Is+Yeast+Alive?


 * Humans use yeast every day. What is yeast, and what are some common uses of yeast?**

The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: baker's yeast in bread production; brewer's yeast in beer fermentation; yeast in wine fermentation and for xylitol production. So-called red rice yeast is actually a mold, //Monascus purpureus//. Yeasts include some of the most widely used model organisms for genetics and cell biology.


 * You can buy yeast to make bread in the grocery store. This yeast consists of little brown grains. Do you think that these little brown grains of yeast are alive? Why or why not?**
 * To find out whether yeast is alive, we first need to think about what makes something alive. What are some characteristics of living organisms?**

Living things can respire and reproduce. They can also react to their surroundings. Yeast cells are alive. However, it takes in oxygen in the form of glucose. Yeast + Glucose -> Alcohol + Carbon Dioxide We know that yeast cells are alive because it produces wastes (alcohol and carbon dioxide) and they reproduce.

To begin to answer the question, "Is yeast alive?", you will test whether the grains of yeast have two characteristics of living things - the ability to grow and the ability to use energy (referred to as metabolism).

**Scientific Experiment for 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 called carbon dioxide as a by-product of this reaction. We will test whether yeast can metabolize sugar and produce a gas in the process. What gas do you think is produced? How do you identify it?**
 * We will carry out an indirect test for metabolism. In other words, we will be indirectly testing whether yeast can use energy, which is one of the characteristics of living organisms.

Carbon dioxide will be produced. We identify it by bubbling it into lime water.

Do you expect yeast to produce a gas when sugar is available?** Yes Yeast + Glucose -> Alcohol + Carbon Dioxide
 * __Hypotheses:__
 * Do you expect yeast to produce a gas when no sugar or other food is available?** No
 * Explain the reasons for your predictions.**

__**Materials & Apparatus provided**__: Yeast powder Boiled yeast suspension Warm glucose solution Methylene blue solution Warm water (around 40 degree celsius) Lime water 2 delivery tubes 4 test tubes Microscope Glass slides & Cover slips Test tube rack Test tube holder 1 Spatula 2 Droppers Labels Ruler

1. Set up 2 test tubes (L&D) of yeast suspension to prov your hypotheses stated above. 2. Fill 1/3 of test tube L with warm glucose solution and test tube D with warm water instead. 3. Add in 3-4 spatula full of yeast powder to teach of the test tube. 4. Shake to dissolve the yeast as much as possible but be very careful not to create too much froth. 5. How do you determine the identity of the gas given? If the yeast cells are capable of metabolism, it will take some time to produce enough gas for change to be visible.
 * Procedure to Test Your Predictions**

__Results:__ To determine the identity of the gas, we bubble the gas into limewater. If the white precipitate forms in the limewater, the gas produced is carbon dioxide. The yeast in test tube L produced carbon dioxide as it turned limewater chalky, whereas the yeast in test tube D did not. Therefore we can conclude that the yeast in test tube L is alive as it can produce waste (carbon dioxide) when it was in contact with sugar, whereas the yeast in test tube D is dead as it cannot produce waste (carbon dioxide), because it is not in contact with sugar.

Methylene blue is a dye that under certain conditions is easily reduced to a colourless compound. This colourless, reduced methylene blue is readily oxidized back to the coloured methylene blue. 1. Add 5cm^3 of the warm yeast suspension to test tubes A and 5cm^3 of the boiled yeast suspension to test tube B respectively. 2. Add 2 drops of methylene blue to each test tube and shake gently after each drop. Observe if there are any differences between test tubes A and B. 3. Take a drop of yeast suspension from each of the test tube and put them on 2 separate glass slides. 4. Leave the test tubes to stand for 5 minutes. Observe. Explain your observations. 5. Place a thumb (or use the stopper) over the mouth of test tubes A and B and shake it vigorously (about 10 times). Observe and give an explanation for your observations.
 * Procedures to test the activities of yeast cells**

__Results:__ The yeast mixture in test tube A turned blue at first but slowly changed back to its original colour and stayed that way. The yeast mixture in test tube B turned blue and remained unchanged. The yeast mixture in test tube A are alive, whereas the yeast mixture in test tube B are dead. __Reactions:__ Methylene blue when reduced turns changes to a colourless methylene blue when it gains electrons, and can be oxidised back to blue when it loses electrons (oxidation). Therefore, it can be deducted that reactions in the yeast cells during respiration reduces blue methylene.

Extra: Methylene blue can be used as an indicator to determine if a cell, in this case yeast, is alive or not. The blue indicator turns colourless in the presence of active enzymes, thus indicating living cells. However, if it stays blue it doesn't mean that the cell is dead - the enzymes could be inactive/denatured. Methylene blue can inhibit the respiration of the yeast as it picks up hydrogen ions made during the process. The yeast cell cannot then use those ions to release energy.

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