Passive transport can't pile up a solute
- The idea
- Predict
- See it
- Why
- Check
The idea
“With enough carriers or channels, a cell can keep pulling a solute in past the outside level.”
Carriers look like they grab molecules and haul them in, so it seems they could keep going. Cells really do hold some solutes far above outside levels, and that costs energy.
The short version
The mistake: With enough carriers or channels, a cell can keep pulling a solute in past the outside level.
What's actually true: Passive transport only moves a solute downhill, so net flow stops once both sides are equal. Holding more inside takes energy.
- In passive transport, molecules move at random. The side with more of them sends more across, so net flow runs from more to less.
- As the inside fills, more molecules cross back out. At equal levels, flow in matches flow out and the levels hold steady. That's equilibrium.
- Holding a solute above the outside level takes energy. A pump uses ATP, and a cotransporter uses an ion gradient.
- For ions, charge counts too, so downhill means down the electrochemical gradient. Channels still only let ions run downhill.
On the exam: If there's more inside than outside, something spent energy. Passive transport only evens things out.