Atlas MCAT Course
Biochemical Integration → Resting Membrane Potential
Resting Membrane Potential
How multiple ions set a cell’s voltage.
01The idea
So far, we’ve reasoned about one ion at a time. Real cells contain many ions simultaneously, all with different gradients and permeabilities. Yet the cell has one membrane potential.
The weights are set by permeability, not concentration alone.
THREAD · Constraints over choices
02Build the model
AThe dominant idea: permeability determines influence
Each ion “wants” to drive the membrane potential toward its own equilibrium (Nernst) potential. But ions differ in how permeable the membrane is to them.
- An ion that can cross the membrane easily has a large influence on membrane potential
- An ion that is mostly blocked has little influence, even with a steep gradient
BWhy potassium dominates at rest
In most resting cells, the membrane is most permeable to K⁺. Why? Many K⁺ leak channels are open at rest, and fewer Na⁺ channels are open. As a result, the K⁺ equilibrium potential strongly shapes resting voltage.
CSodium and chloride: secondary contributors
| Ion | At rest | Result |
|---|---|---|
| Na⁺ | Large inward gradient, low permeability | Small influence on resting potential |
| Cl⁻ | Often passively distributed; may be near equilibrium | Can stabilize membrane potential; direction depends on cell type |
DThe Na⁺/K⁺ pump: maintenance, not creation
The Na⁺/K⁺ ATPase pumps Na⁺ out and K⁺ in, using ATP. Its role is to maintain ion gradients over time.
Instead, it preserves the gradients that allow the potential to exist.
EDynamic equilibrium, not static balance
At rest, ions continue to leak and pumps continue to operate. The system is in steady state, not true equilibrium.
This connects electrochemistry directly back to metabolism.
FPredicting changes: MCAT-style reasoning
You should be able to reason:
- Increasing K⁺ permeability → membrane potential moves toward K⁺ equilibrium
- Increasing Na⁺ permeability → membrane potential becomes more positive
- Blocking the Na⁺/K⁺ pump → slow collapse of gradients
No equations required — only directional logic.
03See the connections
The lesson names these links itself. Read the map left to right.
Each ion pulls toward
- Its own equilibrium potential
Weighted by
- Permeability (open channels)
- K⁺ leak channels dominate at rest
Kept alive by
- Na⁺/K⁺ ATPase
- Constant ATP input (steady state)
Next: chemical equilibrium.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
Why is the resting membrane potential close to the K⁺ equilibrium potential?
Think first.
A drug opens many more Na⁺ channels in a resting cell. What happens to the membrane potential?
Think first.
The Na⁺/K⁺ pump is blocked. What happens?
Think first.
A cell is at its resting membrane potential. Which describes it best?
05MCAT lens
The MCAT emphasizes
Relative permeabilities
Directional predictions
Conceptual understanding
You are rarely asked to calculate exact voltages. You are often asked to predict what happens when permeability changes.
06Common traps
Trap 1: The most concentrated ion dominates
Misconception
Permeability matters more.
Correct model
Influence depends on channel availability.
Trap 2: Pumps create membrane potential directly
Misconception
Pumps maintain gradients.
Correct model
Leak channels shape voltage.
Trap 3: Resting potential means nothing is moving
Misconception
Ions continuously move.
Correct model
Fluxes are balanced.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why K⁺ dominates resting potential
Check against 02 · B: the resting membrane is most permeable to K⁺.
Why Na⁺ has little influence at rest
Check against 02 · C: low permeability means weak influence.
Why pumps are necessary but indirect
Check against 02 · D: they maintain gradients rather than setting voltage.
Why resting potential requires energy
Check against 02 · E: it is a steady state, maintained by constant input.
If something feels unclear, the missing idea is usually thisResting voltage reflects permeability-weighted ion equilibria.
08Remember
REMEMBER · if you keep only a few things
- Resting potential is a permeability-weighted compromise.
- K⁺ dominates at rest because K⁺ leak channels are open.
- Resting potential sits near the K⁺ equilibrium, not on it.
- Na⁺ has a big gradient but little resting influence.
- The Na⁺/K⁺ pump maintains gradients; it does not set voltage directly.
- Rest is a steady state that costs energy.
09Forward hook
You now understand electrochemical gradients, equilibrium potentials, and multi-ion membrane behavior. This completes the electrochemistry block needed before physiology. The next logical step is classical physics applied to biology:
What physical rules govern pressure and flow in the body?
Mechanics, pressure, and flow will let us explain blood circulation, filtration, and respiration. First, one more chemistry idea: equilibrium.
Next lessonMolecular & Cell Biology→