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Yeast cells contain enzymes which catalyse the breakdown of glucose to produce carbon dioxide and water. The carbon dioxide reacts with water and forms a weak acid. Bromothymol blue is a pH indicator and changes colour as shown in Table 1.1.
Table 1.1
[Table_1]
You are required
• to immobilise the yeast cells in sodium alginate beads
• to follow a student's procedure to investigate the independent variable, changing the surface area of the beads.
You are provided with
• 15 cm³ of yeast suspension, labelled Y
• 15 cm³ of 2.0% sodium alginate solution, labelled S
• 50 cm³ of 1.5% calcium chloride solution, labelled C
• 40 cm³ of 2.0% glucose solution, labelled G
• 50 cm³ of bromothymol blue, labelled B
• 20 cm³ of sodium hydroxide solution, labelled A
1. Put 20 cm³ of C into a large test-tube.
2. Put 5 cm³ of S into a small beaker or container.
3. Collect 5 cm³ of Y from below the froth and put it into the same container as S. Mix well.
4. Use a 5 cm³ syringe to collect 2 cm³ of the mixture S and Y.
5. Suspend the 5 cm³ syringe over the large test-tube containing C as shown in Fig. 1.1.
6. Gently press down on the plunger of the syringe with your thumb to release a drop into solution C. The drop should form a bead.
7. Repeat step 6 to make the number of beads that you think you will need.
8. Tip the contents of the large test-tube into a Petri dish or shallow container.
You will need to calculate the mean surface area of the beads. Use blunt forceps to pick up the beads. To do this
• decide on the number of beads you will measure
• use the 2 mm × 2 mm grid to measure each bead
• calculate the surface area of each bead using the formula $\text{surface area} = 4 \pi r^2$ where $\pi = 3.14$, $r =$ radius of a bead
• calculate the mean surface area of the beads.
(a) (i) Prepare the space below to show your measurements and calculations.
Show all the steps in your calculation of the mean.
mean surface area of the beads ............................................ $\text{mm}^2$ [5]
A student suggested that it was possible to investigate the independent variable, surface area, by changing the number of beads. The maximum number of beads used was 20. Decide the other numbers of beads to use and state the different number of beads you will use.
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Carry out the student's procedure.
9. Label as many small test-tubes as you will need with the number of beads for each test-tube.
10. Put 10 cm³ of solution G into each test-tube.
11. Put 1 cm³ of B into each test-tube. Put the bung in each test-tube in turn and mix.
12. If the contents of the test-tube are not blue, add one drop at a time of A to the contents of each test-tube to turn them all the same blue colour.
13. Put the required number of beads into each test-tube.
14. Put the bung in each test-tube in turn and mix contents. Mix every 2 minutes for 6 minutes.
15. Record your observations after each 2 minutes, up to 6 minutes.
(ii) Prepare the space below to record your observations. [7]
(iii) The student realised that there were two independent variables in this procedure. State the two independent variables.
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(iv) Suggest how you would make three improvements to the student's procedure.
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A student set up the apparatus as shown in Fig. 1.2 as another way to measure the carbon dioxide produced by immobilised yeast cells over a period of 75 minutes. The student measured the distance the liquid moved in the capillary tubing.
The student's results are shown in Table 1.2.
Table 1.2
[Table_2]
(b) Describe and explain the results shown in Table 1.2.
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514
525
595
M1 is a slide of a stained transverse section through a plant organ.
(a) (i) Draw a large plan diagram of a half of the specimen as shown in Fig. 2.1.
Label the xylem and the cortex.
(ii) Make a high-power drawing of
• a group of three complete touching xylem vessels
and
• a group of three complete touching cortex cells.
Label a cell wall and a lumen.
xylem vessels
cortex cells
Fig. 2.2 is a photomicrograph showing part of an organ from a plant of a different species.
The actual length of the line X is 1900 $\mu$m.
(b) (i) Calculate the magnification of the specimen shown in Fig. 2.2.
Show all the steps in your calculation.
(ii) Prepare the space below so that it is suitable for you to record the observable differences between the specimens on slide M1 and in Fig. 2.2.
Record your observations in the space you have prepared.
A student investigated water absorption by a plant.
The apparatus was set up by the student as shown in Fig. 2.3.
The volume of water added to top up the water level was recorded each hour for a total of 5 hours.
From this data the student calculated the volume of water absorbed by the plant.
The student’s results are shown in Table 2.1.
[Table_1]
(c) Plot a graph of the data shown in Table 2.1.
[Graph_1]
536
524
545
