Graph
Answer: (B and) C ✓
Markscheme guidance:
Mark the first answer(s)
If the answer is correct and an additional answer is given that is incorrect or contradicts the correct answer, then = 0 marks
Examiner insight: "Most candidates answered this correctly, although some did only mention B and so were not awarded the mark."
Why both B and C?
Let me explain what's happening at each position:
Position A: Resting potential (-70mV) - channels closed
Position B: Early depolarisation - channels OPENING
Position C: Rapid depolarisation - channels FULLY OPEN
Position D: Peak (+30mV) - channels starting to CLOSE
Position E: Early repolarisation - channels CLOSED
Position F: Hyperpolarisation - channels closed
Position G: Return to resting - channels closed
The key point: Voltage-gated sodium channels open during the rising phase of depolarisation. This includes both the early phase (B) and the steep upward phase (C). They're open throughout the depolarisation until the peak is reached.
You must give BOTH B and C to get the mark!
Part (ii): Repolarisation. (1 mark)
Answer: D and E ✓
Markscheme guidance:
Mark the first 2 answers
If the answer is correct and an additional answer is given that is incorrect or contradicts the correct answer, then = 0 marks
IGNORE F
Examiner insight: "Candidates often only stated E, less frequently D alone, while both were required for the mark."
Why both D and E?
Repolarisation is the process of the membrane potential returning from positive back to negative (from +30mV back towards -70mV).
Position D: The start of repolarisation (just after the peak, beginning to fall)
Position E: Continuation of repolarisation (falling steeply)
Position F: This is hyperpolarisation, NOT repolarisation (below -70mV)
Repolarisation is the entire falling phase from peak to resting potential, so includes both D and E.
Common mistake: Students often only give E (the steepest falling part) and forget that repolarisation starts at D (immediately after the peak).
Part (iii): Sodium ions are actively pumped out of the neurone. (1 mark)
Answer: All individual letters A to G / A to G / A – G ✓
OR: F ✓
OR: A and G ✓
Markscheme guidance:
CREDIT all letters A to G as the pump runs continuously
CREDIT F and/or A and G as these are the places where the pump has greatest effect
IGNORE B if given as an additional answer to an otherwise correct answer
Examiner insight: "Candidates did not appreciate that the sodium ion pump is not voltage-regulated and so is actively pumping the whole time. Allowance was made for this in the mark scheme and various combinations of letters were credited."
Why this is tricky:
This question tests a crucial concept: The sodium-potassium pump operates CONTINUOUSLY
Unlike the voltage-gated channels that open and close in response to voltage changes, the Na⁺/K⁺ pump:
Works all the time
Uses ATP constantly
Is NOT voltage-gated
Pumps 3Na⁺ out and 2K⁺ in continuously
Three acceptable answers:
"All letters A to G" - because the pump operates throughout the entire action potential ✓
"F" or "A and G" - because the pump has the greatest visible effect during resting potential and hyperpolarisation when it's restoring the resting potential ✓
Various combinations showing understanding it's always working
The key understanding:
During the action potential:
Voltage-gated channels cause the rapid changes (depolarisation and repolarisation)
The pump works in the background continuously, maintaining the gradients
The pump doesn't cause depolarisation or repolarisation, but without it constantly working, the concentration gradients would eventually run down and action potentials would be impossible.
Perfect answer: "A to G" (showing you know it's always operating)
Also acceptable: "A and G" (showing you know when it's most important for restoring resting potential)
Essential Concepts You MUST Understand
The Resting Potential (-70mV)
What creates it?
Sodium-potassium pump actively transports:
3Na⁺ OUT of the cell
2K⁺ IN to the cell
Uses ATP
Creates concentration gradients
Different permeabilities:
Membrane is MORE permeable to K⁺ (some K⁺ channels open)
Membrane is LESS permeable to Na⁺ (Na⁺ channels closed)
K⁺ diffuses out down its concentration gradient
This makes inside negative relative to outside
Result:
Inside of cell: negative (-70mV)
Outside of cell: positive (0mV)
Membrane is polarised
markscheme:
"Have a resting potential of approximately −70 mV" applies to B (both sensory and motor neurones) ✓
This confirms that all neurones have a similar resting potential of around -70mV.
The Action Potential - Complete Sequence
Phase 1: Resting Potential
Membrane at -70mV
Na⁺ channels: closed
K⁺ channels: some open
Na⁺/K⁺ pump: operating
Phase 2: Depolarisation
Stimulus causes membrane to reach threshold (usually -55mV)
Voltage-gated Na⁺ channels OPEN
Na⁺ rushes IN (down electrochemical gradient)
Membrane potential becomes less negative, then positive
Reaches peak of about +30mV
Na⁺/K⁺ pump: still operating
Phase 3: Repolarisation
Na⁺ channels CLOSE (inactivate)
Voltage-gated K⁺ channels OPEN
K⁺ rushes OUT (down concentration gradient)
Membrane potential becomes negative again
Returns towards -70mV
Na⁺/K⁺ pump: still operating
Phase 4: Hyperpolarisation
K⁺ channels stay open slightly too long
Too much K⁺ leaves
Membrane potential goes below -70mV (e.g., -80mV)
Na⁺/K⁺ pump: still operating
Phase 5: Return to Resting Potential
K⁺ channels close
Na⁺/K⁺ pump restores exact resting potential
Membrane returns to -70mV
Ready for next action potential
Propagation of the Action Potential
How does the action potential move along the axon?
