A lower barrier doesn't make an uphill reaction go
- The idea
- Predict
- See it
- Why
- Check
The idea
“Enzymes and ATP do the same job. They lower the activation energy, which lets an uphill reaction go.”
Enzymes and ATP show up together whenever a cell builds something, so they blur into one helper. They do two separate jobs.
The short version
The mistake: Enzymes and ATP do the same job. They lower the activation energy, which lets an uphill reaction go.
What's actually true: Lowering activation energy only changes speed. An uphill reaction goes when it is coupled to a downhill one, like ATP hydrolysis.
- Activation energy is the hump a reaction has to get over. An enzyme lowers it, so the reaction runs faster. The start and end don't move.
- ΔG is the gap between start and end. When ΔG is positive, the reaction is uphill and won't run forward on its own.
- Energy coupling links an uphill step to a downhill one, usually ATP hydrolysis, at the same enzyme. Their ΔG values add, and a negative total lets the pair go.
On the exam: An enzyme changes how fast, never ΔG. For an uphill step to go, find the downhill reaction it's coupled to.