A flat uptake rate means the carriers are full
- The idea
- Predict
- See it
- Why
- Check
The idea
“When the uptake rate levels off, the gradient must be gone.”
A flat curve looks like something ran out, and the gradient comes to mind first. Here the outside concentration keeps climbing, so the gradient keeps growing.
The short version
The mistake: When the uptake rate levels off, the gradient must be gone.
What's actually true: A rate that levels off while the outside concentration keeps rising means every carrier protein is already busy.
- A carrier protein binds a molecule, changes shape, and releases it on the other side. Each carrier can only make so many trips per second.
- At low concentration many carriers sit empty, so more solute means more trips. Once every carrier is busy, extra solute just waits. That flat top is saturation.
- Initial rates are measured while the inside is still near zero. So the gradient is largest where the curve is flattest.
- At equilibrium, with inside and outside equal, net uptake would fall to zero instead of holding at a high rate.
On the exam: A rate that flattens as the outside keeps rising means full carriers. At equilibrium it would drop to zero.