Biology 2023 · May/June

10 dilution of the enzyme extract.

Describe how the student made a 1:10 dilution of the enzyme extract.

The student studied the effect of pH on the activity of polyphenol oxidase enzyme. Dopamine hydrochloride solution was used as a substrate for the enzyme. The product of this reaction is red in colour. A colorimeter can be used to follow the progress of this enzyme-catalysed reaction. The student set up: buffered solution: pH 3.0, pH 4.0, pH 5.0, pH 6.0, pH 7.0, pH 8.0 The colorimeter tubes were equilibrated at a set temperature. A colorimeter was set up with a filter transmitting light at 470 nm. At time 0 seconds, the substrate solution and the polyphenol oxidase solution at pH 3.0 were mixed in one colorimeter tube. The colorimeter tube was then immediately put into the colorimeter. The absorbance was measured at 15 second intervals over the first three minutes of the reaction. The procedure was repeated for the remaining buffered solutions and the water at pH 7.0.

Identify the independent variable in this experiment.

Identify one variable that the student has standardised.

The student then plotted a graph of the results, showing the increase in absorbance. Absorbance is measured in absorbance units (au). Fig. 1.2 shows the results for the increase in absorbance at pH 5.0. 0.1 0.2 0.3 0.4 0.5 0.6 153045607590 time / s absorbance / au 105120135150165180

Use Fig. 1.2 to calculate the initial rate of this reaction in absorbance units per second (au s Show your working. initial rate of reaction = au s

Using the internet, the student found that pH 7.0 is the optimum pH for polyphenol oxidase. On Fig. 1.2, sketch the line you would expect to get for the rate of reaction at pH 3.0.

The student wanted to determine the effect of the concentration of the substrate, dopamine hydrochloride solution, on the rate of the enzyme-catalysed reaction. The student was provided with a stock solution of 0.2% dopamine hydrochloride solution and the diluted enzyme extract prepared in

. Describe a method the student could use to collect the data that is needed to determine the effect of the concentration of dopamine hydrochloride solution on the rate of the enzyme-catalysed reaction. Your method should be set out in a logical order and be detailed enough to let another person follow it.

The browning of fruit such as bananas can make them more difficult to sell. One way to prevent browning of bananas is to use an anti-browning agent. The student searched the internet and found data from different scientific papers on the inhibitory effect of various anti-browning agents on polyphenol oxidase from different plants. The student processed and ranked these data, as shown in Table 1.1. Table 1.1 anti-browning agent percentage of maximum activity of polyphenol oxidase plant source of polyphenol oxidase heated onion extract 17.0 potato fresh onion extract 44.4 potato L-cysteine 70.0 sorrel potassium sorbate 76.8 potato citric acid 80.4 potato EDTA 88.0 red poppy no anti-browning agent (control) 100.0 potato The student concluded that heated onion extract was the most effective anti-browning agent and that EDTA was the least effective. Use the information to suggest why this conclusion may not be supported.

The student found that a sulfur-containing molecule in the heated onion extract acted as a non-competitive inhibitor to the enzyme polyphenol oxidase. Fig. 1.3 is a graph showing how the substrate concentration affects the rate of polyphenol oxidase activity. On Fig. 1.3 sketch the curve that shows the rate of activity for polyphenol oxidase treated with the heated onion extract. substrate concentration rate of polyphenol oxidase activity [1] [Total: 16]

Air pollution due to motor vehicles is an environmental issue that needs constant monitoring. Lichens consist of protoctists and fungi living in close association. The protoctists and fungi each benefit from this association. Lichens are found in a large range of habitats, including on the surface of tree trunks (tree bark), as shown in Fig. 2.1. tree bark with lichen tree bark without lichen Lichens are sensitive to air pollution. The abundance of lichens decreases with increasing levels of air pollution and so lichens can be used to monitor levels of air pollution. Some scientists investigated the relationship between distance from a road and levels of air pollution. The scientists investigated this relationship by measuring the abundance of lichens on the tree trunks in a woodland beside a busy road. The scientists used the grid shown in Fig. 2.2 to determine the abundance of lichens on the tree trunks. transparent plastic sheet with printed grid circles for sampling 15 cm 15 cm For each tree sampled, the scientists: on the tree

(i) State two variables that the scientists should standardise in this investigation.

State one risk and the safety precaution that the scientists should take when measuring the abundance of the lichens on the tree trunks.

The scientists recorded the presence of lichens by shading in the circles on the grid where lichens were observed. The results are shown in Fig. 2.3. Key: lichen present lichen absent Use the grid in Fig. 2.3 to calculate the percentage cover of the lichen on the tree trunk. Show your working. percentage cover of lichen = %

The scientists measured the percentage cover of lichens on 10 trees, each growing at a different distance from the road. The scientists carried out a Spearman's rank correlation analysis to find out if there was a correlation between the distance from the road and the percentage cover of lichens on the tree trunks. Table 2.1 shows part of the table used to determine the data needed for the Spearman's rank correlation equation.

Complete Table 2.1 for tree number 10. Table 2.1 tree numberdistance from road/ m rank for distance from road percentage cover of lichen rank for percentage cover of lichen difference in rank (D) 10 5.6 7 74 5 [1] The formula for calculating Spearman's rank correlation coefficient (r ) is: = 1 - ( 6 × ΣD - n)

The scientists calculated the value of ΣD Calculate the Spearman's rank correlation coefficient (r ) value to three significant figures. Use this space for working. Spearman's rank correlation coefficient (r ) = [2] Key to symbols: D = difference in rank between each pair of measurements n = number of pairs of items in the sample Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. Local Examinations Syndicate (UCLES), which is a department of the University of Cambridge.

State the conclusions that can be made from the calculated value of Spearman's rank correlation coefficient (r

State how the scientists could use the calculated value of r to determine whether or not the correlation is due to chance. [Total: 14]