3.5.2Inquiry Process
Nerve Impulses — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 133 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5The Resting Potential
Definition
- Definition The difference in electrical potential across the membrane of a neurone not carrying an impulse is about −70 mV, with the inside negative.
Sodium-potassium pump
- Sodium-potassium pump Using ATP, this pump moves 3 Na⁺ out of the cell for every 2 K⁺ taken in, creating concentration gradients.
Potassium leak channels
- Potassium leak channels K⁺ diffuses out through open channels much more easily than Na⁺ diffuses in, making the inside more negative.
Negative ions inside
- Negative ions inside Large negatively charged proteins and other anions cannot leave the cell, adding to the negative charge inside.
Section 2 of 5Spread of an Impulse
1
A stimulus makes the membrane in one region more permeable to Na⁺, so Na⁺ enters and that region depolarises.
2
The inside of that region becomes positive compared with neighbouring areas of the axon.
3
Positive charge flows sideways by local currents to the neighbouring resting region, and its potential rises.
4
If it reaches threshold there, voltage-gated sodium channels open and a new action potential starts.
5
The region just passed is refractory, so the impulse can only move forward along the axon, away from the cell body.
Section 3 of 5Maintaining the Gradients
Maintaining the Gradients
Active transport
- Active transport Pumping is energy-demanding, and neurones use a large share of the body’s ATP to restore ion gradients.
Selective permeability
- Selective permeability The lipid bilayer blocks ions, so only protein channels allow them across, and the channels are specific to one ion.
Electrochemical gradient
- Electrochemical gradient Ions move down gradients of both concentration and charge, and the balance of these forces sets the resting potential.
Oxygen need
- Oxygen need If ATP supply fails the pumps stop, gradients run down and neurones lose the ability to send impulses.
| Ion | Concentration | Main movement at rest |
|---|---|---|
| Na⁺ | High outside | Pumped out; little leaks in |
| K⁺ | High inside | Pumped in; leaks out |
Section 4 of 5Worked Example
Question
- The sodium-potassium pump transfers 3 Na⁺ out and 2 K⁺ in per ATP used.
- If it works 600 cycles in a short time, calculate the net charge moved out and explain its effect on the membrane.
- Na⁺ moved out = 600 × 3 = 1800 ions; K⁺ moved in = 600 × 2 = 1200 ions.
- Each ion carries one positive charge, so net positive charge out = 1800 − 1200 = 600 units.
- Losing net positive charge makes the inside of the cell more negative.
Answer A net loss of 600 positive charges, which contributes to the negative resting potential.
Section 5 of 5Exam Checklist & Tips
3.5.2 – Key Points for the Exam
- Resting potential The potential difference across a neurone membrane when no impulse is passing, about −70 mV.
- Sodium-potassium pump Protein that uses ATP to pump 3 Na⁺ out and 2 K⁺ in.
- Depolarisation Change in membrane potential towards a less negative or positive value.
- Local current Flow of charge along the axon that triggers the next region to depolarise.
Common Mistakes
- Saying the pump alone causes the resting potential; leak channels also matter.
- Believing an impulse is a flow of electrons along the axon; it is a wave of ion movement across the membrane.
- Writing that Na⁺ is pumped in; the pump moves it out.
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