Enzymes lower the barrier. They add no energy.
- The idea
- Predict
- See it
- Why
- Check
The idea
“An enzyme speeds up a reaction by giving the molecules the energy to react.”
With an enzyme, a reaction that took weeks happens in seconds. Speed like that seems to need extra energy, like a push on a stalled car.
The short version
The mistake: An enzyme speeds up a reaction by giving the molecules the energy to react.
What's actually true: An enzyme lowers the activation energy, so molecules react with energy they already have. ΔG and equilibrium stay the same.
- Reactants must get over an energy barrier, the activation energy, to become products. The energy comes from their own motion as they collide.
- An enzyme's active site holds the substrate so bonds break or form more easily. That lowers the barrier, so more molecules get over it.
- The enzyme adds no energy and comes out of each reaction unchanged. It isn't a reactant or product, so ΔG, the free-energy gap between them, stays the same.
- Both directions speed up, so equilibrium comes sooner, in the same place. To drive an unfavorable step, a cell couples it to ATP hydrolysis or removes the product.
On the exam: An enzyme changes how fast a reaction goes, never where it ends up.