Question 128 marks
Question
Photosynthesis occurs in chloroplasts. Students investigated the effect of light intensity on the rate of photosynthesis in a chloroplast suspension. The blue dye DCPIP can be used to investigate photosynthesis. During photosynthesis, the dark-blue colour of the DCPIP changes and gradually disappears so that it becomes colourless. A chloroplast suspension that is made from spinach leaves has a green colour. The students used this method: Step 1 Prepare a chloroplast suspension from fresh spinach leaves. Step 2 Label one test-tube A and another test-tube B. Step 3 Put 1 cm3 of the green chloroplast suspension into test-tube A and into test-tube B. Step 4 Put test-tubes A and B into an ice-cold water-bath for two minutes. Step 5 Put 5 cm3 of the blue DCPIP solution into test-tube A. Put a test-tube rack 10 cm from a bright light. Put test-tube A into the test-tube rack. Step 6 Turn on the light and start the stop-clock. Step 7 Measure the time taken for the blue liquid in test-tube A to return to a green colour. Step 8 Reset the stop-clock to zero. Step 9 Move the test-tube rack so that it is 30 cm from the bright light. Step 10 Put 5 cm3 of the blue DCPIP solution into test-tube B. Put test-tube B into the test-tube rack and start the stop-clock. Step 11 Measure the time taken for the blue liquid in test-tube B to return to a green colour. The times measured in step 7 and step 11 are shown in Fig. 1.1. 1 : 37 5 : 43 time recorded for test-tube A time recorded for test-tube B Fig. 1.1 (a) (i) Prepare a table and record the results of the investigation. Convert the times on the stop-clocks in Fig. 1.1 to seconds and record these times in your table. [4] (ii) State a conclusion for the results. …… [1] (iii) State the dependent variable in this investigation. …… [1] (iv) Identify the possible source of error present in step 7 and step 11. Suggest an improvement to the method to reduce this error. error …… improvement …… [2] (b) DCPIP can be used in food testing. State which substance present in food can be identified using DCPIP. …… [1] (c) Fig. 1.2 is a photograph of a seed from a dandelion plant, Taraxacum officinale. P Q magnification ×15 Fig. 1.2 (i) Draw a large diagram of the whole dandelion seed shown in Fig. 1.2. [4] (ii) Line PQ in Fig. 1.2 represents the length of the achene of the dandelion seed. Measure the length of line PQ in Fig. 1.2. length of PQ …… mm Calculate the actual length of the achene, using the formula and your measurement. length of line PQ in Fig. 1.2 magnification = actual length of the achene Give your answer to two significant figures. Space for working. …… mm [3] (d) Fig. 1.3 shows photographs of a dandelion seed and a seed taken from a milk thistle plant, Silybum marianum. The magnifications of the photographs are not the same. dandelion milk thistle Fig. 1.3 State one similarity and two differences, visible in Fig. 1.3, between the two seeds. Do not include references to size in your answer. similarity …… difference 1 …… difference 2 …… (e) Dandelion roots are used to flavour drinks. Students tested a sample of a colourless drink for starch and reducing sugar. They found that the drink did not contain starch but did contain reducing sugar. (i) State the expected colour of the iodine solution after it was mixed with a sample of the drink. …… [1] (ii) Describe how the students should test the sample of the drink for reducing sugar. …… [2] (f) Some types of weedkiller prevent seeds from germinating. Plan an investigation to determine the effect of weedkiller concentration on the germination of seeds. …… [6] [Total: 28]
Mark scheme
1(a)(i) table drawn with minimum of two columns and a header line ; 4 appropriate column / row headings, with units for time and distance (if given) ; recording two numbers for time in a table ; recording of 97 for, test-tube A / 10 cm and 343 for, test-tube B / 30 cm ; 1(a)(ii) any one from: 1 as light (intensity) increases, (rate of) photosynthesis increases ; (rate of) photosynthesis is, faster / AW, at a high(er) light (intensity) ; 1(a)(iii) time (for colour change) ; 1 1(a)(iv) error: 2 difficult to judge the end point / AW ; improvement: use a, colorimeter / colour standard / colour chart / compare the colour to a test-tube of chloroplast suspension with no DCPIP present ; 1(b) vitamin C ; 1 1(c)(i) outline: single clear line for beak and achene ; 4 size: drawing at least 90 mm long ; detail 1: pappus, beak (stalk) drawn as a double line and achene (seed) with spikes clearly shown ; detail 2: ridge down the achene shown as a double line ; 1(c)(ii) PQ is 19 ±1 mm ; 3 MP1 correct measurement of line PQ 1.3 ;; MP2 dividing their measurement by 15 MP3 answer correctly rounded to two significant figures ecf from previous step if workings shown PQ of 18 mm gives 1.2 PQ of 20 mm gives 1.3 1(d) 3 correct terminology is not expected for description of the parts similarity max one from: both have a parachute / AW ; both have an achene / AW ; both have, an achene / a seed, that is longer than it is wide ; differences max two from: dandelion has a, stem / stalk / AW (and milk thistle, does not / is shorter) ; dandelion (achene) has, spikes / ridges / AW or milk thistle (achene), is smooth ; dandelion (achene shape) is square-ended / blunt-ended or milk thistle (achene shape) is round(ed) ; milk thistle, parachute / AW, is, thicker / more hairs / AW / ora ; dandelion (achene) is light or milk thistle (achene) is dark / mottled ; 1(e)(i) yellow-brown / orange-brown / brown ; 1 A red-brown 1(e)(ii) add Benedict’s (reagent / solution) ; 2 heat ; 1(f) independent variable 6 1 (at least) 2 different concentrations of weedkiller ; dependent variable ; 2 counting, number / percentage, seeds germinated or time taken for, seed / seeds, to germinate 3 and 4 detail of method ;; max two from: • growing medium e.g. cotton wool, filter paper, soil • description of how they can tell the seed has germinated e.g. appearance of plumule • idea of, watering / providing water / maintaining humidity • method to create different concentrations of weedkiller 5, 6 and 7constant variables ;;; max three from: • same time (unless time is the DV) • same temperature • same, species / type / age, of seed / named plant seeds • same pH • same volume of weedkiller • same (type of) weedkiller • same concentration / amount, of oxygen 8 relevant safety precaution ; e.g. gloves / goggles / mask 9 two or more repeats / idea of at least 3 seeds at each concentration if measuring time for germination ;