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Animals | Lesson 5 - Explaining How Animals Grow

Lesson 5: Explaining How Animals Grow

Students model and explain digestion, biosynthesis and cellular respiration using the Three Questions. They learn that the biomass of animals is mainly carbohydrates, fats, and proteins and relate the rearrangement of atoms in cellular respiration to energy release.

Guiding Question

How do animals use food to grow, move and function?

Activities in this Lesson

  • Activity 5.1: Tracing the Processes of Cows Growing: Digestion and Biosynthesis (40 min)
  •  Activity 5.2: Molecular Models for Cows Growing: Digestion and Biosynthesis (40 min)
  • Activity 5.3: Explaining How Cows 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 an animal, including air and food entering the animal, and waste, air enriched in carbon dioxide and water vapor leaving the animal.
  3. Explain the chemical changes that occur when an animal 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 animal biomass.

NGSS Performance Expectations

Middle school

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

High school

  • HS. From Molecules to Organisms: Structures and Processes. HS-LS1-2. Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.
  • HS> From Molecules to Organisms: Structures and Processes. 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.

Background Information

The three activities in this lesson complete the Explanations Phase of the Animals unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of the digestion, and biosynthesis that were drivers of the macroscopic changes they observed in their Mealworms Eating 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 the body of a cow using a poster of a cow.

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.

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 animals break large organic molecules from their food into small organic molecules, and then use the small organic molecules to gain mass. This lesson 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.

Key Carbon-Transforming Processes: Digestion and Biosynthesis

Unit Map

Unit Map Lesson 4

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