The partner reaction has to release enough energy
- The idea
- Predict
- See it
- Why
- Check
The idea
“If an endergonic step is coupled to any exergonic reaction, the step will run.”
Diagrams show ATP driving step after step, so releasing energy looks like the only requirement. The release has to cover the cost.
The short version
The mistake: If an endergonic step is coupled to any exergonic reaction, the step will run.
What's actually true: A coupled pair runs only when the two free energy changes add up to a negative number.
- A reaction proceeds on its own only when its free energy change is negative. Exergonic reactions release free energy. Endergonic ones need it.
- Coupled reactions happen together, often through a shared intermediate. ATP may hand its phosphate to a molecule, which then reacts. The two changes add.
- So the pair runs only if the sum is negative. That's why ATP can drive many steps, but not every one.
On the exam: Add the two free energy changes. Coupling works only when the total is negative.