Anaerobic Cell Respiration by Yeast BACKGROUND: Yeast are tiny single-celled (unicellular) fungi. The organisms in the Kingdom Fungi are not capable of making their own food. Fungi, like any other organism, need food for energy. They rely on sugar found in their environment to provide them with this energy so that they can grow and reproduce.
Yeast, like bacteria grow in or on their food source. They produce and release digestive proteins (enzymes) into their environment where the sugar molecules are found. Complex sugar molecules then break down into monosaccharides that can be absorbed by the yeast and used for food (energy). There are many species of yeast, and each has a particular food source. Certain yeast feed on a variety of natural sources of sugar such as fruits, nectar from plants, and molasses from the plant crop called sorghum. Others break down wood and corn stalks. In doing this, a compound called ethanol is produced. This compound can be used in our cars like gasoline. Another species break down sugar from grain into alcohol. Others break down fruits into wine, which is another type of alcohol. Bread recipes rely on yeast to break down sugar in flour. 1 Yeast is a facultative anaerobe, meaning that it can participate in aerobic respiration when possible, but when this is impossible, it respires anaerobically. When using yeast in making dough, the yeast will use the initial oxygen up very quickly and then start to respire anaerobically. ATP will then be made via glycolysis, which which requires no no oxygen. Without oxygen present, the yeast cells will quickly run out of NAD+ molecules which are vital to the process of glycolysis. To regenerate the NAD+, the yeast will undergo alcoholic fermentation, which converts pyruvic acid into CO 2.as well as ethyl alcohol, with the NADH being oxidized in the the process. Overall, the final final equation for glycolysis plus fermentation fermentation would be: C6H12O6
2CO2 + 2C2H5OH, with 2 ATP also produced.
For the yeast cell, this chemical reaction is necessary to produce the energy for life. The alcohol and the carbon dioxide are waste products produced by the yeast. It is these waste products that we take advantage of. The chemical reaction, known as fermentation can be watched and measured by the amount of carbon dioxide gas that is produced from the break down of glucose.
Yeast Cellular Respiration Investigation Explore: Some cells are able to obtain energy from food without using oxygen. These organisms obtain their energy through fermentation, an energy releasing process that does not require oxygen. There are about 160 known species of yeast. The yeast we are using is baker’s yeast or Saccharomyces cerevisiae. Yeast are single celled organisms that cannot produce their own food either through photosynthesis or chemosynthesis. Their single cell contain mitochondria. Today we are going to perform an investigation that will test whether yeast breaks down sugar to release the energy and determine the products of the reaction. Materials for each group: 1 empty pint size plastic water bottles (16.9 fl. oz. or 500ml), 3 t. table sugar (sucrose), 1 transparent (11” when inflated) balloons, packet of active dry yeast, very warm water (1050- 1150 F), Bromthymol blue solution (make a 50% solution), medicine dropper or 10ml graduate cylinder, measuring spoon and funnel or sheet of paper.
Procedure: 1. Stretch out the balloon by blowing it up repeatedly (a pump may be used), then put it to the side. 2. Fill the 2 empty small plastic water bottles about a 1/3 full with warm water. (Try to make the water level in each bottle as even as possible) 3. Add 3 teaspoonfuls of sugar to one of the water bottles. (You can use a funnel or fold a sheet of paper into a cone.) Place the lid on the water bottle and shake to dissolve the sugar. Remove the lid and let the water settle. Label this bottle S for sugar. 4. Add 1/2 dropper or 1 ml of a 50 % solution of Bromthymol blue to the balloon. Have one person in the lab group hold onto the balloon. 5. One student adds 1 packet of active dry yeast to the bottle, another student quickly places the mouth of the balloon over the top of the bottle being careful not to let any Bromthymol blue into the bottle. Do not shake or swirl at this point. 6. Check to make sure that the balloon is tight over the top of the bottle. Gently swirl the contents. Place the bottle in a warm place or a warm water bath. Hypothesis: What will happen to the balloon? Why do you predict this?
7. As the balloon starts to fill, another student holds the body of the balloon horizontal so that the Bromthymol blue does not drop into the bottle Continue holding the balloon to the side until you can no longer or are told to let go (tape may be used). Every so often, gently rotate the balloon with your fingers so that the Bromthymol blue swishes a bit without dropping into the bottle. 8. Observe what happens. After you observe what is happening in the balloon, you do
not have to hold the balloon any longer.
