At equilibrium, molecules keep crossing
- The idea
- Predict
- See it
- Why
- Check
The idea
“Once the concentrations are equal, molecules stop crossing the membrane.”
A steady concentration looks like nothing is happening, so it's easy to picture the molecules parked. They're still crossing, as many each way.
The short version
The mistake: Once the concentrations are equal, molecules stop crossing the membrane.
What's actually true: At equilibrium, molecules cross both ways at equal rates, so the concentrations stay the same while the traffic goes on.
- Molecules move at random. One that reaches an open channel can cross from either side, and the side with more molecules sends more across.
- At first almost every crossing is inward. As the inside fills, more molecules leave through the same channels.
- Equilibrium is where the two rates match, so the concentrations hold steady. This is called dynamic equilibrium.
- Steady ATP in a cell isn't equilibrium. ATP is remade as fast as it's used, and that takes a steady supply of energy.
On the exam: Equal concentrations mean equal traffic both ways. Zero net change still has molecules crossing.