Cellular Respiration Lab - Wild about Bio - Home



[pic] 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 requires 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 CO2.as well as ethyl alcohol, with the NADH being oxidized in the process. Overall, the final equation for glycolysis plus 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), 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. 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. Do not shake or swirl at this point.

5. 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?

6. Observe what happens.

Observations:

| Bottle with Sugar, Water and Yeast |

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|Observations of what is happening in the bottle: |

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|Observations of what is happening in the balloon: |

Explain in complete sentences:

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?

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