Biology 9700/53 October/November 2024: question paper and mark scheme

Question

Algae are a diverse group of photosynthetic eukaryotes in the kingdom Protoctista. Rhodomonas salina and Skeletonema costatum are two species of alga that live in seawater. Fig. 1.1 shows a scanning electron micrograph of R. salina, which is a unicellular organism. R. salina is red in colour. ×2200 Fig. 1.1 Fig. 1.2 shows a scanning electron micrograph of S. costatum, which is also a unicellular organism. The individual cells can group together in a chain. S. costatum is yellow‑brown in colour. one cell of S. costatum ×450 Fig. 1.2 (a) A student prepared extracts of the photosynthetic pigments from R. salina and S. costatum. The student carried out chromatography to identify and compare the pigments in the two species. The chromatograms are shown in Fig. 1.3. R. salina S. costatum solvent front alloxanthin origin Fig. 1.3 (i) Use Fig. 1.3 to calculate the Rf value of alloxanthin from R. salina. Rf of alloxanthin = …… [1] (ii) Table 1.1 shows the Rf values of some photosynthetic pigments. Table 1.1 photosynthetic pigment Rf value β‑carotene 0.94 chlorophyll a 0.68 chlorophyll b 0.54 chlorophyll c 0.15 diadinoxanthin 0.32 fucoxanthin 0.43 phycocyanin 0.35 R. salina lacks two photosynthetic pigments that are present in S. costatum. Use Fig. 1.3 and Table 1.1 to identify the two photosynthetic pigments that are present in only S. costatum. …… [2] (b) The extract of the photosynthetic pigments from each of the two species was used to obtain absorption spectra as shown in Fig. 1.4. Key R. salina S. costatum 1.8 1.5 1.2 absorbance 0.9 0.6 0.3 0.0 400 450 500 550 600 650 700 wavelength / nm Fig. 1.4 The student planned to investigate the effect of light wavelength on the rate of photosynthesis in R. salina and S. costatum. The student planned to determine the rate of photosynthesis in the two species when they were exposed to three different colours of light: • blue light, peak wavelength of 455 nm • green light, peak wavelength of 550 nm • red light, peak wavelength of 670 nm. Use Fig. 1.4 to compare the expected rates of photosynthesis for the two species in blue, green and red light. blue light …… green light …… red light …… [3] (c) In the investigation, the student used three different filters, blue, green and red, to expose the two species to the three different colours of light. The student immobilised the algae in sodium alginate to form two sets of algal beads, one set for R. salina and one set for S. costatum. The algal beads were the same size. Photosynthesis of the algae is not affected when they are immobilised in the beads. The student placed the algal beads in small bottles for the experiment as shown in Fig. 1.5. small bottle hydrogencarbonate indicator solution algal bead Fig. 1.5 The student used hydrogencarbonate indicator solution to estimate the rate of photosynthesis when the algal beads were exposed to each colour of light. The indicator solution changes colour with pH as shown in Table 1.2. Table 1.2 rate of colour of indicator solution pH CO2 concentration photosynthesis yellow 7.6 yellow‑orange 7.8 increasing orange 8.0 concentration orange‑red 8.2 atmospheric concentration increasing red 8.4 (0.04%) rate red‑magenta 8.6 magenta 8.8 decreasing magenta‑purple 9.0 concentration purple 9.2 The student used pH change as an indirect measure of the rate of photosynthesis. A greater decrease in CO2 concentration indicates a higher rate of photosynthesis. For each bottle tested, the student: • added algal beads to a small bottle containing indicator solution • removed a sample of the indicator solution from the small bottle after some time • used a colorimeter to measure the absorbance of the sample of the indicator solution. The student used a calibration curve to estimate the pH of the samples of the indicator solution. (i) Suggest one reason why using algal beads, instead of placing the algal cells directly into the indicator solution in the bottles, improves the validity of the investigation. …… [1] (ii) Identify the two independent variables and the dependent variable in this investigation. independent variable 1 …… independent variable 2 …… dependent variable …… [3] (iii) In addition to the colour filters, algal beads, small bottles, indicator solution and a colorimeter, the student had access to standard laboratory equipment. The student carried out the investigation in a temperature‑controlled room. Describe a method the student could use to collect sufficient data so that the rates of photosynthesis of both species of alga in the three colours of light can be compared. The description of your method should be set out in a logical way and be detailed enough for another person to follow. You should not include a risk assessment or details of how to prepare the algal beads. …… [8] (iv) Identify a hazard in this investigation and state a risk associated with the hazard and state one precaution that the student should take. …… [1] (d) For each colour of light, the student carried out a statistical test to compare the pH of the samples taken from the small bottles that contained R. salina and S. costatum. The student decided that a t‑test was the most appropriate test to use for these data. (i) Suggest one reason why a t‑test was the most appropriate test to use for these data. …… [1] (ii) State a null hypothesis for this test. …… [1] [Total: 21]

