A fast rate doesn't mean a pump is working
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a solute crosses fast, or its rate keeps climbing, it must be actively transported.”
Active transport sounds like the powered option, so a fast rate looks like a pump. Channels are often faster than pumps, and they use no ATP.
The short version
The mistake: If a solute crosses fast, or its rate keeps climbing, it must be actively transported.
What's actually true: Passive speed depends on the gradient and open paths. Active transport shows up as a solute rising past the outside level.
- Passive transport speeds up with a steeper gradient or more channel and carrier proteins. None of it needs ATP.
- An uncharged solute moves passively until inside matches outside. Then net movement stops.
- A pump moves a solute against its gradient, so the solute can build up past the outside level.
- To spot active transport, look for a solute piling up against its gradient, or uptake that drops without ATP. A high or rising rate shows neither.
On the exam: Only uphill movement needs energy. Check where the solute ends up.