Decomposers | Lesson 4 - Explaining How Decomposers 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)

Unit Map

Decomposers Lesson 4 Unit Map

Target Performances


Target Performance

Lesson 4 –Explaining How Decomposers Move and Function

Activity 4.1: Molecular Models for Fungi Moving and Functioning: Cellular Respiration

Students use molecular models to explain how carbon, oxygen, and hydrogen atoms are rearranged into new molecules in fungus cells.

Activity 4.2: Explaining How Fungi Move and Function: Cellular Respiration

Students explain how matter moves and changes and how energy changes during cellular respiration in fungus cells.

NGSS Performance Expectations

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-LS1-7. 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.

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.

Three-dimensional Learning Progression

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

This lesson focuses on the use of food—as a source of energy—and on the carbon-transforming process that makes food energy available to decomposer cells—cellular respiration. Every living organism, from the smallest bacteria to the largest tree in the forest, needs to acquire a source of chemical energy, which is found in the C-C and C-H bonds in organic matter. Once organic matter is oxidized, the chemical energy found in the high-energy bonds is made available for cell functions such as movement, chemical work, and transport of materials. Ultimately all of this energy becomes heat. The atoms once tied up in organic molecules are rearranged into inorganic water and carbon dioxide. Unfortunately, many students incorrectly see cellular respiration as the way food is converted into energy to move and function. Students even make these matter-energy conversions at the atomic-molecular scale when they learn about ATP (another organic molecule). Students need to develop an explanation of cellular respiration that conserves both matter and energy, and makes the connection between atomic-molecular transformations and macroscopic observations.

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 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 part of 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.

Key Ideas and Practices for Each Activity

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.

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 + 6 O
2 + 6 H2O-> 6 CO2 + 12 H2

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

Content Boundaries and Extensions

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