Hands-on Activity: How to Make Yeast Cells Thrive

Contributed by: Engineering K-PhD Program, Pratt School of Engineering, Duke University

A black and white image shows a cluster of nine circular objects.
Yeast cells seen through a microscope.
copyright
Copyright © genetics.med.harvard.edu/~winston/

Summary

Students set up and run the experiments they designed in the Population Growth in Yeasts associated lesson, using simple yeast-molasses cultures in test tubes. Population growth is indicated by the amount of respiration occurring in the cultures, which in turn is indicated by the growth of carbon dioxide bubbles trapped within the culture tubes. Using this method, students test for a variety of environmental influences, such as temperature, food supply and pH.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

This activity contains biomedical, chemical and environmental engineering and covers elements of biotechnology, applications of experimental and analytical techniques in living systems, food processing and processes of nature. Also included is a section on the interpretation of experimental data versus the interpretation of observations and the importance of using controls in any experiment.

Pre-Req Knowledge

Students should demonstrate ability to meet the learning objectives of the previous lessons and activity of this curricular unit.

Learning Objectives

  • Students will demonstrate an understanding that several environmental factors can contribute to the population growth of simple organisms such as yeasts; these can include food supply, temperature and pH.

More Curriculum Like This

Population Growth in Yeasts

Based on questions that arose during the first lesson and its associated activity, students in this lesson work in small groups to design experiments that determine how environmental factors affect yeast population growth.

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Cellular Respiration and Population Growth

Through two lessons and their associated activities, students explore cellular respiration and population growth in yeasts. Yeast cells are readily obtained and behave predictably, so they are very suitable for use in middle school classrooms.

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Students learn the fundamentals of using microbes to treat wastewater. They discover how wastewater is generated and its primary constituents. Microbial metabolism, enzymes and bioreactors are explored to fully understand the primary processes occurring within organisms.

Dirty Decomposers

Students design and conduct experiments to determine what environmental factors favor decomposition by soil microbes. They use chunks of carrots for the materials to be decomposed, and their experiments are carried out in plastic bags filled with soil.

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Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Summarize, represent, and interpret data on a single count or measurement variable (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Brainstorming is a group problem-solving design process in which each person in the group presents his or her ideas in an open forum. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Design and use instruments to gather data. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Artificial ecosystems are human-made complexes that replicate some aspects of the natural environment. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Document processes and procedures and communicate them to different audiences using appropriate oral and written techniques. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Collect information and evaluate its quality. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Design forecasting techniques to evaluate the results of altering natural systems. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Display numerical data in plots on a number line, including dot plots, histograms, and box plots. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Summarize, represent, and interpret data on a single count or measurement variable (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Understand the processes, structures and functions of living organisms that enable them to survive, reproduce and carry out the basic functions of life. (Grade 7) Details... View more aligned curriculum... Do you agree with this alignment?
  • Understand the composition of various substances as it relates to their ability to serve as a source of energy and building materials for growth and repair of organisms. (Grade 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Analyze photosynthesis and cellular respiration in terms of how energy is stored, released, and transferred within and between these systems. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Analyze the relationships between biochemical processes and energy use in the cell. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Materials List

This lesson requires the same materials as were needed for the previous activity, Yeast Cells Respire, Too (But Not Like Me and You):

  • large test tubes, about 15 cm long and 20 mm in diameter; one per student
  • small test tubes, about 10 cm long and 8 mm in diameter; one per student
  • squares cut from plastic wrap, about 8 cm on a side; one per student
  • several rubber or cork stoppers, size 2
  • test tube racks to hold large test tubes
  • several dropping pipettes
  • five 300-ml beakers
  • 1-liter flask
  • 1-liter graduated cylinder
  • lab thermometer
  • package dry baking yeast (available in grocery stores)
  • 12-ounce bottle molasses (unsulphured)
  • Depending on what experiments students elect to do, you may need several extra flasks (100-300 ml), graduated cylinders (10 and 100 ml), and test tube racks. You may also need a refrigerator and an incubator or other warm location, such as a sunny window sill.
  • If students decide to test the effects of pH, you will need litmus paper, lemon juice or vinegar as a safe means of lowering pH, and sodium carbonate ("washing soda") as a safe means of raising pH. If using any of these chemicals, students will also need eye protection.
  • You will probably also need a few more large and small test tubes for controls, and in order to have adequate sample sizes for the student experiments.

