Hands-on Activity: Yeast Cells Respire, Too (But Not Like Me and You)
Educational Standards :
Pre-Req Knowledge (Return to Contents)
Students should be aware that yeasts respire anaerobically by breaking down molecules of glucose to provide energy for cellular activities, and producing carbon dioxide and ethyl alcohol as by-products.
Learning Objectives (Return to Contents)
After this activity, students shouuld be able to:
Materials List (Return to Contents)
Introduction/Motivation (Return to Contents)
Can you see respiration occur in yeasts? (Listen to student answers. Some students may suggest that with a good microscope someone might be able to see it. As necessary, remind students that yeast cells are microscopic and that cellular respiration involves individual molecules of glucose, carbon dioxide and ethyl alcohol.) The yeast cells convert glucose, the food source, to carbon dioxide and ethyl alcohol through a process called cellular respiration. Energy is also produced during this process, which the yeast cells use to grow and reproduce. Can the individual molecules be seen with a microscope? (Listen to student answers.) No, molecules are too small to be seen with a microscope.
If molecules are too small to be seen, how could we tell if a beaker full of well-fed yeasts living in some nice, warm, water were respiring or not? (Listen to student answers. Give them some time to reason out an answer.) Carbon dioxide is produced by respiring yeasts, and since it is a gas, it should bubble up to the surface of the water. Energy is also produced during cellular respiration, but it is more difficult to measure than carbon dioxide.
So our experimental challenge is to try to determine if yeast cells are respiring or not. (If students seem disinterested, continue on.) What else do respiring yeast cells produce? In the process of observing yeast respiration, you may be interested to know that you will be making a "forbidden" substance—ethyl alcohol. If the experiment works, that is, if you are able to provide the conditions necessary for yeast respiration, the classroom may begin to smell somewhat "beery" after a few days. I hope the principal doesn't decide to drop in! (Laugh; at this point, expect your seventh-graders to be more interested to participate!)
Procedure (Return to Contents)
Before the Activity
With the Students - Day 1
With the Students - Days 2, 3 and 4 (and 5, if necessary)
Have students examine their test tubes to see if anything happened. Depending largely on temperature (but don't tell students that!) only small bubbles may be present in the smaller tube or it may be nearly filled. It is okay if the smaller tubes float up as they fill with carbon dioxide and dislodge the plastic wrap. Be sure to avoid calling the bubbles produced "air" bubbles, since they are not filled with air. Ask students what the difference is between the composition of air and the bubbles produced by their yeast cells. If they do not know the composition of air, make it an assignment to find out.
Each day, have students measure the heights of the gas bubbles (to the nearest millimeter) and record this data. Continue daily until nearly all of the small tubes are completely filled with carbon dioxide. Then have students graph their results in a bar graph or x-y scatter plot, with time on the x-axis (dependent variable) and height of gas bubble on the y-axis (independent variable). Give students a few minutes to compare their graphs with those of their classmates.
With the Students - Concluding Discussion
Ask students to describe what happened physically in their test tubes. Expect students to explain that carbon dioxide was produced, which is one byproduct of cell respiration. Then ask them to describe how what physically happened gives evidence for what was happening with the yeast cells in their tubes.
Some students may have noticed the tiny bubbles floating upward from the bottom of the tubes, but seen that not all of these were trapped inside the smaller, inverted test tube. Some of the tiny bubbles escaped outside the smaller tube and rose to the surface of the molasses solution, where they were released into the air at the top of the chamber. So is the size of the gas bubble inside the smaller test tube an accurate representation of the amount of yeast respiration that occurred? It isn't, but it is safe to assume that the ratio of gas trapped to the gas that escaped is consistent across all the test chambers. This is because the yeast cells, which are denser than the molasses solution, sank to the bottom of the large test tubes when they were first added. The curved bottom of the tube caused them to occupy the center of the tube, where their population was visible, and where most of the carbon dioxide they produced would rise into the smaller inverted tube.
Students might also notice (from their graphs or the bubbles themselves) that during the first 24-48 hours (again, depending on ambient temperature), most of the bubbles grew relatively slowly in height, but then suddenly grew much bigger on subsequent days. Give students some time to think about why that might be. If they need help, ask them how yeasts reproduce. If they do not make the connection, explain that one cell buds a new one to make two cells, those two become four, those four become eight, etc. A diagram sketched on the board helps. Also sketch a graph of a theoretical population size, with time on the x-axis and population sizes of 2, 4, 8, 16 and 32 on the y-axis. Ask if this graph is similar in shape to the graphs of the gas bubble heights. Make sure students make the connection that not only were the yeasts respiring in their test chambers, but because they had an adequate food supply (glucose from the molasses), they were also reproducing.
Ask students how they know that the bubbles produced were made of carbon dioxide produced by the yeasts. Without a chemical analysis of the gas, they do not know. Also ask them how they know that if they created an identical test chamber of molasses solution, but without adding the yeast cells, they wouldn't get the same result - a gas bubble that grew over time. Until they've done the experiment they don't know.
These unknowns are why researchers use controls when they do experiments; controls enable them to rule out other explanations of their results. In this case, the control would be an identical test chamber set up without the addition of yeast cells, and placed alongside the experimental chambers in the rack for the duration of the experiment. If a gas bubble appeared and grew over the next few days, it could not be concluded that the gas bubbles in the test chambers were due to yeast respiration. Instead, it would be more likely that a chemical reaction within the molasses solution produced the bubbles. But if no gas bubble appeared in the control chamber, the most likely explanation for the growing bubble in the test chambers would be the production of carbon dioxide by respiring yeast cells.
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
If materials are prepared correctly and procedures are followed as instructed, expect all of the test tubes to show clear signs of yeast respiration. If gas bubbles are not produced, any of the following may have occurred:
Investigating Questions (Return to Contents)
Ask students the following questions either while they make daily observations and measurements and/or during the concluding discussion.
Assessment (Return to Contents)
Concluding Discussion: Once the activity is done, lead a class discussion to share and compare results and conclusions. See details in the Procedure and Investigating Questions sections. Listen to student answers to gauge their level of comprehension.
Writing Problem: At activity end, present to students the following scenario and ask them to write and turn in their responses for grading. Review their answers to assess their depth of comprehension.
Several loaves of bread that a baker made this morning did not rise as they should have. The result was flat, dense loaves that he cannot sell. He suspects a problem with the yeast, since he knows that it is the yeast that makes the bread dough rise. All of the yeast he used came from his one large supply. How can you help the baker determine if his yeasts are functioning properly?
Activity Extensions (Return to Contents)
Make Yogurt: Like yeasts, some bacteria respire anaerobically. Yogurt is produced when bacteria break down lactose, the sugar contained in milk, to obtain energy for their cellular activities. Lactic acid is produced as a by-product, and this is what gives plain yogurt its tart taste. Plain yogurt is easy to make and does not require a lot of class time, although it does require a warm (about 40 ºC or 100 ºF) location for several hours. Have students conduct an Internet search to find a recipe that the class could make. Afterwards, students might enjoy bringing in flavorings and toppings for when they taste the finished product.
Assign students to research the difference between sourdough and conventional bread.
ContributorsMary R. Hebrank, project and lesson/activity consultant
Copyright© 2013 by Regents of the University of Colorado; original © 2004 Duke University
Supporting Program (Return to Contents)Engineering K-PhD Program, Pratt School of Engineering, Duke University
Acknowledgements (Return to Contents)
This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.