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Yeast: Heterotrophs of Autotrophs?

Life Skills:

  • Thinking & Reasoning
  • Systems Thinking

Curriculum Tie:

Time Frame:
2 class periods that run 30 minutes each.


Have students design experiments as a class to determine if yeast are autotrophic or heterotrophic. Students will utilize a set of materials to address the above question by growing yeast cultures under conditions containing food, light, both and neither.

Main Curriculum Tie:
Science - 6th Grade
Standard 5 Objective 2

Demonstrate the skills needed to plan and conduct an experiment to determine a microorganism's requirements in a specific environment.


  • 4 0.5 gallon or gallon bottles. (Preferably at least two clear bottles)
  • 2 dark shirts or pillow cases.
  • 4 universal carboy bungs and airlocks (balloons could substitute
  • 3 lbs of honey4 packages of bakers yeast

Background For Teachers:
Yeast are heterotrohpic organisms (they need food for energy). When placed in an aqueous environment (water) with simple sugars (honey) they ferment (eat) the sugar into ethanol and C02 (this is why bread rises). The CO2 is released in a gaseous state and can be seen bubbling in the bottle as well as when it passes through and airlock or fills a balloon. In order to successfully grow yeast they need to be in room temperature water and have food. Under these conditions a robust fermentation will usually occur within 24 hours. If these conditions aren't met the yeast will not feed and no bubbling will occur.

Student Prior Knowledge:
Concept of what separates living things from nonliving things. Undergone general introduction to microorganisms and the scientific method.

Intended Learning Outcomes:
1. Use Science Process and Thinking Skills

  • a. Observe simple objects, patterns, and events, and report their observations.
  • . f. Plan and conduct simple experiments.
  • g. Formulate simple research questions.
  • i. Use data to construct a reasonable conclusion.
    3. Understand Science Concepts and Principles
  • c. Solve problems appropriate to grade level by applying science principles and procedures.
    4. Communicate Effectively Using Science Language and Reasoning
  • b. Describe or explain observations carefully and report with pictures, sentences, and models.

    Instructional Procedures:

    1. Pose question to the class “are yeast autotrophic or heterotrophic” and have students write down their hypothesis.
    2. Introduce students to bottles (yeast houses) and add water until 2/3s full in each.
    3. Show them the honey and explain how this is used as food for organisms.
    4. Show students shirts/pillow cases and explain or demonstrate how they can block light (prevent photosynthesis).
    5. Explain how airlocks/balloons work and how we can visualize gas release.
    6. Have students design 4 different yeast habitats to answer proposed question. (Hopefully they come up with food only, light only, both and neither.) Note: amount of honey is relative to amount of liquid 1-2 pounds per gallon for an exciting churning fermentation.
    7. Add yeast and place bottles near window (light source) and maintain at room temperature.
    8. Robust fermentation should begin within 24-48 hours. During which times students can visualize bubbles coming out or airlocks and currents within and record their observations.
    9. Have students share their observations and state their conclusions concerning which habitats were bubbling and whether yeast are autotrophic or heterotrophic.


    Hydrometers can be used to measure the density of honey solutions before and after fermentation. The resulting change in density can be used to calculate the amount of CO2 released and the ethanol percentage of the fermented solution.

    If honey is added at a rate of more than 3lbs per gallon the yeast are incapable of completely fermenting the solution because they can not continue to grow in ethanol concentrations above 15%. This could be used to teach students about the impacts and definition of pollution.

    Holly Godsey
    James Ruff
    Lesson Plans

    Created Date :
    Jan 29 2010 12:27 PM

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