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Decomposers | Lesson 4 - Explaining How Decomposers Move and Function

Lesson 4: Explaining How Decomposers Grow, Move, and Function

Students learn and use a scientific model to explain cellular respiration in decomposers using the Three Questions. They relate the rearrangement of atoms in cellular respiration to energy release and learn that most of the mass of decaying materials is lost to the air as a result of cellular respiration in decomposers.

Guiding Question

How do decomposers use food to move and function?

Activities in this Lesson

  • Activity 4.1: Molecular Models for Decomposers Moving And Functioning: Cellular Respiration (40 min)
    • Note: The steps that have students construct molecular models in this activity are optional if students did molecular modeling for cellular respiration in another unit and performed well on the pretest for items related to cellular respiration.
  • Activity 4.2: Explaining How Decomposers Move and Function: Cellular Respiration (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 function/movement of an organism, including air and food entering the decomposer, and waste, air enriched in carbon dioxide and water vapor leaving the decomposer (focus on growth).
  3. Explain energy transformations during function/movement of an organism: Chemical energy stored in organic molecules is transformed into motion and heat (focus on growth).
  4. Draw and explain movements of materials during movement of an organism, including air and food entering the decomposer, and waste, air enriched in carbon dioxide and water vapor leaving the decomposer.
  5. Identify forms of energy involved in movement of decomposers: chemical energy, movement, and heat energy.
  6. Explain energy transformations during the movement of an organism: chemical energy stored in organic molecules is transformed into motion and heat.
  7. Explain the chemical changes that occur during cellular respiration, representing the changes with molecular models and chemical equations.

NGSS Performance Expectations

Middle School

  • MS. Structure and Properties of Matter. MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
  • MS. 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.
  • MS. 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.
  • MS. 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

  • HS. 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.
  • HS. Chemical Reactions. HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • HS. 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.
  • HS. 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

This lesson helps students develop atomic-molecular scale explanations for what happens when things decay. In this Lesson students also return to the data they collected from the bread mold investigation as a starting point for answering explaining what happens when fungi grow, move, and function at an atomic-molecular scale.

Students will learn that carbohydrates, proteins, and fats are materials that decomposers ingest to give them mass to grow. If students completed the Animals Unit, they know these materials are carbon-based and that they have chemical energy. Students likely cannot trace these substances beyond the decomposer’s body. When these substances (which are polymers) are ingested, they are broken down into monomers in the digestive system. Once they are fully digested into monomers they can be transported across membranes into the bloodstream and then carried to all the cells in the body. At the cell they are transported into the cell body, and rebuilt through various biosynthetic processes back into polymers. The monomers that are the product of digestion can follow different pathways in the body and go through many different processes, but your students need to know a general storyline about what happens in growth: polymers are broken down into monomers through digestion, then rebuilt into polymers that become part of the decomposer’s biomass.

We will consistently focus on the idea that understanding carbon-transforming processes involves answering the Three Questions:

  • The Matter Movement Question: Where are molecules moving? (How do molecules move to the location of the chemical change? How do molecules move away from the location of the chemical change?)
  • The Matter Change Question: How are atoms in molecules being rearranged into different molecules? (What molecules are carbon atoms in before and after the chemical change? What other molecules are involved?)
  • The Energy Change Question: What is happening to energy? (What forms of energy are involved? What energy transformations take place during the chemical change?)

Matter (the Matter Movement and Matter Change Questions). We find that even students who have learned how to balance chemical equations do not appreciate the meaning of the procedure:

  • Conservation of atoms (the Matter Change Question): The numbers of atoms on the left and right side of a chemical equation have to be the same because they are THE SAME ATOMS! A chemical equation just shows how they are being rearranged into new molecules.
  • Conservation of mass (the Matter Movement Question): ALL the mass of any material is in its atoms (and none of the mass is in the bonds, which are just attractive forces between atoms). So the mass of the products is always the same as the mass of the reactants.

Energy (the Energy Change Question). Chemists, physicists, and biologists have many different conventions for describing and measuring chemical energy. We have a deeper explanation of the conventions used in Carbon TIME units and how they relate to conventions used in different scientific fields on the BSCS website in a document called Carbon TIME Content Simplifications. Here are some key points:

  • All bond energies are negative relative to individual atoms. So during a chemical reaction, it always takes energy (the activation energy) to break bonds. Then, energy is released when new bonds are formed.
  • Whether a chemical reaction releases energy or not depends on the total energy of the reactants, compared with the total energy of the products. So energy is released when the total bond energy of the products is lower (i.e., more negative relative to individual atoms) than the energy of the reactants.
  • In systems like our atmosphere, where excess oxygen is always present, the most abundant sources of chemical energy are substances that release energy when they are oxidized (e.g., substances with C-C and C-H bonds).

The two activities in this lesson represent the Explanations Phase of the Decomposers Unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of cellular respiration that was one of the drivers of the macroscopic changes they observed in their Bread Molding investigation in the previous lesson.

Activity 4.1 is the first part of the Explanations Phase of the instructional model (going down the triangle) for cellular respiration. Students construct molecular models of the chemical change that took place during the investigation to help them develop an atomic-molecular explanation for how decomposers use food to move, breathe, and function. Activity 4.1 also simplifies the full story of what happens to matter during the multi-step process of cellular respiration. For a more detailed account, see

The activity uses a standard but simplified formula for the overall chemical change:

C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O

This incorrectly suggests that some of the oxygen atoms O2 in end up in CO2, which is not actually the case. A more accurate formula to represent the multi-step process would be as follows:

     C6H12O6 + 6O2 + 6 H2O --> 6 CO2 + 12 H2 O

Thus all of the oxygen atoms in O2 (bolded in the equation above) end up in H2O, while the oxygen atoms in CO2 all come from glucose or water.

In practice biochemists often do not try to trace individual H and O atoms through biochemical processes, since the processes always take place in environments where water provides H and O atoms.

In Carbon TIME units we explain that the chemical energy released during cellular respiration is used for cell functions and ultimately converted to heat. In more advanced classes, you may choose to include another intermediate step in this story: the energy released by oxidation of glucose is used to convert ADP (adenosine diphosphate) and phosphate into ATP (adenosine triphosphate), which is the immediate source of energy for cell functioning. Some of your students may believe that ATP is a form of energy and not a form of matter or that the matter in glucose is converted to ATP, so pay particular attention to how students describe ATP when learning about cellular respiration. ATP is matter with chemical energy stored in its bonds.

Activity 4.2 is the second part of the Explanations Phase of the instructional model (going down the triangle) for cellular respiration. Students use the Explanations Tool to construct final explanations of what happens when decomposers oxidize small organic molecules to release energy to move and function, and then release water and carbon dioxide. 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.

Key Carbon-Transforming Processes: Cellular Respiration

Lesson Map:

See the Decomposers Unit Read Me document for more details.

Lesson 4 Map

Unit Map

Decomposers Lesson 3 Unit Map

Talk and Writing

At this stage in the unit, the students will be developing Explanations. The table below shows specific talk and writing goals for this 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