A missing product doesn't show which step broke
- The idea
- Predict
- See it
- Why
- Check
The idea
“If the end product is missing, the enzyme that makes it must be broken.”
It's natural to blame the enzyme that makes the missing product. A block at any earlier step leaves it missing too.
The short version
The mistake: If the end product is missing, the enzyme that makes it must be broken.
What's actually true: A block at any step stops everything after it. You find it by what piles up, or by which added substance brings the product back.
- Each step makes the input for the next. When one step fails, every step after it runs dry.
- The substance made just before the block usually piles up, because nothing uses it.
- Or add a later substance directly. If the product comes back, every step from there on works.
On the exam: No product means the chain broke. What piles up, or what added substance fixes it, shows you where.