A-LevelBiologyEnzymes9700/21 · Oct/Nov 2025 · Q310 Marks

The lock-and-key hypothesis and the induced-fit hypothesis are used to describe the interaction of enzymes and their substrates. Describe one similarity and one difference between the lock-and-key hypothesis and the induced-fit hypothesis.

(a)The lock-and-key hypothesis and the induced-fit hypothesis are used to describe the interaction of enzymes and their substrates. Describe one similarity and one difference between the lock-and-key hypothesis and the induced-fit hypothesis.[2]
(b)(i)Phosphorylase enzymes can catalyse the synthesis of starch and the breakdown of starch in some plant tissues. A reaction catalysed by starch phosphorylase is shown in [Figure 3.1]. A student carried out an experiment to study the synthesis of starch by phosphorylase found in potato tissue. The student was provided with a solution, E, extracted from potato tissue. The extract was filtered to remove all the starch grains. Extract E contained biological molecules from the potato tissue including phosphorylase but no starch. Iodine solution was used to confirm that starch was not present in extract E. State the colour observed when iodine solution was added to a sample of extract E.[1]
(b)(ii)The student added a small drop of a dilute starch solution and a solution of glucose 1-phosphate to a test-tube containing extract E. Samples of the reaction mixture in the test-tube were removed every minute and a drop of iodine solution was added to each. The student used a colorimeter to measure the absorbance of the solution in each sample. The results are shown in [Figure 3.2]. Explain the results shown in Fig. 3.2.[3]
(b)(iii)After 12 minutes, the student added a solution containing phosphate ions to the reaction mixture. The student continued taking samples every minute, adding iodine solution to each sample. The absorbance of the solution in each of these samples was measured. The results showed that the absorbance decreased over time. Suggest why the absorbance decreased.[1]
(c)Muscle cells contain glycogen phosphorylase. [Figure 3.3] shows the effect of caffeine on the activity of glycogen phosphorylase at different concentrations of substrate. A student concluded that caffeine acts as a non-competitive inhibitor of glycogen phosphorylase. Explain how the results in Fig. 3.3 support this conclusion.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

3(a) one similarity ; e.g. substrate binds to active site enzyme-substrate complex forms product leaves active site interaction does not alter enzyme for re-use lower activation energy specific to one or very similar substrate(s) interaction can be affected by, action of inhibitors / pH / AW
one difference ; e.g. induced-fit ora for lock-and-key active site not fully complementary to substrate change in, conformation / shape, of active site / AW context of binding substrate or after product release less specificity to substrate
3(b)(i) orange / brown / amber ; A yellow-brown, yellow-orange
3(b)(ii) any three from: phosphorylase (in extract) catalyses addition of glucose 1-phosphate to starch added to extract ;
more starch is synthesised by phosphorylase as time progresses / AW ;
iodine solution added to mixture turns a, blue / black / blue-black, colour (in the presence of starch);
the more starch present in the mixture, the, darker / more intense, the, colour / shade ;
the darker the colour (of the positive result) the less light passes through, mixture / sample;
the darker the colour / AW, the higher the absorbance (reading of the colorimeter);
3(b)(iii) ref. to breakdown of starch catalysed by the enzyme / reaction in Fig. 3.1 goes in the opposite direction;
AVP; e.g. ref to equilibrium shift
ref. to glucose 1-phosphate has run out
decrease in concentration of starch, qualified with ref. to iodine solution
3(c) any three from: reduction in activity of phosphorylase (at all concentrations); ora higher throughout with caffeine
Vmax not reached;
ref. to plateau at a (much) lower activity ;
calculated Km value with caffeine is the same as without caffeine ;
use of data from the graph to support conclusion ;