nO. 34 anaerobic respiration in yeast

introduction

Yeast is a micro organism which, like your muscles, can respire in two ways. You can investigate some aspects of this in a simple experiment.

The indicator Janus Green B changes colour according to the amount of oxygen present.

Oxygen present / Oxygen absent
Indicator oxidised to blue colour / Indicator reduced to pink colour
Apparatus

2 test tubes with delivery tubes

2 collection tubes

2 corks

paraffin, bicarbonate indicator solution

Janus Green b indicator solution

Pipette

20cm glucose solution.

Method

1  Place 20cm3 of glucose solution with some yeast added to in a test tube. Add two drops of indicator solution. The colour produced will tell you whether oxygen is in the mixture.

2  Pour enough liquid paraffin over the mixture to form a layer right over the surface. This layer will prevent more oxygen from the air getting into the mixture.

3  As soon as the colour of the mixture indicates that there is no oxygen present, fix up a delivery tube leading to a small amount of bicarbonate/indicator in another test tube.

4  Design and set up a suitable control. Remember that in this experiment you are trying to find out whether the yeast is respiring in the absence of oxygen.

5  Leave the apparatus for 10-20 minutes and then record your observations.

Fig 84
Results/Discussion

1  Record your results in the form of a table (design your own table).

2  Briefly describe the control you used.

3  Complete the following questions.

Questions

1  What can you conclude from the final colours of?

(a)  the bicarbonate indicator solution?

(b)  The Janus Green Bb indicator solution?

2  What can you conclude from your two answers to question 1?

3  Why is a layer of liquid paraffin used in the experiment?

4  If yeast is left in a sugary liquid for some days, a characteristic smell is produced. This is the result of ethanol accumulating in the mixture. Fermentation is the name given to this process in which yeast turns sugar into ethanol:

Sugar ethanol + carbon dioxide

Suggest a way of discovering whether any energy is released during this process.

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