Biology 9700 · AS & A Level · Respiration

Respiration — practice question

The earliest diagnostic test strip to use immobilised enzymes was a dipstick for estimating glucose concentration in urine. It is a narrow plastic strip with a cellulose pad at one end that contains two enzymes and a colour reagent (chromogen). When the pad is dipped into a urine sample containing glucose, it undergoes a colour change. The colour is then compared with a graduated colour chart to provide a ‘semi-quantitative’ estimate of the glucose concentration in the sample, as shown in Fig. 2.1. The chart includes the colours for a negative result ($-$) and for three progressively more positive results ($+$, $++$ and $+++$).
(a)[2]

Explain why the glucose concentration estimated by this method is described as only ‘semi-quantitative’.

(b(i))[1]

One of the two enzymes fixed in the cellulose pad on the test strip is glucose oxidase, which catalyses the following reaction: $\text{glucose} + \text{oxygen} \xrightarrow{\text{glucose oxidase}} \text{gluconic acid} + \text{hydrogen peroxide}$ Name the other immobilised enzyme in the pad.

(b(ii))[2]

Explain how the reaction catalysed by this enzyme causes the chromogen to change colour.

(b(iii))[2]

A layer of cellulose acetate covers the cellulose pad on the test strip. It allows glucose molecules to pass through, but excludes larger molecules. Suggest why the cellulose acetate layer is used.

(c(i))[4]

Using Fig. 2.2, explain the differences in blood glucose concentration between A and B.

(c(ii))[1]

Suggest what is meant by the term ‘renal threshold’.

(c(iii))[3]

Describe the kidney events, after ultrafiltration, that lead to the rising amount of glucose in B’s urine.

Worked solution & mark scheme

This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Only three positive colours / only a limited range” …

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