Technicians' information Assuming 16 pupils or working groups.
NB The yeast in glucose suspension should be made up 30 minutes before the practical, and be shaken occasionally.
Per class
Yeast in glucose suspension
Enough to 2/3 fill thirty two 22 mm diameter test tubes:
1 dm3 of 10% glucose solution which has been boiled beforehand to remove air then allowed to cool to room temperature, before thoroughly stirring in 80g dried yeast.
Supply of water at approximately 40°C, sufficient for thirty two water baths (see below).
Safety spectacles or goggles.
Per working group
Thermometer.
1 or 2 beakers or cans, to use as water baths. These should be of at least 400 cm3 capacity, to prevent over-rapid cooling.
2 wide test tubes, one fitted with bung and delivery tube.
Supply of methylene blue solution, with means of dropwise delivery.
Access to supply of liquid paraffin.
Access to supply of hydrogen carbonate indicator solution.
Rack, or other support, for test tubes,
You have access to a supply of yeast in glucose suspension. This is glucose solution which has been boiled to remove the dissolved air and which also contains yeast cells.
1 . 2/3 fill one wide test tube with the yeast in glucose suspension.
2. Add 6 drops of methylene blue solution and, wearing eye protection, gently shake the test tube to mix the contents. (Methylene blue is blue when oxygen is present an colourless when it is not). The contents of the test tube should have a very pale blue colour; if they do not, add more drops of methylene blue solution.
3, Pour liquid paraffin on to the mixture in the test tube, so that it forms a very thin layer on the surface. This layer will keep oxygen in the air away from the mixture.
4 Attach the delivery tube to the test tube, then place the test tube in a wann water bath.
5 When the methylene blue has lost its blue colour, place the end of the delivery tube in some hydrogen carbonate indicator in another test tube, as shown in the diagram. Record the time and the colour of the indicator.
7. After about 20 minutes, record your observations of the experiment. Do the same for the control, after the same time. Record the colour of the methylene blue and of the hydrogen carbonate indicator.
8. Explain your results and state the conclusions you can draw.
