A factor that stayed the same can't explain a difference
- The idea
- Predict
- See it
- Why
- Check
The idea
“Cold slows enzymes, so the cold must be why one tube ran slower.”
Cold does slow enzymes, so temperature looks like the answer. Both tubes sat in the same bath, though, so it acted on both at once.
The short version
The mistake: Cold slows enzymes, so the cold must be why one tube ran slower.
What's actually true: Only a factor that differed between the groups can explain a difference. One they all shared can't, even if it matters.
- The tubes differed only in pH.
- Temperature was a controlled variable, held the same for both tubes. Changing it moves both results together, so it can't explain why B was slower.
- Far from its best pH, some of the enzyme's amino acid side chains change charge. That disrupts the bonds holding the active site's shape, so it binds substrate less well.
On the exam: Before you blame a factor, check that it differed between the groups. A shared one can't explain the gap.