AS & A Level Biology - 9700 Paper 3 2009 Winter Zone 1

You are required to estimate the concentration of sugar in an extract from the phloem of a flowering plant.
You are provided with:
• 2 cm³ of a solution extracted from phloem in a test-tube, labelled P
• a range of sucrose solutions of known concentration in beakers, labelled W to Z, as shown in Table 1.1
[H] • 20 cm³ of Benedict’s solution, labelled B
[H] • 20 cm³ of dilute hydrochloric acid, labelled H
• 50 cm³ of sodium hydrogen carbonate solution, labelled S.

Table 1.1 shows the concentration of sucrose in each beaker.

[Table_1]

1. Set up a water-bath with hot water. The water-bath should not be more than one third full. Heat the water until it boils.

While you are waiting for the water to boil, carry on to instructions 2 to 5 and prepare the space at (a)(i) to record your results.

2. Label four test-tubes, W to Z.
3. Use the large syringe to add 2 cm³ of:
• solution W to test-tube W
• solution X to test-tube X
• solution Y to test-tube Y
• solution Z to test-tube Z.
4. Use the small syringe to add 1 cm³ of dilute hydrochloric acid to tubes W, X, Y, Z and P.
5. Gently shake each test-tube.
6. When the water is boiling, place all the test-tubes into the water-bath and leave them in the boiling water for three minutes.
7. After this time, remove the test-tubes from the water-bath using a test-tube holder and place them in the test-tube rack.
8. Use the large syringe to add 5 cm³ of sodium hydrogen carbonate solution to each test-tube. The mixture will fizz and rise up the test-tube.
9. Make sure that the temperature of the water-bath is between 80°C and 90°C.
10. When the fizzing stops, use the small syringe to add 1 cm³ of Benedict’s solution to test-tube W and place the test-tube in the hot water-bath. Immediately start timing.
11. Observe the test-tube very carefully for the first sign of a colour change. As soon as you see this, record the time taken for the colour to change.
12. Remove the test-tube from the water-bath and put it in the test-tube rack.
13. Repeat steps 9 to 12 for test-tubes X, Y, Z and P.
(a) (i) Prepare and use the space below to record all your results.

(ii) Use your results to estimate the concentration of sugar in P. ............................................................................................. ............................................................................................. [2]

(b) You used syringes to measure the volumes of the solutions used.
State the volume of the smallest division on the small syringe ……...
State the degree of uncertainty in using the small syringe to measure the volumes ........................................................................................................................................ [1]

(c) (i) Identify a significant source of error in estimating the sugar concentration in P.
........................................................................................................................................ ................................................................................................................................. [1]

(ii) Suggest how you would improve the investigation.
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(d) A student obtained another sample from the phloem and tested it using the same method. The concentration of sugar was higher than you found.
Suggest one reason why the concentration of sugar in the phloem is not always the same.
........................................................................................... .............................................................................................. [1]

(a) Draw a large, labelled plan diagram of the part of the stem shown in Fig. 2.1.

On your diagram add two annotations to describe the visible appearance of two tissues in Fig. 2.1.

[6 marks]

Fig. 2.1 is a photomicrograph of part of a transverse section of a stem of a flowering plant.



(b) Using Fig. 2.2 make a large drawing of cell X and all the cells that are touching it. Start with the phloem and include a companion cell.

Label cell X on your drawing.

[6 marks]

Fig. 2.2 is a photomicrograph of part of a stem of a different flowering plant viewed under high-power.

Methylene blue stains dead cells but is decolourized by living cells so they will appear white or clear.

You are required to observe the colour of methylene blue when added to

• boiled yeast, A1
• yeast in a high concentration of sodium chloride solution, A2
• yeast in a glucose solution, A3

1. Label three microscope slides A1, A2 and A3.
2. Place one drop of A1 onto slide A1 and add one drop of methylene blue.
3. Use a glass rod to mix carefully.
4. Leave for five minutes.
5. Repeat steps 2 to 4 with solutions A2 and A3.
6. Add a coverslip to each slide.
7. Use the paper towel to dry off any liquid around the coverslip.
8. View the slides using the microscope.

(a)
(i) Prepare the space below and record your observations. [2]

(ii) Explain the appearance of the yeast cells in A1 and A3. [1]

In a similar investigation, a student recorded the activity of yeast cells in a glucose solution to which different concentrations of sodium chloride had been added. The student counted the number of bubbles of carbon dioxide released in three minutes.

The student's results are shown in Table 3.1.

Table 3.1
[Table_1]

(b)
(i) Complete Table 3.1 by calculating the missing value for the mean activity of yeast.
Show all the steps in your calculation.
Write your answer in Table 3.1. [2]

(ii) Plot a graph of these data shown in Table 3.1. [4]

(iii) Describe the results shown in your graph. [2]
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(iv) From your graph estimate the mean activity of yeast in a 2.0 % sodium chloride solution. [1]
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(v) Explain the difference in the activity between
0.0 % and 1.5 % sodium chloride solution
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3.0 % and 5.0 % sodium chloride solution.
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