Lab+Experiment+-+Is+Yeast+Alive?

Procedure to test for metabolism
When yeast, humans, and other living organisms use energy, they break down high-energy molecules like sugar to get the energy they need and give off a gas calld carbon dioxide as a by-product of this reaction. __Hypotheses:__ //vailable?// //1. Do you expect yeast to produce gas when sugar is available?// //vailable?// Yes/hai. //2. Do you expect yeast to produce gas when no sugar or other food is available?// No/eii //3. Explain the reasons for your choice.// Since glucose is needed for respiration, the yeast only produced carbon dioxide when it has synthesized the glucose. //4. How would you measure the amount of gas given? How do you determine the identity of the gas?// Since the gas produced by yeast during respiration is likely to be carbon dioxide, we can bubble the gas through limewater. In this way, the white precipitate formed in the calcium hydroxide will provide us with the identity of the gas - that of carbon dioxide.

Procedures to test the activities of yeast cells
//1. Observations of test tubes A and B after adding 2 drops of methylene blue:// Test tube A turns bluish-green while test tube B turns turquoise. //2. After leaving the test tubes for 5 minutes, observe and explain your observations:// Test tube A (which contains the warm and live yeast solution) bubbles, while test tube B (which contains the boiled and dead yeast solution) remains unchanged. This is because the live yeast contains active enzymes which are able to ferment the glucose solution, producing ethanol and carbon dioxide in the process, causing the solution to foam. //3. Place a thumb (or use the stopper) over the mouth of the test tubes A and B and shake it vigorously (about 10 times). Observe and give an explanation for your observations.// The entire test tube A turns bluish-green while the entire test tube B turns turquoise (homogenous mixture).

Procedures to examine yeast cells close up
//1.Examine the yeast cells from test tubes A and B and note down the difference.// The yeast cells from test tube A are mostly colourless, and are moving fluidly. The yeast cells from test tube B are stained with methylene-blue, and they do not move at all.

Discussion
//1. Discuss the results you obtained with your partner. How do you interpret the results?// //2. What are the evidences that prove that the yeast cels are alive?// If methylene blue-stained yeast cells remain blue, it shows that they are dead. This is because alive yeast cells contain active enzymes which may reduce the methylene blue (coloured) into reduced methylene blue (colourless). //3. When you make bread, if you just mix flour, sugar and 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?// // Through a process known as fermentation, it converts food to carbon dioxide. This carbon dioxide produced by the yeast will cause the bread dough to rise. // //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. //

Key notes
1. A warm temperature is needed to break up the yeast granules and allow the thousands of yeast cells to be released into the water. This creates a yeast suspension. 2. Process where methylene blue is reduced: Oxidization - gaining electrons (blue), Reduction - losing electrons (colourless). Within the yeast cells during its respiration, it reduces the oxygen to become colourless. The boiled yeast cells do not cause the methylene blue to become colourless, hence the solution remains blue.
 * uniform colour would prove the yeast cells to be dead or alive