Overview
This lecture explains how the sodium-potassium pump establishes a cell's resting membrane potential and how cells use the resulting electrochemical gradient for active transport, especially in secondary active transport with glucose.
Sodium-Potassium Pump & Resting Membrane Potential
- The sodium-potassium pump actively moves 3 sodium ions (Na⁺) out and 2 potassium ions (K⁺) in per cycle.
- This pump creates a higher concentration of sodium outside and potassium inside the cell.
- The resulting charge difference helps establish the resting membrane potential but is not the sole factor.
- Potassium ions tend to diffuse back out of the cell through channels, balancing charge and concentration gradients.
Electrochemical Gradient
- Sodium ions accumulate outside, creating both a concentration and electric (charge) difference across the membrane.
- An electrochemical gradient combines the chemical (concentration) gradient and electric gradient (potential difference).
- Sodium ions naturally want to move back into the cell due to both gradients, creating potential energy.
- Cells can use the energy stored in these gradients, especially the sodium electrochemical gradient.
Secondary Active Transport & Simporters
- Simporters use the sodium electrochemical gradient to transport other molecules into the cell.
- A common example is the sodium-glucose simporter, which brings glucose into the cell against its concentration gradient.
- Glucose transport in this way is called secondary active transport because it uses energy stored from another active transport process (the sodium-potassium pump).
- Glucose moves into the cell together with sodium, utilizing the sodium gradient for energy.
Key Terms & Definitions
- Resting Membrane Potential — The electrical potential difference across a cell membrane at rest, mainly due to ion gradients.
- Sodium-Potassium Pump — A protein that uses ATP to move sodium out and potassium into the cell, creating gradients.
- Electrochemical Gradient — A combined effect of concentration and electric charge differences across a membrane, driving ion movement.
- Simporter — A protein that transports two substances 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.
Action Items / Next Steps
- Review how active and secondary active transport differ.
- Study the structure and function of the sodium-potassium pump.
- Prepare for questions on electrochemical gradients and their role in cellular transport.