Biology 9700 · AS & A Level · Carbohydrates and lipids
Carbohydrates and lipids — practice question
The enzyme catechol oxidase makes a brown colour appear when slices of many fruits, such as apples, are left exposed to air. It catalyses this reaction: $\text{catechol} + \text{oxygen} \xrightarrow{\text{catechol oxidase}} \text{quinone} + \text{water}$. The quinone is then rapidly oxidized further in air to form a brown-coloured substance. Catechol and quinone are colourless. A student studied how fast this reaction occurs under different conditions.
(a)[1]
State how the student could track the progress of this reaction.
(b)[5]
Explain the results shown in Fig. 4.1.
(c(i))[2]
In a second investigation, the student repeated the experiment, but this time added the competitive inhibitor para-hydroxybenzoic acid (PHBA) to every reaction mixture. Sketch the results that would be obtained for this second investigation on Fig. 4.1.
(c(ii))[2]
Explain the effect PHBA will have on the action of phenoloxidase.
(d)[2]
Lemon juice contains citric acid. Adding even a small amount of diluted lemon juice to apple slices delays the appearance of the brown colour. Suggest an explanation for this observation.
Worked solution & mark scheme
This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Noting when brown colour first appears” …