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Biology · Oct / Nov 2024 · Paper 62 · Question 1

0610

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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 DCPlP 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 DCPlP 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. minute:second time recorded for test-tube A 1 † ‡ˆ minute:second time recorded for test-tube B Š † ‹‡ Fig. 1.1

Read automatically from the paper; the printed page above is exact.

Show the answer and mark scheme

  • 1(a)(i)[4]

    table drawn with minimum of two columns and a header line ; 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)[1]

    any one from:

    as light (intensity) increases, (rate of) photosynthesis increases ; (rate of) photosynthesis is, faster / AW, at a high(er)

    light (intensity) ;

  • 1(a)(iii)[1]

    time (for colour change) ;

  • 1(a)(iv)[2]

    error:

    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)[1]

    vitamin C ;

  • 1(c)(i)[4]

    outline: single clear line for beak and achene ;

    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)[3]

    PQ is 19 ±1 mm ;

    1.3 ;;

  • 1(d)[3]

    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)[1]

    yellow-brown / orange-brown / brown ;

  • 1(e)(ii)[2]

    add Benedict’s (reagent / solution) ;

    heat ;

  • 1(f)[6]

    independent variable

    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 ;

    9

    two or more repeats / idea of at least 3 seeds at each concentration if measuring time for germination ;

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