Required Practical 9 - Rate of Respiration Yeast

Question 1

Suggest how colour change is caused in methylene blue

Answer 1

• accepts electrons from coenzymes involved in respiration
• methylene blue is reduced
• changes colour from blue to colourless

Question 2

Describe a control for this investigation and explain its purpose

Answer 2

• methylene blue and glucose (plus boiled yeast)
• shows that glucose did not cause colour change
• shows that RESPIRING yeast cause colour change

Question 3

Suggest why test tubes containing yeast solutions were left in the water baths for five minutes before the reaction

Answer 3

• allow them to reach required temperature

Question 4

Suggest why it is important not to shake the reaction mixture again once methylene blue is added

Answer 4

• mixes with oxygen from air

- oxidises methylene blue

Question 5

Suggest how you would ensure the temperature of the glass beaker water baths remains constant

Answer 5

• use a thermometer to measure at regular intervals

- add hot or cold water to adjust temperature

Question 6

Suggest why a glass beaker water bath is used rather than an electric water bath

Answer 6

• allows colour change to be observed

- easily/while test tube is in water bath/without removal