BioEd Online

Teacher Background

All living things on Earth require energy to move, grow and maintain themselves. Some organisms, especially plants and algae, are able to build all of the materials they need from very simple substances. Using energy from light, these organisms, known as producers, are able to make food in the form of carbohydrates from water and carbon dioxide. All other organisms, called consumers, rely on producers for food. Food provides energy and other raw materials necessary for life.

When used by organisms, food is broken down and energy is released. Oxygen is consumed during this process, and carbon dioxide is given off as a waste product. Some energy in living things is used to maintain their bodies and conduct the reactions necessary for life. During these processes, some of the energy also escapes as heat.

This activity is designed to introduce students to the relationship between food and energy. Students will observe what happens when yeast, a single-celled fungus, is provided with food (table sugar).

Objectives and Standards

Life Science

• All organisms are composed of cells—the fundamental unit of life. Most organisms are single cells.

• Cells carry on the many functions needed to sustain life. They grow and divide, thereby producing more cells. This requires them to take in nutrients, which they use to provide energy.

• Reproduction is a characteristic of all living systems. Some organisms reproduce asexually.

• Decomposers, primarily bacteria and fungi, are consumers that use waste materials and dead organisms for food.

Physical Science

• Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, and the nature of a chemical.

Science, Health and Math Skills

• Observing

• Measuring

• Graphing

• Using a microscope

Materials and Setup

Materials per Group of Students

• 100 mL of water at room temperature

• 2 250-mL beakers or plastic cups

• 2 craft sticks or plastic spoons

• 2 pkgs of rapid rising yeast

• 2 tsp of sugar (or 2 single serve sugar packets)

• Plastic ruler, metric (mm marked)

• Sheet of graph paper

• Tape, paper and marker (for labeling)

• Copy of student sheet (see Lesson pdf)

Optional Materials (see Setup, Items 4 and 5)

• 12-in. laboratory thermometer or temperature probe

• Dropper or plastic pipette

• Glass or plastic slides and coverslips

• Microscope

Setup

1. Adjust the temperature of the water to room temperature.

2. Read “Using Cooperative Groups in the Classroom” (page 3), and prepare name tags for each group of four students.

3. Place all materials in a central location for each group of students to collect.

4. If you have access to 12-in. laboratory thermometers or electronic probes to measure temperature, have students also measure the starting temperature of the yeast mixtures and record the temperatures at 10 minute intervals. OR set up a demonstration with a temperature probe inserted in the yeast, sugar and water mixture. Students will be able to observe that the temperature of the water in which the yeast is growing will increase between 0.5–1.5°C during the class period. Have students construct a graph showing the change in temperature over time.

5. If microscopes are available, have students observe a drop of water containing yeast cells from the beakers containing sugar. Students should place a drop of solution on a slide and cover it with a cover slip.

Procedure and Extensions

1. Begin a class discussion of energy and living things by asking questions such as, What are the basic needs of living things? Do all organisms need exactly the same things to live? What do plants need? What about animals? Do animals need the same things as plants? Mention that plants are able to manufacture everything they need from very basic raw materials (carbon dioxide from air, water, nutrients from soil and energy from sunlight) through the process known as photosynthesis. Also mention that plants and other photosynthetic organisms are called producers, and that animals, fungi and others that rely on photosynthetic organisms for food are known as consumers.

2. Tell the Materials Manager to measure 50 mL of water into each of the 250-mL beakers. Have the Materials Manager from each group collect all of the supplies and take them to his or her own work area.

3. Tell students that they will be investigating the behavior of a common fungus (baker’s yeast) when it is fed. Ask students to share anything they know about yeast. Students will follow the instructions and record their observations on their student sheet.

4. Before beginning, each group should label one beaker as “Sugar” and the other beaker, “No Sugar.”

5. Have each group predict what will happen when yeast and water are combined. They also should predict what might happen when yeast, water and sugar are combined. Let the groups add yeast to the water in each of the beakers and stir the mixture gently. Groups should observe the appearance of the mixtures and record their observations.

6. Next, have students add approximately two teaspoons of sugar to the beaker labeled “Sugar.”

7. Have students observe the appearance of the yeast mixtures at 5-minute intervals and record their observations. They may gently stir the mixtures periodically with separate craft sticks or plastic spoons.

8. Once some of the yeast cultures have accumulated a thin layer of foam, ask students, What is happening to the yeast? Help students understand that the yeast cells have begun to grow and multiply in the presence of water and food (sugar). The gas being produced is carbon dioxide, the same waste product that we give off when food is processed inside cells in our bodies.

9. Next, students will observe the production of carbon dioxide gas by yeast. Have students measure and record the height of the foam in each beaker at 5-minute intervals. (The beaker labeled “No Sugar” may not produce any foam at all.) Students should record their observations on their student sheets.

10. Conclude by leading students in a discussion of yeast growth. Help them understand that they used different kinds of evidence to show that the yeast were using the sugar as food. First, the yeast were breaking down sugar to obtain energy. In the process, the yeast released observable carbon dioxide gas (visible as bubbles). Second, the yeast mixture became warmer. Heat was released as a by-product of the energy conversions happening inside the yeast cells. The yeast in the beaker without sugar did not have food to grow, so these reactions did not occur in this beaker.

Extensions

• Challenge students to come up with other ways to measure yeast growth and development.

• Have students use an acid/base indicator solution (such as bromothymol blue) to detect the presence of carbon dioxide in the air they exhale. Students should blow through a straw into a glass of water and use the indicator to observe whether the water becomes more acidic from the presence of dissolved carbon dioxide (forms a weak acid in water). OR have them make their own indicator by boiling purple cabbage to create a dark blue or purple liquid. This liquid turns pink in the presence of acids, and green or blue in the presence of bases.

• Challenge students to compare and contrast the use of sugar by yeast and the burning of a candle. What are the similarities between the two processes? What are the differences?

Handouts and Downloads

Download color images from The Science of Food and Fitness Teacher's Guide.