Question

A group of ecologists investigated the biodiversity of two peat bog ecosystems, A and B. Peat bogs are wetland ecosystems that often contain rare species. Fig. 2.1 shows an example of a peat bog. Fig. 2.1 (a) The ecologists sampled the plant species in peat bog A and peat bog B. The ecologists decided to use two suitable indices of biodiversity: • Simpson’s index of diversity • Shannon diversity index. The ecologists had read that the two indices can lead to different conclusions. The Shannon diversity index gives values in the range 1.5–3.5, where 1.5 is low biodiversity and 3.5 is high biodiversity. The results are shown in Table 2.1. Table 2.1 peat bog A peat bog B species number of individuals species number of individuals L 34 L 30 M 10 M 15 N 11 N 18 O 15 O 24 P 8 P 16 Q 7 Q 8 R 4 R 20 S 1 S 4 T 3 T 0 U 3 U 0 V 2 V 0 W 1 W 0 Simpson’s index (D) = 0.82 Simpson’s index (D) = 0.85 Shannon index = 2.03 Shannon index = 1.96 The ecologists concluded that one index indicated peat bog A had more biodiversity, but the other index indicated peat bog B had more biodiversity. Suggest how the differences in the data in the two samples, shown in Table 2.1, may have led to the different conclusions. …… [2] (b) The ecologists sampled the invertebrate species in peat bogs A and B and calculated invertebrate biodiversity using Simpson’s index of diversity. (i) The ecologists sampled invertebrate species in peat bog A: • between 13:00 and 16:00 on three different days within a 30‑day period • at 10 random sites along the edge of a path in the peat bog • by sampling the invertebrates within a 1 m2 quadrat placed at each site • by using an identification key to identify the species • by counting the number of individuals of each species. Describe how this sampling method could be improved to make sure the results are more representative of all invertebrates in the whole of peat bog A. …… [3] (ii) Table 2.2 shows the results of the invertebrate sampling in peat bog A. The formula for Simpson’s index of diversity (D) is: J J N2N n D = 1 – KK/ KK OO OO N L L PP n = number of individuals of each species present in the sample N = total number of all individuals of all species present in the sample Complete Table 2.2 and use the formula provided to calculate Simpson’s index of diversity. Table 2.2 species n n/N (n / N)2 Chartoscirta cocksii 11 0.071 0.005 Eristalis cryptarum 72 0.462 0.213 Glyphesis cottonae 8 0.051 0.003 Glyphesis servulus 6 0.038 0.001 Loxocera nigrifrons 29 0.186 0.035 Stenus argus 10 0.064 0.004 Trechus rivularis 7 0.045 0.002 Tipula limbata 13 …… …… |= …… Simpson’s index of diversity (D) = …… [3] (iii) The ecologists calculated a D value of 0.710 for peat bog B. Use your value of D from (b)(ii) to compare the biodiversity of peat bog A and peat bog B. …… [1] [Total: 9] 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.