Yeast obtains energy for growth and reproduction by breaking down sugar (glucose) through anaerobic respiration:
glucose → carbon dioxide + alcohol
In human food, sugar is added to give a sweet taste. If the diet contains too much sugar, diseases can develop. Sugar substitutes are available and taste as sweet as sugar.
A student chose to find out whether yeast could use a sugar substitute for respiration.
Three test-tubes were arranged as shown in Table 1.1.
Each test-tube was given a label so that it could be recognised.
The level of the mixtures in each test-tube was marked with a water-resistant marker. This showed the starting level.
When yeast respires, bubbles of gas are produced and trapped in the mixture, causing the level of the mixture to rise up the test-tube.
The student used a large beaker as a water-bath to keep the temperature of the test-tubes constant in this investigation. The temperature of the water at the start was $35\,^{\circ}\text{C}$. The intention was to keep this temperature the same throughout the investigation by adding hot or cold water.
The three test-tubes were placed in the water-bath, and a timer was started. At 5 minutes, 10 minutes and 15 minutes, the distances moved by the mixtures from the starting levels were measured and recorded. The temperature of the water-bath was also recorded.
Fig. 1.1 shows the lower parts of the test-tubes at 5 minutes and the water-bath thermometer reading at 5 minutes.
Fig. 1.2 shows the results for 10 and 15 minutes recorded in the student’s notebook.
(a(i))[1]
State which labels you would use on the three test-tubes.
Test-tube containing sugar: [BLANK]
Test-tube containing sugar substitute: [BLANK]
Test-tube with no addition: [BLANK]
(a(ii))[1]
Explain why the stirring rod had to be cleaned after it had been used in each test-tube.
(b(i))[5]
Complete Table 1.2.
• Fill in the headings.
• Using Fig. 1.1, measure the distances moved by the yeast mixtures from the starting levels in the three test-tubes and record them. Read the temperature on the water-bath thermometer in Fig. 1.1 and record it.
• Use the information in Fig. 1.2 to complete Table 1.2.
(b(ii))[1]
Explain how the test-tube with no addition worked as a control in this investigation.
(b(iii))[1]
State the conclusion you can draw from the results of this investigation.
(b(iv))[1]
Use the information in Fig. 1.1 and Fig. 1.2 to evaluate how well the student controlled the water-bath temperature.
(b(v))[1]
Suggest why the water-bath temperature at 15 min was $39\,^{\circ}\text{C}$.
(c(i))[4]
Measure and note the length of the line between A and B.
Calculate the increase in volume of the mixture in this test-tube after 10 minutes.
Use $3.14$ as the value of $\pi$. Give your answer to 1 decimal place. Show your working.
(c(ii))[2]
The apparatus used in this investigation is shown in Fig. 1.1. Suggest a piece of apparatus that could have been used in this investigation to directly measure the volume of the mixture. Describe how you would determine the increase in volume at each time interval using this apparatus.