Observations: Bottle with Sugar, Water and Yeast
Observations of what is happening in the bottle:
Observations of what is happening in the balloon:
Explain: 1. What are the products of fermentation? Is there evidence for each product? 2. How can we determine in this lab whether fermentation occurred? 3. If you were asked to set up controls for this investigation, what would you do? 4. How is fermentation different from aerobic respiration? 5. How is fermentation similar to aerobic respiration? Conclusion: Was your hypothesis correct? Why or why not? What have you learned from this investigation? Model: Using diagram(s) and labels, show and explain what happened to the balloon and its contents and to the contents of the bottle. Elaborate: For further investigation. 1. Add a bit of yeast to a teaspoon of warm water, place a drop on a slide with a cover slip and examine under a microscope at 430x. Make a large drawing of a single yeast cell and label what you observe. 2. In a zip lock sandwich bag, add ½ cup flour, 1/4 pkg. active dry yeast that has been added to 1/8 cup of warm water, and 1 teaspoon of sugar. Seal the bag carefully making sure there is no air inside the bag. Rub the plastic bag with your fingers to thoroughly mix the substances. Place the bag in a warm place and observe after 15 minutes or so. 3. Perform the Anaerobic Cell Respiration by Yeast investigation on the next page. For further Investigation:
Anaerobic Cell Respiration by Yeast Investigation Do you think that the rate of carbon dioxide production during fermentation would be affected by the availability of simple sugars? Explain.
OBJECTIVE: In this lab, we will observe the effect of food source on the process of cellular respiration by yeast. You will attempt to determine whether a yeast “bread dough” contains only flour or flour and sugar, based on the rate of CO2 production. You will assess CO2 production by measuring how much the dough rises in a set period of time. MATERIALS: • Two plastic cups per group • Two plastic spoons per group • Marker • Several packages yeast • Warm Water • Table sugar • Flour • Plastic wrap PROCEDURE: 1. Get two plastic cups and label them A and B. 2. Place flour only in cup A and a mixture of flour and sugar in cup B. 3. Add 45 ml of warm water-yeast solution into each cup. 4. Mix each cup with a separate spoon until all the flour is moistened. Be sure to check the bottom of the cup to make sure NO DRY flour remains. 5. Continue to mix the dough for 2 to 3 minutes. 6. Stop mixing when you see the mixture forming gluten “threads” as you pull it apart. These gluten threads make the dough stretchy enough to capture bubbles of CO2, resulting in puffy dough. 7. Use the spoons to gently push the dough down to a relatively flat surface in each cup. Mark the level of the dough on the side of the cups. Use a ruler to measure 8. the approximate height in cm of the dough from the bottom of the cup. Record the starting height in the group data section. 9. Write your names on the cup and cover it with plastic wrap. Wash the spoon and then place the cup in the warm water bath. Wait at least 35 minutes for the dough to rise. After Rising 10. Mark the level of your dough now on the side of the cup and measure the change in the height. 11. Convert this number to a percentage of the starting height . To do this, divide the change in height by the original height. An increase in height would be a positive number,
while a decrease would be negative. Record your data in the group data section, and put your % change data in the class data table.
RESULTS:
Group data: Starting height
Change in height
% change in height
Cup A
Cup B
Class Data: Percent Change in Height Cup A
Cup B
Calculate the average % change for the class data for each cup. Cup A average % change: Cup B average % change: Analysis Questions:
1. Sugar was added to either the cups marked A or the cups marked B. Given what you know about anaerobic respiration, which cup had the added sugar? Explain you answer in terms of the energy source(s) available to the yeast. 2. At what point will the dough stop growing? Why? 3. Was anything else produced during the fermentation process? If so, where is it? 4. Why was it necessary to knead the dough? 5. What was the purpose of the warm water bath? 6. Most recipes for bread have you add a small amount of sugar or molasses to the mixture. Looking at the results, what purpose do you think this may serve? 7. If yeast utilized lactic acid fermentation, what would happen to the dough in contrast to what we observed in class? 8. Outline another method that you might have used to accomplish the same information about yeast respiration.
