Binding triggers the carrier. It doesn't pay for the move
- The idea
- Predict
- See it
- Why
- Check
The idea
“Binding to the carrier gives the solute the energy to cross.”
Binding comes first and the move follows, so binding looks like the push. It works like a key. It lets the carrier change shape, and something else pays.
The short version
The mistake: Binding to the carrier gives the solute the energy to cross.
What's actually true: Binding only triggers the carrier's shape change. The energy comes from the gradient, from ATP, or from another ion's gradient.
- A carrier binds its solute, changes shape, and releases it on the other side. Binding makes the carrier specific and sets off the shape change.
- Each cycle leaves the carrier as it started, so binding and release add no net energy.
- To go uphill, a carrier needs another source. A pump spends ATP. A cotransporter uses another ion's gradient, like sodium's.
On the exam: Binding is the trigger, never the fuel. Find the gradient or the ATP.