practical 9

Question 1

practical nine

Answer 1

1. Set up a water bath at 35°C.
2. Add 5cm3 of the yeast and glucose solution to three test tubes. Place test tubes
   in the water bath and leave them, for the solution to equilibrate for 10 minutes.
3. Add 2cm3
   of methylene blue to the test tubes and start the timer. Shake for 10
   seconds and place test tube back in water bath. Record how long it takes for the
   methylene blue to turn colourless for each test tube.
4. Repeat the experiment using temperatures of 40°C, 50°C, 60°C and 70°C.
5. Find the mean of the results for each temperature and use to calculate the
   average rate of respiration.

The removal of the blue colour is caused by the glucose which, under alkaline conditions, is reducing the methylene blue to a colourless form. Shaking the solution admits oxygen, which re-oxidises the methylene blue back to the blue form.

Question 2

practical nine

Answer 2

1. Set up a water bath at 35°C.
2. Add 5cm3 of the yeast and glucose solution to three test tubes. Place test tubes
   in the water bath and leave them, for the solution to equilibrate for 10 minutes.
3. Add 2cm3
   of methylene blue to the test tubes and start the timer. Shake for 10
   seconds and place test tube back in water bath. Record how long it takes for the
   methylene blue to turn colourless for each test tube.
4. Repeat the experiment using temperatures of 40°C, 50°C, 60°C and 70°C.
5. Find the mean of the results for each temperature and use to calculate the
   average rate of respiration.

The removal of the blue colour is caused by the glucose which, under alkaline conditions, is reducing the methylene blue to a colourless form. Shaking the solution admits oxygen, which re-oxidises the methylene blue back to the blue form.