By Heather Ferguson
Product Developer

Use unicellular organisms to demonstrate essential cellular processes such as molecular transport, reproduction, protein synthesis, metabolism, and cell signaling. A readily available, easily cultured example is Saccharomyces cerevisiae , baker’s yeast. Yeasts are fungi, heterotrophic organisms with cell walls of chitin. While S. cerevisiae can easily be viewed under a microscope, adding a stain such as Congo red or methylene blue helps highlight cellular processes.

In this activity, students run a series of experiments with test tubes containing either boiled or unboiled yeast to determine which tube contains living cells. In the process, students observe molecular transport, reproduction, and metabolism. This high school activity takes approximately 45 min and is easily adjusted for various class sizes.

National Science Education Standards

Grades 9–12

Unifying Concepts and Processes

• Systems, Order, and Organization
• Form and Function

Science as Inquiry

• Abilities Necessary to Do Scientific Inquiry
• Understandings About Scientific Inquiry

Life Science

• The Cell
• Matter, Energy, and Organization in Living Systems
• Behavior of Organisms

Safety

Ensure that your students understand and adhere to safe laboratory practices when performing these activities, including use of appropriate PPE (goggles, gloves, and aprons). Even though the yeast used poses no known human health hazard, have students wash their hands thoroughly after handling these or any cultures.

Materials

• Baker’s Yeast
• Microscopes, Slides, and Coverslips
• Laboratory Markers
• Test Tubes and Racks
• Graduated Cylinder
• Pipets
• Beaker
• Hot Plate
• Congo Red or Methylene Blue Stain
• Small Balloons
• Water
• Sugar Packets (like those at restaurants)
• Small Rubber Bands
• Paper Towels (for cleanup)

Teacher preparation

1. Divide students into groups of 2 or 3.
2. Locate the materials needed to set up lab stations for each group of students.
3. Prepare 1 boiled and 1 unboiled yeast solution for each lab station.
1. Add 15 mL of water to each test tube.
2. Add 20 to 30 granules of baker’s yeast to each tube. (Soaking activates the dried yeast.)
3. Prop half of the test tubes in a beaker containing water. Place the beaker on a hot plate and heat it until the yeast solution boils. Allow the yeast to boil for 5 min.
   Caution: Keep hands, hair, and clothing away from the heat source.
4. Turn off the hot plate. Allow the heated test tubes to cool for a few minutes before handling them.
5. Label the boiled tubes “1” and the unboiled tubes “2.”
4. Distribute the following items to each lab station:
   • Compound microscope
   • 2 microscope slides
   • 2 coverslips
   • 1 test tube of boiled yeast and 1 test tube of unboiled yeast
   • Test tube rack
   • 2 plastic pipets
   • Congo red or methylene blue
   • Small packet of table sugar (a restaurant packet)
   • 2 small balloons
   • 2 small rubber bands
5. Before you begin the activity, review cellular processes with your students. Make sure that students are familiar with active and passive transport of molecules. To help orient students in their microscopic observations, you might want to show them a picture of budding yeast.

Student procedure

1. Prepare a slide of the yeast from tube 1. With a clean plastic pipet, mix the yeast and place a drop of solution onto a microscope slide.
2. Add a drop of Congo red stain on top of the yeast and cover with a coverslip.
3. Make an initial observation of the yeast cells under a microscope, using the 40× objective lens.
4. Using a different, clean pipet, follow the same process to prepare and view a slide of the yeast culture from tube 2.
5. Label the slides (“1” and “2”) and set them aside for 5 min.
6. After 5 min, observe how the yeast from tube 1 reacts to the stain. Be sure to examine the slide for at least 1 min. Look for reproducing, or budding, yeasts. A budding yeast looks like 2 yeast cells that are joined but share a cell membrane.
7. Observe how the yeast from tube 2 reacts to the stain. Be sure to examine the slide for at least 1 min. Again, look for budding yeasts.
8. Add half a packet of sugar to each test tube. Place your thumb over the top of the tube and invert the tube 2 or 3 times to mix.
9. Fasten a small balloon over the top of each test tube and secure it with a rubber band.
10. Allow the test tubes to sit overnight.
11. Check them the following day. Record your observations of each tube.
12. After completing your observations, make a table and record whether you observed the yeast in each tube perform any of these cellular processes: reproduction, metabolism, molecular transport.
13. On the basis of your observations, decide which test tube contains live yeast.

Discussion of results

Discuss the differences between the 2 yeast cultures with your students after they have completed the activity. Boiling denatured the proteins in the yeast, preventing many essential cellular processes from being carried out. Ask students to share and describe what they observed and to propose explanations for their observations of molecular transport and metabolism. Active yeasts carry out active transport to remove stain from inside their cells, so they will become colorless during the period of observation. Denatured yeast cells absorb the stain. The test tube containing active yeasts produces carbon dioxide as the cells use the sugar as an energy source in respiration. Students probably observed some reproduction in the denatured yeast because the budding process had already begun before the yeasts were killed, at which point budding stopped.

An activity similar to this one can be found in the Active Transport in Yeast Cells Kit (item #201108). An alternative to using yeast cells is to use protozoa cultures to study cellular processes. Amoeba, Stentor , and Paramecium are large protozoa that allow for observations of homeostasis, endocytosis, and reproduction. The Homeostasis and the Amoeba: An Exploration of Osmoregulation kit (coming soon) uses easy-to-view Vitachrome® amoebas to explore the mechanism by which amoebas regulate the amount of water inside the cell.