Membrane Potential & Nernst Equation
Calculate the resting membrane potential using the Goldman-Hodgkin-Katz equation. Visualize Na⁺ and K⁺ leak channels, actively tweak permeabilities, and trigger simulated action potentials.
WHAT IS MEMBRANE POTENTIAL?
The **membrane potential** is the difference in electrical charge (voltage) between the inside and outside of a cell. This potential is generated by the unequal distribution of ions, primarily and , across the plasma membrane. In a 'resting' neuron, the inside is typically negative (around -70mV) relative to the outside, creating a form of potential energy used for cellular work.
THE NERNST EQUATION & ION GRADIENTS
Two forces govern the movement of ions: the **chemical gradient** (concentration) and the **electrical gradient** (charge). Together, these form the **electrochemical gradient**. The **sodium-potassium pump** ( ATPase) actively maintains these gradients by pumping 3 out and 2 in for every ATP consumed. Because the membrane is more permeable to at rest (via leak channels), diffuses out, leaving behind a negative charge.
HOW TO USE THIS VISUALIZATION
1. **Adjust Ion Concentrations**: Change the levels of and inside and outside the cell. 2. **Open Channels**: Toggle leak channels to see how permeability affects the voltage. 3. **Run the Pump**: Activate the pump and watch the energy (ATP) count increase as the gradient is established. **Try This**: Open all Potassium leak channels. Notice how the membrane potential becomes more negative as exits. This demonstrates why is the primary determinant of the resting potential.
CORE FORMULAS
AP EXAM CONNECTION
Unit: Unit 2: Cell Structure and Function (Topic 2.6)
Learning Objective: ENE-2.C
COMMON MISCONCEPTIONS
- Thinking the membrane is equally permeable to all ions.
- Confusing the pump (active) with leak channels (passive).
- Believing the cell is positive on the inside at rest.
KEY TAKEAWAYS
- Membrane potential is an electrochemical gradient.
- Active transport (the pump) creates the gradient.
- Passive transport (leak channels) establishes the resting voltage.
- Potential energy is stored in the ion separation.
PRACTICE QUESTIONS
Q1 (QUANTITATIVE): If the concentration of is equal on both sides of the membrane and all other ions are ignored, what is the membrane potential?
Show Answer & Explanation
Answer: 0 mV.
Explanation: According to the Nernst equation, if , the log of 1 is 0, resulting in an equilibrium potential of 0 mV.
Q2 (CONCEPTUAL): What is the primary role of the Sodium-Potassium pump in a resting neuron?
Show Answer & Explanation
Answer: To maintain the concentration gradients of Na+ and K+.
Explanation: The pump uses active transport to counteract the constant leakage of ions, ensuring the cell remains 'charged' and ready to fire an action potential.
