Overview
This lecture explains how the sodium-potassium pump helps establish a cell's resting membrane potential and introduces the concepts of electrochemical gradients, and secondary active transport involving the sodium-glucose symporter.
Sodium-Potassium Pump and Resting Membrane Potential
- The sodium-potassium pump actively moves 3 sodium ions out of the cell for every 2 potassium ions pumped in.
- This 3:2 ion exchange helps generate a charge difference across the membrane, contributing to the resting membrane potential.
- Potassium ions can diffuse back out down their concentration gradient, but the positive charge outside limits how much leaves.
Electrochemical Gradient
- Sodium ions accumulate outside the cell, creating both a concentration gradient and a more positive charge outside.
- If unrestricted, sodium would move into the cell due to both its chemical (concentration) and electric (charge) gradients.
- The combined effect of these two forces is called the electrochemical gradient.
- Electrochemical gradients represent stored potential energy cells can use.
Secondary Active Transport & Symporters
- Cells use the sodium electrochemical gradient as energy to transport other molecules against their own gradients.
- A symporter (specifically, a sodium-glucose symporter) uses the energy from sodium moving into the cell to bring glucose in at the same time.
- Glucose is transported into the cell against its concentration gradient, which requires energy.
- This process is called secondary active transport because it uses energy stored in the sodium gradient, originally created by the sodium-potassium pump.
Key Terms & Definitions
- Resting membrane potential — the voltage difference across a cell membrane when the cell is not active.
- Concentration gradient — difference in the concentration of a substance across a space or membrane.
- Electrochemical gradient — the combined effect of concentration gradient and electric charge difference across a membrane.
- Symporter (simporter) — a protein that transports two molecules together in the same direction across a membrane.
- Secondary active transport — transport of a molecule against its gradient using energy stored in another molecule's electrochemical gradient, not direct ATP.
Action Items / Next Steps
- Review how the sodium-potassium pump establishes ion gradients in the cell.
- Understand the distinction between primary and secondary active transport.