Is+yeast+alive?

Scientific Experiment to Test for Metabolism

Test if gas is given off in the presence of sugar

We can bubble the gas in limewater and if a white precipitate is formed, the gas is carbon dioxide. A glowing splint can be put above the test tube giving off the gas, if it sparks into flames, the gas produced is oxygen.

Results: Gas from test tube L formed a precipitate in the limewater whereas the gas from test tube D did not. Interpretation: Yeast mixed with glucose solution respires and hence gives off carbon dioxide.

Test the activities of yeast cells

After adding the drops, the contents of test tube A, containing the warm yeast solution, turned blue and slowly changed back yellow, the methylene blue turning colourless. Whereas the contents of test tube B turned blue.

Interpretation: Yeast when mixed with glucose solution causes reduction to happen and hence, turns the methylene blue colourless.

Examine yeast cells close up

http://bugs.bio.usyd.edu.au/learning/resources/CAL/Microconcepts/images/Topics/Diversity/buddingYeastCells.jpg

2. What are the evidences that prove that the yeast cells are alive?

They respire and multiply.

3. When you make bread, if you just mix flour, sugar ad water, the dough does not rise, and the bread will be flat and hard. If you include yeast in the bread dough, then the dough rises and the bread is bigger and fluffier. Can you explain how the yeast helps the bread dough to rise?

The yeast feeds on the sugar and starts to respire at a rapid rate, producing large amounts of carbon dioxide. The air then starts to trap in the dough and creates air bubbles, forming a complex structure inside the interwoven dough.

4. If we were to place a few grains of the yeast in a yeast culture, do you think you will be able to see any growth after a day or two?

Yes.