A scientist adds saliva boiled to 75°C to starch solution, then tests for starch and gets a positive result. Why?
AThe boiling converted the starch into maltose
BThe amylase was denatured, so starch remained
CThe saliva contained no amylase to begin with
DThe heat made the amylase work far faster
Answer & Solution
Correct answer: B. The amylase was denatured, so starch remained
1. Salivary amylase normally hydrolyses starch to maltose, which would give a negative starch test.
2. Boiling to 75°C is above the enzyme's optimum temperature.
3. The added kinetic energy makes the enzyme's molecules vibrate, breaking the weak bonds holding its shape.
4. Losing its three-dimensional shape, the active site no longer fits starch, so the enzyme is denatured.
5. The starch is therefore left unconverted, which is exactly why the starch test comes back positive.
_Source: NECTA ACSEE 2022 Biology 133/1, Question 1: Cytology_
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