Required Practical 9

Question 1

You were told to leave the tubes in the water bath for 5 minutes, explain why

Answer 1

To equilibrate the yeast and glucose mixture

So the yeast reaches the required temperature

Question 2

You were told to make sure that the water bath temperature stayed at 35degrees. describe how you monitored the water bath temperature and kept it constant at 35 degrees

Answer 2

Use thermometer to measure at regular intervals throughout the investigation (eg every 30 seconds)
Add cold or hot water

Question 3

There was no control experiment in this investigation. Describe the control experiment you would set up. Explain why this control experiment would be important.

Answer 3

Methylene blue,boiled yeast and glucose

Control is important to show that colour change was produced by the yeast

Question 4

You were told not to shake the tubes again, shaking the tubes would have caused the methylene blue to go blue again. Suggest why

Answer 4

Mixing with oxygen

Oxygen oxidises methylene blue

Question 5

Respiration by the yeast caused the methylene blue to go colourless. Explain how

Answer 5

Methylene blue accepts electrons

Electrons from glucose

Question 6

Suggests one reason why it was important that the student left the apparatus for one hour after the yeast culture reached a constant temperature

Answer 6

So the oxygen is used

Question 7

Explain why a log scale is used to record the number of cells

Answer 7

Large range

Question 8

Many yeast cells die during the death phase. Suggest one reason why

Answer 8

Decrease in substrate

Question 9

Describe how you would use the apparatus shown to compare the effect of using each of the three sugars, fructose glucose, lactose on the rate of respiration in yeast (6)

Answer 9

1- Same volume and concentration of yeast suspension for each sugar
2- same volume and concentration of sugar solution for each sugar
3- mixture kept at a suitable constant temperature
4- mixture of sugar and yeast drawn into syringe (using plunger)
5- join syringe to glass tube via rubber tubing
6- adjust plunger so that meniscus is on scale
7- measure time taken to displace a set volume
8- repetitions and calculate mean
9- clean apparatus and rinse with new sugar and yeast mixture

Question 10

How do you know if a result is an anomaly

Answer 10

Does not fit pattern
Does not lie ON curve
Curve changes direction

Question 11

Suggest a possible cause for this anomalous result

Answer 11

Temperature change
Misread value
Dirt on glass tube causes meniscus to ‘stick’

Question 12

How do you check for an anomaly

Answer 12

Repeat experiment for glucose