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Yeast cells contain enzymes, which catalyse the breakdown of glucose to produce ethanol and carbon dioxide. These products change the environment of the yeast cells and can affect their activity and survival. The carbon dioxide when dissolved forms a weak acid so the more carbon dioxide that is released the more acid will be formed.
You are required to investigate the effect of different concentrations of ethanol on the activity of yeast cells by measuring the change in pH, using Universal Indicator paper.
You are provided with:
• five labelled tubes each containing 0.7 g of dried yeast
• at least 50 cm$^3$ ethanol, labelled E
• at least 100 cm$^3$ 20% glucose solution, labelled G
• at least 100 cm$^3$ distilled water, labelled W
Ethanol is harmful and highly flammable. If any comes into contact with your skin, wash immediately under cold water. It is recommended that you should wear eye protection. Keep the ethanol covered when you are not using it.
It is important to find the pH of the glucose solution and the ethanol before starting the investigation.
Place a small piece of the Universal Indicator paper on a white tile. Using a clean pipette, place a drop of glucose solution onto the paper. Use the colour chart to identify the pH. Repeat, after cleaning the pipette, to find the pH of ethanol.
(a)
(i) Record the colour of the Universal Indicator paper and the pH for the glucose solution and the ethanol below. [2]
Clean and dry the tile. You are going to change the independent variable, the concentration of ethanol. Table 1.1 shows how to make up two of the concentrations you should use.
[Table_1]
(ii) Using the information in Table 1.1, decide which other concentrations to make and complete Table 1.2 including the concentrations from Table 1.1.
[Table_2]
Tubes 1 to 5 each contain the same mass (0.7 g) of dried yeast.
- Adding the water before the ethanol, use the syringes provided to make up the ethanol concentrations in the correct tubes.
- Use the beaker, or other container provided, to make a water bath with warm water between 45°C and 50°C.
- Shake the tubes carefully to thoroughly mix the ethanol and water.
- Place the tubes into the warm water and leave them for at least 5 minutes.
- Use the marker pen provided to label the white tile as shown in Fig. 1.1.
- Arrange two rows of small pieces of Universal Indicator paper on the white tile as shown in Fig. 1.2.
- After the tubes have been in the water bath for at least five minutes start a stopwatch or stop clock or note the time on a clock.
- Use a clean 10 cm$^3$ syringe to put 10 cm$^3$ of the glucose solution into each tube starting with tube 1. Each time shake the tube well and return it to the warm water bath.
- When the clock shows one minute, use the glass rod to remove a drop from the contents of tube 1 and place the drop onto the correct piece of Universal Indicator paper.
- Clean the glass rod and use it to remove a drop as described in step 9 from the other four tubes.
- When the clock shows 10 minutes, use the glass rod to take further drops as described in step 9.
(b)
(i) Identify a significant source of error in this investigation.
.....................................................................................................................................................
..................................................................................................................................................... [1]
(ii) You used syringes to measure the volumes of ethanol and water.
State the volume of the smallest division on the syringe .................
State the degree of uncertainty .............................................. [1]
A student decided to investigate the effect of temperature on the activity of enzymes in yeast. The student measured the activity of the enzymes by counting the number of bubbles of carbon dioxide, which were released in three minutes.
The results of the student’s investigation are shown in Table 1.3.
[Table_3]
(c)
(i) Plot a graph of the data shown in Table 1.3. [4]
(ii) From the graph, estimate the enzyme activity at 25°C.
..................................................................................................................................................... [1]
(iii) Suggest how the student should make sure that the results of this investigation are as accurate as possible,
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.....................................................................................................................................................
as reliable as possible.
.....................................................................................................................................................
..................................................................................................................................................... [3]
In carrying out this investigation the student made the hypothesis that:
The activity of the enzymes in yeast increases as temperature increases.
(d) State whether you think that this hypothesis is supported by the student’s results.
Explain your answer.
.....................................................................................................................................................
..................................................................................................................................................... [2]
551
598
517
J1 is a slide of a stained transverse section of a plant organ.
(a) Draw a large, low power, plan diagram of a quarter of the specimen as shown in Fig. 2.1.
[Image_1: Fig. 2.1]
Label the phloem and xylem in a vascular bundle.
(b) Fig. 2.2 is a photomicrograph showing some cells from a transverse section of a root viewed under high-power.
[Image_2: Fig. 2.2]
(i) Calculate the mean width of the cells shown in Fig. 2.2 in micrometres (μm).
Mark clearly on Fig. 2.2 the cells which you used in your calculation.
Show your working. $.......................... \text{μm}$ [4]
(ii) Fig. 2.3 is an outline of a longitudinal section of a root.
Label with a label line and the letter \( X \), the area on Fig. 2.3 from which the section shown in Fig. 2.2 may have been cut.
[Image_3: Fig. 2.3] [1]
Fig. 2.4 is a photomicrograph showing some cells from a transverse section from another part of the same root taken under high-power.
[Image_4: Fig. 2.4]
(c) From Fig. 2.4 make a large, labelled drawing of three complete cells which are touching and include at least one cell with a nucleus.
Show clearly on Fig. 2.4 the three cells, which you have drawn. [5]
(d) Prepare the space below so that it is suitable for you to show the differences between the cells shown in Fig. 2.2 and Fig. 2.4.
Record your observations in the space which you have prepared. [4]
535
584
546
