Cambridge A Level Biology 9700 — 2023 May/June Paper 5 · Variant 3

1 Hydrogencarbonate indicator is a water-soluble solution that can act as a source of carbon dioxide for aquatic photosynthetic organisms. The solution changes colour depending on the concentration of carbon dioxide in the solution. These colours are related to different pH values, as shown in Table 1.1. Table 1.1 colour of hydrogencarbonate pH concentration of carbon dioxide indicator solution in the solution yellow 7.6 increasing carbon dioxide yellow-orange 7.8 concentration orange 8.0 orange-red 8.2 red 8.4 atmospheric concentration red-magenta 8.6 decreasing carbon dioxide magenta 8.8 concentration magenta-purple 9.0 purple 9.2 Chlorella vulgaris is a protoctist that is single-celled, aquatic and photosynthetic. It can be immobilised in alginate beads. Alginate beads with immobilised C. vulgaris can be used to measure the rate of photosynthesis. (a) A student noticed that a colour change occurred, from red to magenta, when the alginate beads with immobilised C. vulgaris were left in a container of hydrogencarbonate indicator solution and exposed to light. Explain why this colour change occurred. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) The student used the alginate beads with immobilised C. vulgaris in hydrogencarbonate indicator solution to investigate the rate of photosynthesis in different light intensities. Fig. 1.1 shows some of the apparatus and reagents the student used. water small beaker large test-tube hydrogencarbonate indicator solution alginate beads containing C. vulgaris Fig. 1.1 (i) Identify the independent variable in this investigation. ..................................................................................................................................... [1] (ii) The student was provided with a supply of alginate beads containing immobilised C. vulgaris. Describe a method the student could use to collect data to determine the effect of light intensity on the rate of photosynthesis of C. vulgaris using hydrogencarbonate indicator and the experimental set-up in Fig. 1.1. Your method should be set out in a logical order and be detailed enough to let another person follow. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [8] (c) The student set up a large test-tube containing alginate beads with immobilised C. vulgaris in hydrogencarbonate indicator solution at pH 8.4 (red). The student kept this set-up in the dark for 12 hours. Predict and explain the results that will be observed after 12 hours in the dark. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (d) Some scientists wanted to culture cells of C. vulgaris on a large scale for use as a biofuel. To determine the optimal growing conditions for C. vulgaris, the scientists needed to determine the number of cells per cm3 of suspension to monitor the population growth. They tried two methods to determine the number of cells per cm3 of suspension. The first method used a Secchi stick, as shown in Fig. 1.2. 10 ruler 9 8 7 6 5 4 3 2 1 cm black and white circle 0 Fig. 1.2 The Secchi stick is lowered into the suspension of cells until the black and white circle is not able to be seen from above. The depth in cm is recorded from the ruler, as shown in Fig 1.3. 10 ruler 9 8 depth in cm on ruler 7 6 5 beaker 44 suspension of C. vulgaris 33 22 11 0cm0cm black and white circle is not able to be seen from above Fig. 1.3 The log10 (l g) of the number of cells is determined from a graph of log10 of cells counted per cm3 suspension against Secchi depth (cm), as shown in Fig 1.4. 5.8 5.7 5.6 5.5 log10 cells counted per 5.4 cm3 suspension 5.3 5.2 5.1 5.0 1.4 1.6 1.8 2.0 2.2 2.4 Secchi depth / cm Fig. 1.4 (i) When the scientists inserted the Secchi stick into a sample from their cell suspension, the circle (on the Secchi stick) was not able to be seen at a depth of 1.9 cm. Using the graph in Fig. 1.4, calculate the actual number of cells per cm3 of suspension. Show your working and give your answer to the nearest 1000 cells. number of cells per cm3 of suspension ......................................................... [2] The second method used a counting chamber to determine the number of cells per cm3 of suspension. Fig. 1.5 shows a section of a counting chamber with cells present, as viewed using the high power of a light microscope. The depth of the 1 mm × 1 mm counting chamber is 0.1 mm. The scientists counted the number of cells in several sections of a counting chamber. 1 mm cells to be counted are within the 1 mm × 1 mm 1 mm section of the counting chamber Fig. 1.5 (ii) Count the number of cells in the 1 mm × 1 mm section of the counting chamber shown in Fig. 1.5. Use your answer to calculate the number of cells per cm3 of the suspension. Show all your working. number of cells per cm3 of suspension ......................................................... [3] (iii) The scientists decided that using the Secchi stick was a less accurate method for determining the number of cells per cm3 of suspension. Give two reasons why using the Secchi stick is less accurate than using a counting chamber. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 20]

