Key IGCSE Concepts
Lock and Key Model: The enzyme's active site has a specific shape that only fits one type of substrate — like a lock and key. When the substrate fits into the active site, an enzyme-substrate (ES) complex forms.
Optimum Temperature: At low temperatures, molecules move slowly — few collisions, low rate. As temperature rises, more collisions occur and rate increases. Above the optimum (~37°C for human enzymes), the enzyme denatures — its shape changes permanently and substrates can no longer fit.
Optimum pH: Each enzyme works best at a specific pH. E.g. pepsin works at pH 2 (stomach acid), amylase at pH 7 (saliva). Extreme pH changes the enzyme's shape and denatures it.
Substrate Concentration: More substrate = more chance of collision with enzyme = higher rate. But once all active sites are occupied, adding more substrate has no effect (saturation).
Denaturation: The permanent loss of an enzyme's 3D shape. The active site no longer fits the substrate. Caused by high temperature or extreme pH. It is irreversible.
Amylase: Breaks down starch into maltose. Found in saliva and pancreatic juice. Optimum pH 7, optimum temperature 37°C.