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Biology · Oct / Nov 2020 · Paper 61 · Question 1

0610

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The enzyme lipase catalyses the break-down of fats into fatty acids and glycerol. A student investigated how the concentration of lipase affected the break-down of fat in milk. An increase in the concentration of fatty acids in the milk will change the pH of the milk. The student used a pH indicator, bromothymol blue, and observed the colour changes of the indicator in the milk and lipase solution. Table 1.1 shows the colour of bromothymol blue indicator at different pH values. Table 1.1 pH 6 7 8 colour yellow green blue

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  • 1(a)(i)[1]

    0.5(%) ;

  • 1(a)(ii)[3]

    table drawn with a minimum of two columns and a header line ; column and row headings with units ;

    correct time recorded in seconds for each concentration ;

  • 1(a)(iii)[1]

    as the, enzyme / lipase, concentration / percentage, increases the time to change colour, decreases / is faster ; ora

    the higher the enzyme concentration the faster the, fat / milk, is broken down /AW ;

  • 1(b)(i)[2]

    control: distilled water / L4 / 0% lipase ;

    explanation: to show that lipase / enzyme is causing the change (of colour in the indicator / of pH) ;

  • 1(b)(ii)[1]

    L1 = 6 and L4 = 8 ;

  • 1(b)(iii)[1]

    lipase / enzyme, concentration ;

  • 1(b)(iv)[2]

    any two from:

    volume of milk ;

    total volume, of (enzyme) solution ;

    volume of sodium carbonate solution ;

    number of drops of, indicator / bromothymol blue, used ;

  • 1(b)(v)[1]

    to give enough time for the (contents of the) test-tubes to, reach the temperature of the water-bath / equilibrate AW ;

  • 1(b)(vi)[1]

    idea of: colour change is subjective / end-point is hard to judge ;

  • 1(c)[3]

    add biuret, reagent / solution (to lipase solution) ;

    colour change (from blue) to purple / lilac / violet / mauve ;

    wear gloves / safety glasses / goggles ;

  • 1(d)[6]

    independent variable

    1

    at least three temperatures ;

    2

    one below 37°C, one above 37°C, and one at 37°C ;

    new method

    3

    method of maintaining temperature described ;

    4

    named alternative sources of fat ;

    5

    correct reference to emulsion test

    6

    correct reference to use of a different indicator, e.g. UI

    7 and 8 max 2 from given method ;;

    same, volume of sodium carbonate
    same number of drops of indicator
    same volume of milk
    measure time taken for indicator to change colour
    idea of equilibrate enzyme and milk before mixing / AW 9 and 10 max 2 from variables kept constant ;;
    same volume of indicator
    same type of milk / fat
    same volume of fat ;
    same amount / volume / concentration, of lipase

    11 ref. to two or more repeats ;

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