Mark scheme: 1(a) (red to magenta shows) increase in, pH / decrease in, concentration of, carbon dioxide ; (in the light C. vulgaris) uses up / AW, carbon dioxide for photosynthesis ; 2 1(b)(i) light intensity ; 1 1(b)(ii) any eight from: 1 use, lamp / AW, at minimum of 5 different distances from plant or use, lamp / AW, with minimum 5 different power ratings / wattages (at same distances) ; 2 use, a light meter / an app, to measure light intensity ; 3 idea that carry out investigation, in a darkened room / with no other light source ; 4 same / stated, mass / number / size, of alginate beads ; 5 same / stated, volume / concentration, of hydrogencarbonate indicator or same starting, colour / pH, hydrogencarbonate indicator ; 6 use a method to reduce heating effect of lamp ; 7 method to measure, colour / pH, of hydrogencarbonate indicator ; 8 measure / note / record, the colour/pH (of the indicator) after a, set / stated, time or measure / note / record, the time taken (for the indicator) to be a, set / stated, colour / pH ; 9 use at least three measurements for each light intensity and calculate a mean ; 10 safety: hazard and risk and mitigation ; 8 Question Answer Marks 1(b)(ii) hazard risk mitigation hydrogencarbonate indicator irritant / allergy gloves / goggles / mask / PPE / medication alginate beads / C. vulgaris irritant / allergy gloves / goggles / mask / PPE / medication heat (from bulb) burns / injury gloves / do not touch 11 use of a suitable control ; 1(c) will turn orange / yellow or pH decreases ; respiration, produces carbon dioxide ; 2 1(d)(i) correct working ; 229 000 ; 2 1(d)(ii) 1 correct number of cells counted / 24 ; 2 correct working ; 3 correct calculation of number of cells per cm3 / 240 000 ; 3 Question Answer Marks 1(d)(iii) any two from: Secchi stick is less accurate: 1 subjective / difficult to determine when circle is no longer visible ; 2 error of  0.5 mm in the ruler ; 3 Secchi stick, not held vertical ; 4 no replicates / no repeats of investigation / no mean calculated ; 5 idea that Secchi stick used is, different / AW, from Secchi stick used for calibration curve ; 6 idea that inserting Secchi stick, mixes / disturbs / AW, suspension ; 2

More questions on Investigation of limiting

2 Lichens consist of protoctists and fungi living in close association. Both types of organism benefit from this association. Lichens are found in a large range of habitats, including on the trunks of trees. Fig. 2.1 shows an example of a lichen, the salted shield lichen, Parmelia saxatilis, on the trunk of a tree. salted shield lichen tree trunk with no lichen Fig. 2.1 The red deer, Cervus elaphus, is found in many parts of the world including north-west Europe. Red deer are found in a large range of habitats, including mixed woodlands. Mixed woodlands contain many different tree species. Fig. 2.2 shows a male red deer. Fig. 2.2 Red deer have increased in numbers in the mountainous regions of Scotland in north-west Europe since the 1960s. Scientists think that grazing of plants by red deer has affected the abundance of certain lichens found growing on the trunks of trees in mixed woodlands. To investigate this, the scientists excluded red deer from certain mixed woodland areas (exclosures) in 1995, allowing the exclosures to remain ungrazed. In 2013, the scientists compared the abundance of the salted shield lichen in the exclosures and in the grazed areas. The scientists used the grid shown in Fig. 2.3 to determine the abundance of the salted shield lichen on the tree trunks. transparent plastic sheet with printed grid 15 cm circles for sampling 15 cm Fig. 2.3 For each tree sampled: • The grid was placed against the tree trunk at three different heights: base, middle and upper. • The scientists counted the number of circles that contained the salted shield lichen. • The number of circles that contained the salted shield lichen was used to calculate the percentage cover of salted shield lichen on the tree. This study was carried out over the course of the three months (June, July and August) in the summer of 2013 in the exclosures and the grazed areas. (a) (i) Suggest two variables that the scientists should have standardised in this investigation. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) State one risk and the safety precaution that the scientists should take when measuring the abundance of salted shield lichen on the tree trunks. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) The scientists calculated a percentage cover for the salted shield lichen as 63.3% on one of the trees, using the grid in Fig. 2.3. Calculate how many circles on the grid contained salted shield lichen. Show your working and give your answer to the nearest whole number. number of circles with salted shield lichen = ......................................................... [2]

Mark scheme: 2(a)(i) any two from: height (of grids) above ground ; side / aspect, of tree ; age / size of tree ; type / species, of tree ; grid placed horizontally / orientated as diamond shape ; Question Answer Marks 2(a)(ii) named hazard and risk ; matching precaution ; hazard and risk – 1 mark precaution – 1 mark plants – leaves, bark, pollen toxic / scratches / irritant / allergy suitable PPE / medication / don’t ingest animals, animal waste dangerous / allergy / bites / toxoplasmosis medication / work in a group / travel with an expert / suitable PPE fungi, lichen toxic / irritant / allergy suitable PPE / medication inhaling fungal spores (lung) infection wear a mask woods, falling branches / trees, holes, trip hazards injury / getting lost look where you’re stepping / boots / hard hat / don’t climb trees / GPS / map / travel with an expert climbing trees (upper height grid) injury (from trips / falls / falling branches) use a ladder / use ropes / hard hat / training / use an expert 2 2(b) 63.3( / 100)  90 / 0.633  90 ; 57 (circles) ; 2 2(c)(i) there is no, difference, in the abundance of (salted shield) lichen between, exclosures and grazed areas ; 1 Question Answer Marks 2(c)(ii) any one from: 1 no, significant difference, between exclosures and grazed areas, for base of tree ; or there is a, significant difference, between exclosures and grazed areas, for middle and upper parts of tree ; 2 the, percentage cover / abundance, of (salted shield) lichen is greater, as height of grid increases ; ora 3 percentage cover / abundance, of (salted shied) lichen is greater, in grazed areas (than in exclosures) ; ora 1 2(c)(iii) any two from: 1 check named abiotic factor is similar (in exclosures and grazed areas) ; 2 check named biotic factor is similar (in exclosures and grazed areas) ; 3 repeat the investigation, in other years (after 2013) ; 2

More questions on Biodiversity