Introduction/Motivation

Read and comment on the written proposals submitted by each group at the end of the preceding lesson. Combine any groups that proposed similar experiments. Any combined groups need to agree on the details of the procedures they will follow to set up their experiment, and all groups need to organize themselves so everyone knows his or her role. When you are satisfied that the class is ready, have the groups set up and start their experiments.

Procedure

Students will:

  • set up their experiments
  • make daily observations of the respiration chambers and measurements of the gas bubbles collected in the inverted test tubes
  • after 3-4 days, determine mean daily bubble sizes for each test condition and graph the results
  • present their results and conclusions to the rest of the class

Body of Activity:

Day 1

Working in their teams, have students set up their test chambers according to the plans they made earlier. It is best to set up the experiment early in the week, so there will be several consecutive days for students to make observations and measurements.

Days 2, 3 and 4

Allow time each day for students to observe their test chambers and record the heights of the gas bubbles.

On the last day, have student groups pool their data (within their groups) in order to calculate mean bubble sizes each day for each test condition. Then have them prepare graphs of these data, in the same manner as was done in Lesson 1. Establish a set of scales for the y-axes (bubble height) of the graphs that everyone in the class will use, and provide identical graph paper for everyone. This way the effects of the different environmental conditions can be easily compared between groups.

Have a spokesperson from each group give a very brief presentation to the rest of the class. The presentation should state what environmental factor was tested, and what the results were.

Then ask students what they can conclude about the most and least favorable conditions for yeast population growth. Make sure the data really do support their conclusions; it is not uncommon for students to conclude that what they expected to happen did happen, even if the data suggest otherwise. Also, be sure to ask students how what happened in their control chambers supports, or doesn't support, their hypotheses.

If students tested the effects of different molasses concentrations, they probably expected that the greater food supply available at higher concentrations would favor population growth. Instead, they most likely found that high concentrations were not favorable for yeast growth. Ask them for ideas on why that might have happened. The answer is probably due to the very high concentration gradient between the internal environment of the cells and the molasses solution. This would cause too much water to leave the yeast cells, and then cell death. The concepts of diffusion and osmosis are rather complicated for seventh graders, but you can give a simple explanation such as, "When there is too much sugar surrounding the cells, water leaks out of them and they shrivel up and die." The idea is the same as people putting salt on slugs that are garden pests. It is also why jams and jellies, which are made from fruits but contain large amounts of sugar, rarely spoil in a refrigerator, but plain fruits eventually will.

Safety Issues

  • As students set up their respiration chambers, remind them not to push too hard on the rubber stopper. If the test tube breaks, it could cut into a student's fingers or hand. Be sure to have rubber gloves and clean paper towels on hand in case of accidents. Follow whatever first aid procedures are in place at your school.
  • Do not allow students to taste the yeast-molasses solution at experiment end, since it could be contaminated with other microbes.

Investigating Questions

As students make daily observations and measurements, ask open-ended questions such as:

  • What has happened in your test tubes so far, that is, what have you observed, as opposed to what is your interpretation of your observations?
  • So far, does it look like your hypothesis is being supported? Why or why not?

Assessment

Present the following situation to students, and ask them to write down their responses:

Since she knew that humans need to have some salt in their diets, a student wanted to find out if adding salt to the molasses solution would result in faster population growth in yeast cells. She set up her experiment the same way she had done in class, using 10% molasses solution. She added ordinary table salt to the test chambers as follows:

  • one-half gram of salt was added to each of three test tubes;
  • 1 gram of salt was added to each of three test tubes;
  • 2 grams of salt were added to each of three test tubes;
  • 4 grams of salt were added to each of three test tubes; and
  • no salt was added to each of three test tubes.

Starting the next day, the student recorded her data in the table below:

Data table

Based on these observations, what do you think the student concluded about the effects of salt on yeast cell populations?

Activity Extensions

Some yeasts and other types of fungi are pathogenic to humans. Ask students what environmental conditions they think would favor such pathogens. They should be able to realize that such yeasts and other fungi would be adapted to thrive at human body temperature (about 37º C). If a student has ever had athlete's foot, he or she might recognize that a warm, moist environment favors growth of the fungus. Have students do some library or internet research to find out what some common fungal infections are, how they are treated, and how they can sometimes be prevented.

Contributors

Mary R. Hebrank, project and lesson/activity consultant

Copyright

© 2013 by Regents of the University of Colorado; original © 2004 Duke University

Supporting Program

Engineering K-PhD Program, Pratt School of Engineering, Duke University

Acknowledgements

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.

Last modified: August 23, 2017

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