Resting Potentials (AQA A Level Biology): Revision Note

Establishing resting potential

• Neurones that are not actively transmitting nerve impulses have a difference in charge across their cell surface membranes that is known as resting potential

  • The difference in charge across a membrane can be described as:

    • potential difference

    • membrane potential

    • membrane voltage

• During resting potential the membrane potential is around -70mV

  • This means that the inside of an axon has a charge that is 70 mV more negative than the outside

• Two factors contribute to establishing and maintaining resting potential:

  • sodium-potassium pumps in the cell surface membrane

  • outward flow of potassium ions

Sodium-potassium pumps

• Carrier proteins called sodium-potassium pumps use ATP to actively transport:

  • 3 sodium ions (Na⁺) out of the axon for every

  • 2 potassium ions (K⁺) that they actively transport in

• The result of this is a larger concentration of positive ions outside the axon than inside the axon

Outward flow of potassium ions

• The action of sodium-potassium pumps sets up an electrochemical gradient between the inside and outside of the axon

  • There are more potassium ions inside the axon than outside

  • There are more sodium ions outside the axon than inside

• Potassium ions diffuse out of the axon by facilitated diffusion, via open potassium ion channels in the cell surface membrane

  • Note that sodium ion channels in the axon are closed at this point, so the inward movement of sodium ions cannot occur

  • The difference in permeability to sodium and potassium at this point is described as differential membrane permeability

• This further increases the concentration of positive ions outside the axon and a difference of -70 mV is achieved