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Biology · May / June 2020 · Paper 62 · Question 1

0610

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The question as text

A student investigated the effect of temperature on diffusion. Dialysis tubing was used to represent a cell membrane. Cell membranes are partially permeable. Step 1 One test-tube was labelled C and another test-tube was labelled H. The student drew a small X on the outside of each test-tube as shown in Fig. 1.1. C H X X Fig. 1.1 Step 2 The student was given two pieces of dialysis tubing. A knot was tied at one end of each piece of dialysis tubing to form two bags. Step 3 Starch suspension was put into each dialysis tubing bag until each bag was approximately three-quarters full. Step 4 One of the dialysis tubing bags containing starch suspension was put into test-tube C. The open end of the dialysis tubing was folded over the top of the test-tube. It was secured with an elastic band as shown in Fig. 1.2. open end of the dialysis tubing bag folded over the top of the test-tube dialysis tubing bag containing starch suspension test-tube closed end of the dialysis tubing bag X elastic band C Fig. 1.2 3 Step 5 Step 4 was repeated with the other dialysis tubing bag and test-tube H. Step 6 Cold water was added to test-tube C until it reached the level of the elastic band. Step 7 Hot water was added to test-tube H until it reached the level of the elastic band. Step 8 The temperatures of the water in test-tube C and test-tube H were measured. The temperatures are shown on the sections of thermometers in Fig. 1.3. thermometer in test-tube H thermometer in test-tube C 60 70 80 10 20 30 Fig. 1.3 Step 9 1 cm3 of iodine solution was added to the water in each test-tube. Step 10 A stop-clock was started. Step 11 The student observed the X on each test-tube by looking through the dialysis tubing bags as shown in Fig. 1.4. X drawn on the opposite side of the test-tube eye looking at X through the dialysis tubing bag that is inside the test-tube water containing iodine solution Fig. 1.4 4 Step 12 The student continued to observe the X until the contents of the dialysis tubing bag became blue-black in colour. At this point the X was no longer visible. The student recorded the time taken for the X to stop being visible in each test-tube. The student’s results are shown in Fig. 1.5. test-tube C 8 minutes 36 s test-tube H 1 minute 23 s Fig. 1.5

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Show the answer and mark scheme

  • 1(a)(i)[3]

    table drawn with minimum of two columns and a header row ;

    column headings with units: temperature / oC and time taken / s ;

    recording of two temperatures: 18 and 78 and two times: 516 and 83 ;

  • 1(a)(ii)[1]

    at higher temperatures, the iodine solution diffuses faster / the cross disappears more quickly / AW ;

    ora

  • 1(a)(iii)[1]

    temperature ;

  • 1(a)(iv)[2]

    conclusion:

    starch does not pass through the dialysis tubing ;

    evidence:

    the liquid around the bag remains brown / does not becomes blue–black ;

  • 1(a)(v)[2]

    error must match apparatus:

    error

    temperature of the water not controlled / temperature changes AW ;

    apparatus

    thermostatically controlled water-bath / insulation / Bunsen burner ;

    OR

    error

    difficult to tell when the cross had disappeared ;

    apparatus

    compare with a colour standard / compare with one that had already changed colour / colorimeter ;

    OR

    error

    volume of starch not measured ;

    apparatus

    measuring cylinder / burette / pipette ;

  • 1(b)(i)[3]

    method:

    add Benedict’s, solution / reagent ;

    heat ;

    result: red / orange / yellow / green ;

  • 1(b)(ii)[6]

    method:

    1&2 use of Benedict’s reagent to measure reducing sugar produced, at set time / at regular intervals ;

    ref. to use of hot water-bath / use of (thermostatically controlled) water-bath to maintain temperatures ; OR

    use of iodine solution to determine starch remaining ;

    ref. to drops on spotting tile at regular intervals ;

    independent variable:

    3

    ref. to using at least 2 different concentrations of amylase ;

    variables kept constant - two from mp 4-8:

    4

    same type of enzyme (amylase) ;

    5

    concentration of starch suspension ;

    6

    temperature ;

    7

    same pH ;

    8

    incubating enzymes and starch suspension for same time ;

    dependent variable:

    9

    concentration of reducing sugar produced / time taken for drops and iodine solution to remain brown ;

    10

    ref. to at least two further replications / repeat investigation two more times ;

    11

    ref. to relevant safety precaution ;

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