Here's why:
* Enzymes are highly specific: Each enzyme has a unique active site that is shaped to fit a specific substrate (the molecule it acts upon). This is like a lock and key mechanism – only the correct key (substrate) will fit into the lock (active site).
* Starch and sucrose have different structures: Starch is a complex carbohydrate made of many glucose molecules linked together. Sucrose is a simple sugar made of one glucose molecule and one fructose molecule. These structural differences mean they cannot bind to the same active site on an enzyme.
Therefore, the enzyme that digests starch (likely amylase) has an active site designed for the shape of starch molecules. It won't be able to fit sucrose into its active site, hence it won't be able to break it down.
