Models of how enzymes work
The way in which enzymes actually work is extremely complicated so we use models to think about them.
A good model tries to represent the known facts about something.
A model about digestive enzymes needs to try to represent these fact:
Enzymes turn large molecules into smaller ones.
Enzymes do not get used up as they carry out their tasks.
Enzymes change shape as they work.
Each enzyme only works on one particular type of molecule.