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Decomposers | Lesson 5 - Explaining How Decomposers Grow

Lesson 5: Explaining How Decomposers Grow

Students learn and use a scientific model to explain digestion and biosynthesis using the Three Questions.

Guiding Question

How do decomposers use food to grow?

Activities in this Lesson

  • Activity 5.1: Tracing the Processes of Decomposers Growing: Digestion and Biosynthesis (40 min)
  •  Activity 5.2: Explaining How Decomposers Grow: Digestion and Biosynthesis (40 min)
  • Activity 5.3: Explaining How Fungi Grow: Digestion and Biosynthesis (40 min)


  1. Describe systems and processes in a hierarchy of scales, including atomic-molecular, macroscopic, and large scale (connecting atomic molecular and macroscopic scales).
  2. Draw and explain movements of materials during growth of a decomposer, including air and food entering the decomposer, and waste, air enriched in carbon dioxide and water vapor leaving the decomposer.
  3. Explain the chemical changes that occur when an decomposer digests food and creates new biomass.
  4. Explain energy transformations during growth: chemical energy stored in food is preserved as polymers are broken down to monomers then reassembled as decomposer biomass.

NGSS Performance Expectations

Middle School

  • Structure and Properties of Matter. MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
  • Chemical Reactions. MS-PS1-5. Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
  • Matter and Energy in Organisms and Ecosystems. MS-LS1-7. Develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism.
  • Matter and Energy in Organisms and Ecosystems. MS-LS2-3. Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.

High School

  • Chemical Reactions. HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends on the changes in total bond energy.
  • Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • Matter and Energy in Organisms and Ecosystems. HS-LS1-6. Construct and revise an explanation based on evidence for how carbon, hydrogen, and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.
  • Matter and Energy in Organisms and Ecosystems. HS-LS1-7. Use a model to illustrate that cellular respiration is a chemical process whereby the bonds of food molecules and oxygen molecules are broken and the bonds in new compounds are formed resulting in a net transfer of energy.

Background Information

The three activities in this lesson complete the Explanations Phase of the Decomposers unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of the digestion, and biosynthesis that were the drivers of the macroscopic changes they observed in their Bread Molding investigation in Lesson 3.

Activity 5.1 is the first part of the Explanations Phase of the instructional model (going down the triangle) for digestion and biosynthesis. Students trace the chemical changes of digestion and biosynthesis in a decomposer on a poster of a mushroom.

Activity 5.2 is a 2-turtle activity appropriate for advanced middle school or high school students and classes. If you decide not to teach 5.2, you can move directly from 5.1 to 5.3. In 5.2, students model the chemical changes of digestion and biosynthesis using paper molecules. This activity introduces and uses the vocabulary of polymer and monomer, as well as the names of specific monomers.

Digestion and biosynthesis are chemically the most complicated processes in the Carbon TIME units. Although we believe that with enough effort many middle school students could understand the chemical details included in Activity 5.2, we do not recommend it for middle school. We feel that middle school students need to understand four key points:

  1. Fungi use food for one of two purposes:
    1. Growth (digestion and biosynthesis), OR
    2. As a source of energy (digestion and cellular respiration)
  2. The food that fungi digest and absorb and the bodies of fungi are both made of large organic molecules.
  3. Digestion breaks large organic molecules into smaller organic molecules.
    1. Digestion occurs OUTSIDE the cells of fungi, when they secrete digestive enzymes into organic materials that are their food source.
    2. Digestion is different from cellular respiration, which combines small organic molecules with oxygen to release energy.
  4. Biosynthesis makes small organic molecules into large organic molecules.
    1. Biosynthesis occurs in every cell of a fungus, after the cell gets small organic molecules from its food source or other cells.
    2. Biosynthesis is how cells grow bigger so that they can divide and the fungus can grow.

Activity 5.3 is the second part of the Explanations Phase of the instructional model (going down the triangle) for digestion and biosynthesis. Students use the Explanations Tool to construct final explanations of what happens when decomposers break large organic molecules from their food into small organic molecules, and then use the small organic molecules to gain mass. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale. This activity is appropriate for students who did only 5.1 and students who did both 5.1 and 5.2, but the vocabulary used to describe the molecules will be different depending on what activities were taught. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale.

This basic story omits many complications. Although we hope that students will come to appreciate the vast number and complexity of biomolecules, our emphasis in Carbon TIME is on helping students understand that fungi can build their bodies out of decaying materials through a few basic processes in digestion and biosynthesis.

Some key complexities that you as a teacher may wish to be aware of include the following;

  • Fungi often modify monomers before using them for biosynthesis. For example, they have limited abilities to modify fatty acids (e.g., changing unsaturated to saturated fatty acids) and to modify amino acids.

The large organic molecules (sometimes called macromolecules) in plants, animals, and decomposers are actually much larger than the molecules shown in the presentation and during the construction of molecular models:

  • Fatty acids typically contain 10 to 25 carbon atoms.
  • Proteins can consist of hundreds of amino acids. Humans are typical of many organisms in that our proteins contain 21 different kinds of amino acids.
  • Starch and cellulose molecules can be made of hundreds or thousands of monomers. Different kinds of starch and fiber can also include other 5-carbon and 6-carbon sugars besides glucose.

Key Carbon-Transforming Processes: Digestion and Biosynthesis

Unit Map

Decomposers Lesson 3 Unit Map

Talk and Writing

This lesson of the unit represents the fading portion of the Explanations Phase. This means that students are expected to develop explanations for carbon-transforming processes they studied in this unit in new and novel contexts. The table below shows specific talk and writing goals for the Explanations phase of the unit.

Talk and Writing Goals for the Explanations Phase

Teacher Talk Strategies That Support This Goal

Curriculum Components That Support This Goal

Examine student ideas and correct them when there are problems. It’s ok to give the answers away during this phase! Help students practice using precise language to describe matter and energy.

Let’s think about what you just said: air molecules. What are air molecules?

Are you talking about matter or energy?

Remember: atoms can’t be created. So that matter must have come from somewhere. Where did it come from?

Let’s look at the molecule poster again… is carbon an atom or a molecule?

Molecule Poster

Three Questions Poster


Focus on making sure that explanations include multiple scales.

The investigation gave us evidence for what was happening to matter and energy at a macroscopic sale. But what is happening at an atomic-molecular scale?

What is happening to molecules and atoms?

How does energy interact with atoms and molecules during chemical change?

Why doesn’t the macroscopic investigation tell us the whole story?

Let’s revisit our scale poster… what is happening to matter at the molecular scale?

Molecular Models

Molecular Modeling Worksheets

Explanations Tool

PPT Animation of chemical change

Powers of Ten Poster

Encourage students to recall the investigation.

When did this chemical change happen during our investigation?

How do we know that? What is our evidence?

What were the macroscopic indicators that this chemical change took place?

Evidence-Based Arguments Tool

Investigation Video

Elicit a range of student explanations. Press for details. Encourage students to examine, compare, and contrast their explanations with others’.

Who can add to that explanation?

What do you mean by _____? Say more.

So I think you said _____. Is that right?

Who has a different explanation?

How are those explanations similar/different?

Who can rephrase ________’s explanation?

Explanations Tool