IB Biology Topic C2.2.2: Generation of the Resting Potential by Pumping Notes

The Na⁺/K⁺ Pump and Membrane Permeability Hold the Neuron at About −70 mV

1. Resting potential is the electrical charge difference across a neuron's plasma membrane when it is not transmitting a signal.
2. It is typically about −70 mV, with the inside of the neuron more negative than the outside.
3. It is established and maintained by the sodium-potassium pump (Na⁺/K⁺ pump) and by the membrane's selective permeability to ions.

The Na⁺/K⁺ Pump Uses ATP to Move 3 Na⁺ Out for Every 2 K⁺ In

1. The sodium-potassium pump is a transmembrane protein that actively transports ions against their concentration gradients.
2. For every 3 sodium ions (Na⁺) pumped out of the neuron, 2 potassium ions (K⁺) are pumped in.
   1. This unequal exchange gives a net loss of positive charge from inside the neuron.
3. The pump uses ATP, so moving these ions against their gradients is active transport.

Three Factors Make the Inside of the Neuron Negative

1. Unequal ion exchange: the pump moves more positive ions out (3 Na⁺) than in (2 K⁺), creating a charge imbalance.
2. Selective permeability: the membrane is more permeable to K⁺ than Na⁺, so K⁺ leaks out through potassium leak channels and takes positive charge with it.
3. Fixed negative charges: large negatively charged proteins are trapped inside the neuron and cannot cross the membrane.

The Pump Cycles Through Five Steps to Build the Ion Gradients

1. Step 1: the pump binds 3 Na⁺ ions from inside the neuron.
2. Step 2: ATP is hydrolysed, adding a phosphate group that makes the pump change shape.
3. Step 3: the pump releases the 3 Na⁺ ions to the outside of the cell.
4. Step 4: the pump binds 2 K⁺ ions from the extracellular space.
5. Step 5: the phosphate group leaves, the pump returns to its original shape, and the 2 K⁺ ions move into the neuron.
6. This keeps a high Na⁺ concentration outside the cell and a high K⁺ concentration inside.
   1. These concentration gradients store the potential energy the neuron uses to fire.

Resting Potential Keeps the Neuron Ready to Fire

1. The negative resting potential sets the stage for the rapid charge changes of an action potential.
2. The pump preserves the Na⁺ and K⁺ gradients so the neuron can respond quickly to a stimulus.

When a stimulus is strong enough, this stored charge is released as an action potential, covered in the next topic.