Step 1: Action potential occurs at one region of axon membrane
Step 2: Na⁺ ions entering at this point create local currents
Na⁺ ions move sideways inside the axon
This causes depolarisation of the adjacent membrane region
Step 3: Adjacent region reaches threshold
Voltage-gated Na⁺ channels open in this new region
New action potential generated
Step 4: Process repeats along the axon
Action potential appears to "move" along axon
Actually, it's a wave of depolarisation
Each section generates its own action potential
Step 5: Why doesn't it go backwards?
Refractory period prevents this
After an action potential, Na�+ channels are inactivated
They cannot open again immediately
This ensures one-way transmission
Top Tips Based on Markscheme Guidance
1. The sodium-potassium pump ALWAYS operates:
Don't say it "switches on" during repolarisation
Don't say it only works at certain phases
It runs continuously using ATP
This came up in multiple questions (Q12, Q18)
2. Be precise about "membrane potential decreasing":
Decreasing = numerical value getting smaller
+30 → -70 is a decrease (even though it's more negative)
This is tested in Question 17
3. Know your definitions exactly:
Depolarisation = less negative/more positive
Repolarisation = returning to resting potential (more negative)
Hyperpolarisation = MORE negative than resting potential
Question 13 specifically tests this
4. Specify what ions do:
Don't just say "ions enter" - say which ions!
"Na⁺ cannot enter" (not "cannot enter membrane")
Question 14 markscheme is specific about this
5. For graph questions with multiple letters:
Some processes occur over multiple positions (like depolarisation at B and C)
Read carefully whether you need one letter or several
Question 18 requires multiple letters for several parts
6. Understand the cause-effect sequence:
TTX blocks Na⁺ channels → Na⁺ can't enter → no depolarisation → no action potential
This logical chain is essential for Question 14
What NOT to Write - Common Mistakes from Markschemes
Topic Don't write Do write Question Sodium entry "Cannot enter membrane" "Cannot enter neurone/cell" Q14 Channel notation "Na channels" "Na⁺ channels" / "sodium ion channels" Q18 Pump operation "Pump switches on during repolarisation" "Pump operates continuously" Q12, Q18 Membrane potential change "Potential increases during repolarisation" "Potential decreases during repolarisation" Q17 Hyperpolarisation position "Hyperpolarisation at position 6" "Hyperpolarisation at position 5" Q13 Voltage-gated channels "Channels open due to pressure" "Stretch-sensitive channels / mechanoreceptors" (Different topic)
Understanding Extended Response Questions
Question 15 is a 6-mark question comparing action potentials in different neurones.
Markscheme uses level descriptors:
Level 3 (5-6 marks):
"Comprehensive description of differences with explanations"
"Well-developed line of reasoning, clear and logically-structured"
"Uses scientific terminology appropriately"
Level 2 (3-4 marks):
"Good description with limited explanation"
"Some structure and appropriate scientific language"
"Information mostly relevant"
Level 1 (1-2 marks):
"Limited description with attempted explanation"
"Little structure"
"Inappropriate use of technical terms"
What this means for you:
To get Level 3 (5-6 marks) you MUST:
Describe what you see (e.g., "The dopamine neurone has a longer action potential duration")
Explain why this happens (e.g., "This is because voltage-gated potassium channels open more slowly")
Use correct terminology (depolarisation, voltage-gated channels, etc.)
Structure logically (use paragraphs or linking words like "furthermore")
Simply describing what you see on a graph = maximum Level 2 (4 marks)
You need BOTH description AND explanation for full marks!
Practice Questions to Test Yourself
Based on the markscheme insights, try these:
1. State three ways the sodium-potassium pump is essential for action potentials. (3 marks)
2. Explain why the membrane potential goes below -70mV during hyperpolarisation. (2 marks)
3. A student says "stronger stimuli produce bigger action potentials." Explain why this is incorrect and describe how stimulus strength is actually coded. (3 marks)
4. Describe the role of voltage-gated potassium channels during an action potential. (2 marks)
5. Explain why action potentials can only travel in one direction along an axon. (2 marks)
Model answers available in the markscheme principles we've covered!
Final Exam Checklist
Before your exam, make sure you can:
✓ Explain resting potential (-70mV) in terms of pump activity and membrane permeability
✓ Describe each phase of the action potential with correct terminology
✓ State what happens to each type of channel at each phase
✓ Explain that the pump operates continuously (not just during certain phases)
✓ Identify phases on a graph (depolarisation, repolarisation, hyperpolarisation)
✓ Understand "all-or-nothing" - same size action potentials regardless of stimulus strength
✓ Explain stimulus coding - frequency of action potentials represents stimulus intensity
✓ Describe propagation - local currents, sequential depolarisation, refractory period
✓ Explain the refractory period - ensures one-way transmission
✓ Apply knowledge to novel situations - like TTX blocking sodium channels
Summary: The Big Picture
The action potential is a carefully orchestrated sequence of events:
Resting potential maintained by continuous pump activity and differential permeability
Threshold reached by stimulus causing some depolarisation
Positive feedback as voltage-gated Na⁺ channels open → more depolarisation → more channels open
Rapid depolarisation as Na⁺ floods in
Na⁺ channels inactivate at peak, preventing further entry
K⁺ channels open causing repolarisation as K⁺ leaves
Hyperpolarisation as K⁺ channels close slowly
Pump continues working to maintain gradients for the next action potential
Local currents propagate the depolarisation along the axon
Refractory period ensures one-way transmission
Master this sequence and you'll be able to answer any question on this topic!
Remember: examiners reward precision, correct sequence, and clear explanations that show you understand the mechanisms, not just memorised facts.
Good luck with your revision! ⚡🧠
Pro tip: Draw the action potential graph from memory daily until you can label every phase, every channel opening/closing, and every ion movement without thinking. This is one of the most examined topics in A Level Biology!