Biology 9700/52 May/June 2025: question paper and mark scheme

Question

Phosphatases are enzymes that catalyse the removal of phosphate from other molecules, releasing inorganic phosphate (Pi ). A phosphatase can be extracted from mung bean seedlings, Vigna radiata. Fig. 1.1 shows mung bean seedlings. mung bean seedlings Fig. 1.1 A student found a published method to extract phosphatase from mung bean seedlings. • Use a pestle and mortar to grind mung bean seedlings, with a small volume of distilled water, to make a paste. • Add distilled water to the paste to make a mixture with a total volume of 50 cm3. • Filter the mixture and put the filtrate into a clean centrifuge tube. • Centrifuge the filtrate until a solid pellet is formed, as shown in Fig. 1.2. • Pour the liquid extract containing phosphatase into a clean test-tube. lid of centrifuge tube plastic centrifuge tube liquid phosphatase extract solid pellet Fig. 1.2 (a) State two other variables that should be standardised in the published method so that extracts with the same concentration of phosphatase can be produced. …… [2] (b) The student read that the optimum pH for phosphatase extracted from mung bean seedlings is less than pH 7.0. The student decided to investigate phosphatase activity, using the substrate phenolphthalein phosphate (PPP). In the first four steps of the method, the student: • mixed 5 cm3 of pH 6.0 buffer solution with 1 cm3 of a 1.0% solution of PPP in a test-tube • added 1 cm3 of the phosphatase extract to the test-tube and started a timer • incubated the test-tube for 10 minutes in a water-bath at 30 °C • stopped the enzyme reaction after 10 minutes, by adding 5 cm3 of 10.0% solution of sodium carbonate. Sodium carbonate solution is alkaline. (i) The buffered PPP solution and the phosphatase extract were mixed and then placed in the water-bath. The student identified this as a source of error in the method. Explain why this is a source of error and state how you would modify the method to remove this source of error. explanation modification [2] (ii) Suggest how the addition of 10.0% solution of sodium carbonate stops the enzyme reaction. …… [1] (c) The phosphatase catalyses the removal of inorganic phosphate from PPP as shown by: phosphatase PPP phenolphthalein + inorganic phosphate The 10.0% solution of sodium carbonate added at the end of the experiment also causes any phenolphthalein to turn pink. The intensity of the pink colour is an indication of the concentration of phenolphthalein. To estimate the concentration of phenolphthalein produced by the reaction, the student decided to make a proportional dilution using a 2.0% stock solution of phenolphthalein. The student made 50 cm3 of each diluted solution. Describe a method the student could use to make a proportional dilution of the 2.0% stock solution of phenolphthalein to get a range of concentrations. …… [2] (d) Describe how the student could use the dilutions from (c) and a colorimeter to estimate the concentration of phenolphthalein in a reaction mixture. …… [2] (e) The student decided to determine the optimum pH for phosphatase extract from the mung bean seedlings. Describe a method the student could use to determine the optimum pH for phosphatase. Your method should be set out in a logical order and be detailed enough to allow another person to follow it. Details of how to extract phosphatase from the mung bean seedlings and how to make the dilutions of phenolphthalein solutions should not be included. …… [6] (f) The student investigated the effect of a competitive inhibitor on the activity of a phosphatase. Fig. 1.3 shows a graph of the initial rates of enzyme reaction against PPP concentration for the enzyme with no inhibitor and the enzyme with inhibitor. Key enzyme with inhibitor enzyme with no inhibitor 0.020 0.018 0.016 0.014 initial rate of 0.012 enzyme reaction 0.010 / mmol dm–3 s–1 0.008 0.006 0.004 0.002 0.000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 PPP concentration / mmol dm–3 Fig. 1.3 (i) One of the data plots in Fig. 1.3 is anomalous. Circle the anomalous data plot in Fig. 1.3 and explain why you think it is anomalous. …… [2] (ii) Use Fig. 1.3 to determine the Michaelis–Menten constant (Km) for the enzyme with no inhibitor and for the enzyme with inhibitor. Include the correct units in your answers. Km for the enzyme with no inhibitor = …… Km for the enzyme with inhibitor = …… [1] (iii) Calculate the percentage increase in the Km that occurs in the presence of the inhibitor. Show your working. percentage increase = …… % [2] [Total: 20]

Question

Sponges are immobile, aquatic animals that live on rocks and sediment at the bottom of salt water environments. Fig. 2.1 shows an example of a sponge, growing on the seabed, in a marine habitat. scuba diver photographing sponges sponge Fig. 2.1 There are many different species of sponge. Most species of sponge are sensitive to environmental stress. Human activity causes some environmental stress. Scientists sampled the marine habitats along a length of coastline in Algeciras Bay in southern Spain to study the species diversity of sponges. The scientists selected 12 sampling stations, A to L, as shown in Fig. 2.2. The scientists noted the land use or human activity along the coast next to each sampling station. Key to diagram: sampling type of land use station or human activity A natural habitat B natural habitat G H C tourist beach I F J D housing land E shipping port E K F tourist beach land G thermal power D L station H oil industry C Algeciras Bay I ship building and B marine habitat repairs J tourist boats A sampling stations K tourist boats L natural habitat Fig. 2.2 The sampling stations were chosen to compare the effects of land use or human activity on the species diversity of sponges in the bay. At each marine sampling station, the scientists: • placed permanent line transects, 50 m in length, on the seabed • photographed all sponges sighted at a distance of 1 m either side of the transect • sampled each transect for the same length of time • sampled each transect four times a year. (a) The scientists standardised some variables. State two other variables that the scientists should standardise in this investigation. …… [2] (b) The scientists compared the species diversity of sponges between different sampling stations using an index of diversity known as beta diversity. The higher the beta diversity index, the higher the species diversity at a sampling station. Fig. 2.3 shows a graph of the beta diversity index for each sampling station. 1.7 1.6 1.5 1.4 1.3 beta diversity 1.2 index 1.1 1.0 0.9 0.8 0.7 0.6 A B C D E F G H I J K L sampling station Fig. 2.3 A simplified formula for beta diversity is shown: n beta diversity = x – 1 Key to symbols: n = the number of sponge species recorded at a sampling station x = the mean number of sponge species across all sampling stations At sampling station A, 37 sponge species were recorded (n = 37). Use Fig. 2.3 and the formula for beta diversity to calculate the mean number of sponge species across all sampling stations (x). Show your working. Write your answer to the nearest whole number. mean number of sponge species across all sampling stations (x) = …… [2] (c) (i) A student concluded that the species diversity of sponges was higher in the sampling stations where there was less human activity. Using the information in Fig. 2.2 and in Fig. 2.3, evaluate this conclusion. Use the values in Fig. 2.3 to support your answer. …… [3] (ii) Use the information in Fig. 2.2 and in Fig. 2.3 to state two other conclusions about the species diversity of sponges in Algeciras Bay. Use the values in Fig. 2.3 to support your answer. …… [3] [Total: 10] The boundaries and names shown, the designations used and the presentation of material on any maps contained in this question paper/insert do not imply official endorsement or acceptance by Cambridge Assessment International Education concerning the legal status of any country, territory, or area or any of its authorities, or of the delimitation of its frontiers or boundaries.