A rate that levels off means every enzyme is busy
- The idea
- Predict
- See it
- Why
- Check
The idea
“When the rate levels off, the substrate is used up and the reaction is done.”
A graph of product over time really does flatten when the substrate runs out. So when a rate graph flattens, it looks like the same ending.
The short version
The mistake: When the rate levels off, the substrate is used up and the reaction is done.
What's actually true: The rate levels off when nearly every active site is filled. Product keeps forming at that top speed.
- Each active site holds one substrate molecule at a time. As substrate rises, more active sites are filled at any moment, so the rate climbs.
- Once nearly every active site is filled, extra substrate just waits its turn. The enzyme is saturated and running at its maximum rate.
- Initial rate is measured at the start, before much substrate is used. A flat rate still makes the same amount of product every minute.
- Adding more enzyme raises that ceiling, because it adds active sites.
On the exam: A flat rate at high substrate means every active site is busy. Product is still being made.