Enzymes are biological catalysts. They speed up the reactions in living cells. Each enzyme has an active site. The active site has a set shape. Only one kin...

This question uses the lock-and-key idea to explain enzyme specificity and the effect of temperature and pH on amylase.

(a) Amylase breaks starch down into maltose (a sugar) [1].

(b) Amylase breaks down starch but not protein because the active site of amylase has a specific shape [1]; only starch (its complementary substrate) fits into that active site [1]; protein molecules have a different shape, so they cannot bind to the active site and are not broken down [1].

(c) As the temperature falls below 37 °C the enzyme and substrate molecules have less kinetic energy [1]; so they move more slowly and collide less often [1]; this means fewer enzyme-substrate complexes form per second, so the rate of reaction falls [1]. (Note that the enzyme is not denatured by cooling; activity returns if it is warmed again.)

(d) Amylase stops working in the stomach because the stomach is very acidic (a low pH) [1]; this is far below the optimum pH for amylase [1]; the low pH denatures the enzyme, changing the shape of its active site [1]; so the starch substrate can no longer bind to the active site and no reaction occurs [1].

(e) Any one variable that must be controlled when investigating temperature [1]: pH; enzyme (amylase) concentration; substrate / starch concentration; or the volume of the solutions used.

Exam tip: distinguish the two effects carefully: cold slows an enzyme by reducing collisions (reversible), while extreme pH or high heat denatures it by changing the active site (usually permanent).