Cambridge IGCSE Biology (9-1) 0970 — 2019 May/June Paper 6 · Variant 1

1 A student measured the distance moved by different concentrations of citric acid solution through agar jelly. The agar contained Universal Indicator which changed colour in the presence of acid. The agar mixed with Universal Indicator was green at the beginning of the investigation. Step 1 Three test-tubes were labelled A, B and C. Three different concentrations of citric acid solution were made. Table 1.1 shows the volumes of 5% citric acid solution and distilled water that were used to make each solution. Table 1.1 solution A B C volume of 5% citric acid solution / cm3 1.0 2.0 10.0 volume of distilled water / cm3 9.0 8.0 0.0 percentage concentration of citric acid solution 0.5 1.0 5.0 Step 2 The base of a Petri dish containing agar and Universal Indicator was labelled A, B and C. Three holes were cut into the agar. This is shown in Fig. 1.1. A agar and Universal Indicator Petri dish base hole cut into the agar C B not to scale Fig. 1.1 Step 3 The student was provided with one dropping pipette. Three drops of solution A were placed into the hole in section A of the Petri dish.

Mark scheme: 1(a)(i) table drawn with a minimum of two columns and a line separating headings from data ; headings with correct units: percentage concentration of citric acid (solution) and distance (travelled) / cm or mm ; three correct distances recorded ; 3 1(a)(ii) from (edge of) hole to (edge of) the (red or yellow) circle ; diameter / radius of (red or yellow) circle ; 1 1(a)(iii) the higher the concentration (of solution) the further (the acid) moves / AW; 1 1(a)(iv) 6.5 ;; 2 1(a)(v) green agar pH: 7 ; red agar pH: 1 / 2 ; 2 A value in range 7–8 / neutral A value lower than 7/ acidic 1(b)(i) number of drops / three drops (of citric acid / solution in the holes) ; agar / agar concentration / depth of agar / volume of agar ; size of, holes / wells / AW ; type of indicator / concentration or volume of indicator ; time / 30 minutes ; temperature ; AVP ; e.g. one type of acid / same stock solution of citric acid 2 Question Answer Marks Guidance 1(b)(ii) error effect on results drop sizes vary ; larger volume produces greater diffusion distance ; ora no repeats ; unable to identify anomalous results ; one dropping pipette used for all three solutions / contamination; a weaker solution would produce a smaller diffusion distance ; longer diffusion time / citric acid added at different times / circles measured at different times ; greater / lesser distance travelled ; AVP; e.g. edge of colour change difficult to judge / subjective matching AVP effect; e.g. over/under estimation of diffusion distance 2 AW throughout mark as a pair, effect must match the source of error Question Answer Marks Guidance 1(c) any two from given method ;; • agar plates used • indicator (in agar) • holes made in agar • (citric) acid added • left for fixed (stated) time • measure distance moved/diameter/radius any three from novel method ;;; • using range of at least 2 temperatures • temperatures specified and all less than 70 (°C) • idea of keeping temperature constant • ref to temperature equilibration time prior to adding citric acid • measure time taken to reach specified diameter any additional points: same concentration of citric acid ; wear gloves / goggles ; repeating the investigation at least twice / three holes in each plate / use three dishes at each temperature ; AVP ; 6

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2 (a) Fig. 2.1 is a photomicrograph of a cross-section of part of a leaf. A B magnification ×100 Fig. 2.1 (i) Draw a large diagram to show the layers present in the leaf section shown in Fig. 2.1. Do not draw any cells. [3] (ii) Measure the thickness of the leaf along the line AB on Fig. 2.1. length of line AB ....................................... Calculate the actual thickness of the leaf using your measurement and the formula. Include the units. length of line AB on Fig. 2.1 magnification = actual thickness of leaf ................................................................ [3] (iii) Fig. 2.2 shows a photomicrograph of cells from one type of tissue found in leaves. magnification ×300 Fig. 2.2 Label the layer on your drawing, with the letter X, to show where this type of tissue is found. [1] (b) Scientists carried out an investigation into the effect of light on the growth of leaves. Plants of the same species (A) were grown in three different light intensities. The plants were grown in the same soil and kept in glasshouses with automatic watering. A sample of 100 leaves was selected at random and collected from plants in each of the three different light intensities. A total of 300 leaves were collected. The scientists studied the variations in the size and structure of the leaves in each sample. (i) Suggest why the scientists used large samples of leaves. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) Suggest why the leaves in each light intensity were selected at random. ........................................................................................................................................... ..................................................................................................................................... [1] (iii) A grid, divided into millimetre squares, was used to measure the surface area of the leaves. Outline how the grid could have been used. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iv) State the variable that was changed (independent variable) in this investigation. ..................................................................................................................................... [1]

Mark scheme: 2(a)(i) line: clear single continuous lines without shading ; size: occupies at least half the space available ; detail: layers in correct proportions ; 3 2(a)(ii) length of line AB = 40 mm ; actual length =0.4 mm ;; 3 A ±1 mm A ±0.01 mm 2(a)(iii) X written on the spongy mesophyll of drawing ; 1 2(b)(i) to obtain a representative leaf size ; to identify anomalous results ; 1 2(b)(ii) to avoid bias / gain a representative sample / AW ; so that a comparison can be made (between the different light intensities) ; 1 A leaves at different heights may be different sizes 2(b)(iii) draw round the outline of the leaf on a grid / place leaf under a (transparent) grid ; count the squares ; include any squares more than half covered / other valid method described ; 2 2(b)(iv) light intensity ; 1 2(c)(i) 67(%) ;; 2 Question Answer Marks Guidance 2(c)(ii) axes labelled with units: light intensity / arbitrary units or au and average leaf area / mm2 and species A and B labelled / key given ; scale and size: even scale for leaf area sequential for x-axis bars/plotting area to occupy at least half the grid in both directions ; plots: 6 values plotted accurately ± ½ small square ; 4 bars: bars the same width (at least 1 small square wide) gaps present between bars / pairs of bars ; 4 2(c)(iii) species A: as the light intensity decreases the (average) leaf area increases / ; ora species B: (average) leaf area increases with increasing light intensity (to maximum at 50 au) and then decreases ; ora 2 2(c)(iv) measure leaf growth at a narrower range of light intensities around 50 (au) ; 1

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