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Plants | Lesson 5 - Explaining How Plants Grow

Lesson 5: Explaining How Plants Grow

Students develop a story about how the high-energy glucose molecules created during photosynthesis are transformed into larger organic polymers during biosynthesis in plants.

Guiding Question

How can a potato plant make a potato?

Activities in this Lesson

Note: There are multiple pathways to choose from in Lessons 4-5. Please see the Plants Unit Read Me document, the Student Challenges and Teacher Choices in the Plants Unit document, and/or the Background Information section below for clarification in making this instructional decision.

  • Activity 5.1: Tracing the Process of Potatoes Growing: Biosynthesis (40 min)

  • Activity 5.2: Explaining How Plants Grow: Biosynthesis (40 min)
  • Activity 5.3: Explaining How Plants Grow: Biosynthesis (40 min)


  1. Measure changes in dry mass of plants.
  2. Construct arguments that use evidence about mass gain in plants, and carbon dioxide concentration in air to defend claims about movements of atoms and chemical changes during plant growth and functioning.
  3. Find patterns in data collected by multiple groups about changes in mass or gas exchange in plants.
  4. Draw and explain movements of materials in a growing plant, include:
    • Carbon dioxide, oxygen, water, and minerals entering a plant
    • Sugar, water, and minerals moving within a plant, and
    • Carbon dioxide, oxygen, and water exiting the plant.
  5. Describe molecules of key materials in plant processes, including atmospheric gases, soil minerals, water, and organic materials.
  6. Explain how atoms are rearranged into new molecules in biosynthesis in plants.
  7. Identify forms of energy at different stages of plant growth and life processes.
  8. Explain transformation and conservation of energy during biosynthesis in plants.

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-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
  • 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. Structure, Function, and Information Processing. MS-LS1-3. Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells.
  • 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 non-living parts of an ecosystem.

High School

  • HS. Matter and its Interactions. HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon 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. 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. 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.

Background Information

The three activities in this lesson represent the Explanations Phase of the Plants Unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of biosynthesis that answer the question: where do the atoms come from that make up a plant? Plants are different from animals because animals take in organic materials for food. Most of the atoms in a plant come from CO2 in the air, and a few atoms come from water and minerals in the soil, like the nitrogen from ammonia.

This lesson is about molecules and how all the molecules of a plant are made during biosynthesis. During biosynthesis, plants use the glucose produced by photosynthesis and soil minerals to produce other small organic molecules or monomers (including amino acids, fatty acids, other simple sugars) to construct large organic polymers: primarily proteins, fats, and carbohydrates. Most of the chemical energy stored in the bonds is transferred from monomers to polymers. These polymers are used to construct the plant’s cells and organelles. Thus most of the dry mass of plants originates in CO2 taken in from the air. Coming into this unit, students may incorrectly think that plants either create mass themselves (e.g., through cell division) or build most of their mass using molecules from soil and water. The activities in this lesson help students revise these ideas.

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? )
  • 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).

Our research has consistently showed that these ideas are extremely difficult for students who have not formally studied chemistry. We therefore use the convention of twist ties to identify bonds that release energy when they are oxidized.

The investigations in all units will make use of two essential tools:

  • Digital balances. Students can detect movement of atoms (the Matter Movement Question) by measuring differences in mass. In this activity, students observe a difference in the plants systems.
  • Bromothymol blue (BTB) is an indicator that changes from blue to yellow in response to high levels of CO2. Thus changes in BTB can partially answer the Matter Change Question by detecting whether there is a chemical change that has CO2 as a reactant or product.

Activity 5.1is the first part of the Explanations Phase of the instructional model (going down the triangle) for Lesson 5. Students trace the process, on a poster of a plant, of the chemical change that took place during the investigation to help them develop an atomic-molecular explanation for how plants gain mass.

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 Activity 5.2, you can move directly from Activity 5.1 to Activity 5.3. In Activity 5.2, students model the chemical changes of biosynthesis using paper molecules. This activity introduces and uses the vocabulary of polymer and monomer, as well as the names of specific monomers.

The modeling focuses on the building of polymers inside the cells. Plants rearrange the atoms of glucose, and soil minerals (especially nitrogen in ammonia) to first build small organic molecules (monomers): amino acids, glucose, fatty acids, and glycerol (this step is not included in the tracing in Activity 5.1). The energy that is stored in the C-C and C-H bonds of the glucose molecules is conserved and passed along from the glucose molecules to the small organic molecules. These small organic molecules are then used to build large organic molecules (proteins, carbohydrates, and fats), which are called polymers.

Activity 5.3 is the second part of the Explanations Phase of the instructional model (going down the triangle) for Lesson 5. Students use the Explanations Tool to construct final explanations for biosynthesis. 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. By this point in the unit, the students will have completed at least one of each of the process tools: Expressing Ideas, Predictions, Evidence-Based Arguments, and the three Explanations Tools for photosynthesis, cellular respiration, and biosynthesis.

There are multiple pathways from which to choose when teaching the carbon-transforming processes in Lessons 4 and 5. You may choose to go in the order presented here. You may choose to follow an order that makes more sense instructionally to you. Or you may choose to go in an order based on the types of questions your students are asking. Please see the Plants Unit Read Me document or the Student Challenges and Teacher Choices in the Plants Unit document for clarification in making this instructional decision. Remember that if you choose a different order than what is presented here, you will need to be aware of what you have and have not yet taught your students, and you may want to make small adjustments to the activities accordingly.

A note on mass and weight: Grams and kilograms in the SI (metric) system are units of mass—the amount of matter in a system. On the other hand, pounds and ounces in the English system are units of weight—the force of gravity on a particular mass. As long as gravity doesn’t change, these units are interconvertible: The force of gravity on a 1 kg mass is about 2.205 pounds. Since most American students are more familiar with the English units of weight, we sometimes use “weight” and “weight,” especially when encouraging students to express their own ideas. When referring to measurements in grams, we use “mass” as both a verb and a noun.

Key Carbon-Transforming Processes: Biosynthesis

Lesson Map:

See the Plants Unit Read Me document for more details.

Unit Map for Lesson 5 Unit Map

unit map lesson 5

Talk and Writing Goals for the Observations Phase

Teacher Talk Strategies That Support This Goal

Curriculum Components That Support This Goal

Help students discuss data and identify patterns.

What patterns do we see in our data?

How do you know that is a pattern?

What about ______ data. What does this mean? 

Class Results Poster

Class Results Spreadsheet

Encourage students to compare their own conclusions about the data and evidence with other groups and other classes.

What about this number? What does this tell us?

How is group A’s evidence different from Group B’s data?

How do our class’s data differ from another classes’ data?

Class Results Spreadsheet

Class Results Poster

Investigation Video (selected segments)

Make connections between the observations and the data/evidence.

It says here that our BTB turned colors. What does that mean?

You recorded that your plant gained mass. What does that mean?


Have students consider how their predictions and results compare.

Let’s revisit our predictions. Who can explain the difference between our class predictions and our results?

Who had predictions that were similar to our results? Has your explanation changed? How?



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