Atlas MCAT Course
Biochemical Integration → Membrane Potentials & Equilibrium Gradients
Membrane Potentials & Equilibrium Gradients
Quantifying ion movement.
01The idea
You now understand that gradients store energy and that charge separation is a form of potential energy. When this charge separation occurs across a membrane, it creates something measurable: electrical potential (voltage).
Membranes don’t stop ions forever — they constrain them.
THREAD · Gradients and flow
02Build the model
AWhy membranes are essential
A membrane separates charge, limits ion movement, and allows selective permeability. If ions could move freely, gradients would collapse and no electrical potential would persist.
BChemical vs. electrical forces: a reminder
An ion experiences two simultaneous forces:
- Chemical force: pushes ions down their concentration gradient
- Electrical force: pushes ions toward opposite charge
The actual movement depends on the net force. This is why ions can move against concentration gradients, or remain stationary despite steep gradients.
CEquilibrium: when forces balance
An ion is at electrochemical equilibrium when the chemical force exactly balances the electrical force. At equilibrium there is no net ion movement, even though gradients still exist.
DThe Nernst concept, without math first
The Nernst relationship answers a specific question: what membrane potential exactly counteracts an ion’s concentration gradient? That value is the equilibrium potential for that ion.
- Large concentration gradient → large equilibrium potential
- Small concentration gradient → small equilibrium potential
EInterpreting the Nernst potential
For a given ion:
- If membrane potential equals its Nernst potential → no net movement
- If membrane potential is different → the ion will move
Direction depends on which force is stronger at the moment.
FWhy different ions have different equilibria
Each ion has a different concentration gradient and a different charge. Therefore each ion has its own equilibrium potential. This explains why Na⁺, K⁺, and Cl⁻ behave differently, and why changing permeability alters membrane potential.
GResting membrane potential: a preview
In cells, multiple ions contribute simultaneously, and the membrane potential reflects a weighted average. Ions that are more permeable have greater influence.
This will matter enormously in neurophysiology.
03See the connections
The lesson names these links itself. Read the map left to right.
Forces on an ion
- Chemical (concentration)
- Electrical (charge)
Balance point
- Equilibrium potential (Nernst)
- One per ion
In a real cell
- Several ions at once
- Weighted by permeability
Next: resting membrane potential.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
An ion is at its equilibrium potential. Which statement is true?
Think first.
Ion X has a much larger concentration gradient across the membrane than ion Y. What does that suggest about their equilibrium potentials?
Think first.
The membrane potential differs from an ion’s Nernst potential. What happens if the ion can cross?
Think first.
Why does a membrane potential require selective permeability rather than total impermeability or free movement?
05MCAT lens
The MCAT emphasizes
Direction of ion movement
Qualitative Nernst reasoning
Comparing potentials
You are rarely asked to calculate Nernst values numerically. You are often asked to predict whether an ion will move in or out.
06Common traps
Trap 1: Equilibrium means ions stop caring about gradients
Misconception
Forces still exist.
Correct model
Forces balance.
Trap 2: Concentration gradient alone determines movement
Misconception
Electrical forces matter.
Correct model
Net electrochemical force rules.
Trap 3: All ions behave the same way
Misconception
Charge and gradients differ.
Correct model
Each ion has its own equilibrium.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why membranes generate voltage
Check against 02 · A: selective permeability lets charge separate without the gradient collapsing.
Why equilibrium does not mean equal concentrations
Check against 02 · C: equilibrium is zero net flux while the gradient remains.
Why different ions have different equilibrium potentials
Check against 02 · F: different gradients and charges.
Why permeability determines influence
Check against 02 · G: the membrane potential is a permeability-weighted average.
If something feels unclear, the missing idea is usually thisMembrane voltage exists because gradients are constrained, not eliminated.
08Remember
REMEMBER · if you keep only a few things
- Membrane potential comes from constrained ion movement.
- Two forces on every ion: chemical and electrical.
- Equilibrium potential: the voltage that exactly balances an ion’s gradient.
- Equilibrium = zero net flux, not equal concentrations.
- Compare membrane potential to equilibrium potential to predict movement.
- More permeable ions have more influence.
09Forward hook
You now understand electrochemical gradients and equilibrium potentials. The next question is:
How do multiple ions together determine a cell’s actual membrane potential?
That leads directly to mixed-ion systems and resting membrane potential logic.
Next lessonResting Membrane Potential→