(i)     Pepsin is a protein which functions as an enzyme in human stomachs. Describe the mechanism of the catalytic activity of an enzyme.

(ii)     Discuss two differences in the catalytic action of an enzyme such as pepsin and an inorganic catalyst such as nickel metal.

(i)     E–S complex forms / substrate attracted to active site of enzyme / substrate held in active site by intermolecular forces;

induced fit / lock and key / OWTTE;

product released (from active site) after reaction;

Award [1 max] for answers giving at least two characteristics of catalysts in general.

(ii)     enzyme specific and Ni non-specific/can catalyse many reactions;

enzyme works in narrow temperature range/37–42°C and Ni is effective at high temperatures/wide temperature range;

enzyme is homogeneous and Ni is heterogeneous;

enzyme produces intermediate with reactant and Ni adsorbs reactant molecules onto its surface / Ni weakens bonds in reactants;

enzyme denatured by high pH and Ni is not;

Award [1 max] for any two correct catalytic action characteristics for either the enzyme or nickel.

Do not accept a difference based on reaction rate, such as “enzymes increase reaction rates anywhere from 10³ up to 10²⁰, whereas inorganic catalysts increase rates much less”.

Many students misread the first part of the question and gave details of how chromatography is carried out. Those that did read the question often failed to gain full credit either through forgetting to mention that the protein needs heating with the acid, or though failing to mention which bonds were being hydrolyzed. Candidates generally scored quite well on the remaining parts of the question.