A-LevelBiologyBiological molecules9700/31 · Oct/Nov 2024 · Q122 Marks

Milk is sometimes contaminated with substances such as starch. You will determine the concentration of starch in a sample of contaminated milk, using a range of starch standards. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume/cm³ M milk none 50 SM 1.0% starch solution in milk (starch-milk) none 20 CM contaminated milk none 20 iodine iodine solution irritant 20 If any solution comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. You will need to carry out a serial dilution of the 1.0% starch-milk solution, SM, to reduce the concentration by one fifth between each successive dilution. You will need to prepare four concentrations of starch-milk solution in addition to the 1.0% starch-milk solution, SM. You must use the milk, M, to dilute the starch-milk solution. Do not use water for the dilution. After the serial dilution is completed, you will need to have 8cm³ of each concentration available to use.

(a)(i)Complete Fig. 1.1 to show how you will prepare your serial dilution. Each beaker should have: • a labelled arrow to show the volume of starch-milk, SM, solution transferred • a labelled arrow to show the volume of milk, M, added • a label under the beaker to show the concentration of the starch-milk solution.[3]
(a)(ii)Record your results in an appropriate table.[5]
(a)(iii)To determine the concentration of starch in the contaminated milk, CM, you will need to test a sample of CM. State the volume of CM that you will use. volume =[1]
(a)(iv)Record the colour score for test-tube CМ. colour score for CM[1]
(a)(v)Use your results in (a)(ii) and (a)(iv) to determine the concentration of starch in the contaminated milk, CM. concentration of starch in CM[1]
(a)(vi)Identify one source of error in step 6.[1]
(a)(vii)Suggest why milk is used to dilute the starch-milk solution rather than water.[1]
(a)(viii)Milk is sometimes contaminated with glucose. Suggest how you would modify this investigation to determine the concentration of glucose in a sample of contaminated milk.[2]
(b)(i)Plot a graph of the data in Table 1.2 on the grid in Fig. 1.3. Use a sharp pencil.[4]
(b)(ii)Describe and explain the shape of the graph in Fig. 1.3. description explanation[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[3] 1 states 0.2%, 0.04%, 0.008%, 0.0016 ;
[3] 2 shows transfer of 2 cm³ to each beaker from the previous beaker ;
[3] 3 shows 8 cm³ of W added to each beaker ;
[5] 1 heading for independent variable: % concentration of starch-milk;
[5] 2 heading for dependent variables: colour score ;
[5] 3 states results for all concentrations ;
[5] 4 correct trend for results (the colour score for 1% starch-milk is higher than the colour score at the lowest concentration of starch-milk);
[5] 5 records results using colour score ;
[1] adds 1 cm³ of contaminated milk ;
[1] records the colour score for contaminated milk (5–0);
[1] estimates concentration of starch in contaminated milk using results in (a)(ii) and (a)(iv) ;
[1] identifies the difficulty of matching the colour in the test-tube with a colour score as a source of error ;
[1] states that water would dilute the colour ;
[2] 1 use Benedict's solution and heat to greater than 80 °C or boil ;
[2] 2 measure time to first colour change ;
[4] 1 x-axis: labelled 'concentration of lactose / mmol dm³ and_y-axis: labelled 'rate of reaction / arbitrary units';
[4] 2 scale on x-axis: 40 to 2 cm_scale, on y-axis: 0.25 to 2 cm, labelled at least every 2 cm;
[4] 3 correct plotting of 6 points with cross or dot in circle ;
[4] 4 smooth, thin line, joined point to point, through all the points;
[3] 1 rate increases as lactose concentration increase and then plateau ;
[3] 2 rate increases as more substrate available ;
[3] 3 active sites saturated;

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