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 absent
Indicator reduced to pink colour
Oxygen present
Indicator oxidised to blue 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 20cm
3
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
2 oxygen is in the mixture.
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
3 mixture.
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
5
Fig 84
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.
Leave the apparatus for 10-20 minutes and then record your observations.
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Results/Discussion
1 Record your results in the form of a table (design your own table).
2
3
Briefly describe the control you used.
Complete the following questions.
Questions
1 What can you conclude from the final colours of?
2
(a) the bicarbonate indicator solution?
(b) The Janus Green Bb indicator solution?
What can you conclude from your two answers to question 1?
3
4
Why is a layer of liquid paraffin used in the experiment?
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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