Specialized Membranes
High-Yield Summary
- Membrane potential = voltage difference across a membrane from uneven ion distribution. Ions leak down their gradient through leak channels, so maintaining the potential requires active pumps (e.g. sodium-potassium pump).
- Nernst equation gives the equilibrium potential for a single ion: (61.5 ÷ ion charge) × log(concentration outside ÷ concentration inside) — the voltage at which that ion has no net movement.
- Goldman-Hodgkin-Katz (GHK) equation extends this to multiple ions at once, weighting each by the membrane's relative permeability to it (e.g. higher K⁺ permeability → K⁺ dominates resting potential).
- Sodium-potassium pump (Na⁺/K⁺ ATPase): moves 3 Na⁺ out, 2 K⁺ in per ATP-powered cycle — electrogenic (net positive charge moved out), directly contributing to negative resting potential.
- Mitochondrial membranes are structurally specialized: outer membrane is permeable (large pores, smooth); inner membrane is highly selective, cholesterol-free, folded into cristae (maximize surface area for electron transport chain + ATP synthase), enclosing the matrix.
Key Terms
- Membrane potential
- Voltage difference across a membrane from uneven ion distribution inside vs. outside.
- Leak channel
- Channel allowing ions to passively diffuse down their concentration gradient, requiring active pumps to counteract.
- Equilibrium potential
- Voltage at which a specific ion has no net movement across the membrane (given by the Nernst equation).
- Electrogenic pump
- A pump (e.g. Na⁺/K⁺ ATPase) whose action creates a net movement of charge across the membrane.
- Cristae
- Folds of the inner mitochondrial membrane that increase surface area for the electron transport chain and ATP synthase.
- Mitochondrial matrix
- Space enclosed by the inner mitochondrial membrane, where key metabolic reactions occur.
Nernst Equation (Simplified Biology Form)
Equilibrium potential = (61.5 ÷ ion charge) × log(concentration outside ÷ concentration inside)
- Ion charge = The charge of the ion in question (e.g. +1 for K⁺, +2 for Ca²⁺)
- Concentration outside/inside = Ion concentration on each side of the membrane
- Gives the voltage at which diffusion outward and inward for that one ion are perfectly balanced (no net movement).
- The Goldman-Hodgkin-Katz (GHK) equation extends this to account for multiple ions and their relative membrane permeabilities simultaneously.
Mitochondrial Outer vs. Inner Membrane
| Feature | Outer vs. Inner Membrane |
|---|---|
| Permeability | Relatively permeable (small molecules/ions) / Highly selective, needs specific transporters |
| Structure | Smooth / Folded into cristae |
| Cholesterol | Present / Absent |
| Function | General boundary / Houses ETC + ATP synthase, encloses matrix |
Common MCAT Trap
- The Nernst equation handles ONE ion at a time; use the GHK equation when a question involves multiple ions and their relative permeabilities together (e.g. resting membrane potential).
- The Na⁺/K⁺ pump moves 3 Na⁺ out for every 2 K⁺ in — an unequal exchange, which is exactly why it's electrogenic (net positive charge leaves the cell) rather than charge-neutral.
- The inner mitochondrial membrane lacks cholesterol (unlike most other cellular membranes) — a detail that's easy to overlook but distinguishes it structurally from the outer membrane and plasma membrane.
Quick Recall
What does the Nernst equation calculate?
Why is the sodium-potassium pump described as electrogenic?
What structural feature of the inner mitochondrial membrane maximizes surface area for ATP production?
Which equation accounts for multiple ions' relative permeabilities in determining membrane potential?