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Plants | Lesson 4 - Explaining How Plants Make Food, Move, and Function

Lesson 4: Explaining How Plants Make Food, Move, and Function

Students use molecular models to learn how matter and energy are transformed in plants during photosynthesis and cellular respiration. The focus of this lesson is on developing explanations for how plants make food, move, and function in the light and in the dark.

Guiding Question

What happens to atoms, carbon, and energy during photosynthesis and cellular respiration?

Activities in this Lesson

  • Activity 4.1: Molecular Models for Potatoes 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 Plants Move and Function: Cellular Respiration (40 min)
  • Activity 4.3: Molecular Models for Potatoes Making Food: Photosynthesis (60 min)
  • Activity 4.4: Explaining How Plants Make Food: Photosynthesis (40 min)

Objectives

  1. Describe plant systems and processes in a hierarchy of scales, including atomic-molecular, macroscopic, and large scale.
  2. Draw and explain movements of materials in a growing plant, including: carbon dioxide, oxygen, water, and minerals entering and exiting a plant
  3. Describe molecules of key materials in plant processes, including atmospheric gases, soil minerals, water, and organic materials.
  4. Explain how atoms are rearranged into new molecules in photosynthesis and cellular respiration in plants.
  5. Identify forms of energy at different stages of plant growth and life processes.
  6. Explain transformation and conservation of energy during photosynthesis and cellular respiration 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. Matter and Energy in Organisms and Ecosystems. MS-LS1-6. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy among living and non-living parts of an ecosystem.
  • 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. Chemical Reactions. 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 the 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. Energy. HS-PS3-1. Create a computational model to calculate change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
  • HS. Structure and Function. 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-LS1-5. Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.
  • 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.
  • HS. Matter and Energy in Organisms and Ecosystems. HS-LS2-5. Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.

Background Information

This lesson helps middle school and high school students understand why plants “breathe” (i.e., exchange gases with the air) differently in the light and in the dark and how the mass of plants can come mostly from the air. As they model photosynthesis, they learn how to explain plant gas exchange and growth in a way that follows the key rules about matter and energy—atoms last forever and energy lasts forever (in chemical changes). As they model cellular respiration, they learn how to explain this carbon-transforming process that makes food energy available to plant cells.

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 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 (Matter Movement and Matter Change). 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 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 four 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 photosynthesis and cellular respiration that were the drivers of the macroscopic changes in CO2 concentration that they observed in their Plants in the Light and Dark 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 plants get energy to move. Plants use the energy released from cellular respiration to grow and function (for biosynthetic processes and other cellular functions) as well as to move. Plants engage in cellular respiration in both the light and the dark. If your students did cellular respiration molecular modeling as part of the Animals or Decomposers Units, and did well on questions about cellular respiration on the pretest, you may want to skip the cellular respiration modeling steps in this Activity.

Activity 4.3, is the first part of the Explanations Phase of the instructional model (going down the triangle) for photosynthesis. Students construct molecular models of the chemical change that explains how the plants give off oxygen in the light.

Activity 4.4, is the second part of the Explanations Phase of the instructional model (going down the triangle) for photosynthesis. Students use the Explanations Tool to construct final explanations of how plants make their own food. Ideally, at this phase their explanations will combine evidence from the macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale. The questions in this Activity should also help students notice two important relationships:


A note on the chemical change formulas for photosynthesis and cellular respiration. Activities 4.1 and 4.3 simplify the full story of what happens to matter during the multi-step processes of cellular respiration and photosynthesis. For a more detailed account, see http://dqc.crcstl.msu.edu/node/2027..

In Activity 4.1, we use a standard but simplified formula for the overall chemical change occurring in cellular respiration:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2 O

This incorrectly suggests that some of the oxygen atoms in O2 end up in CO2, which does not happen directly during the multi-step process of cellular respiration. A more accurate formula to represent the multi-step process would be as follows:

C6H12O6 + 6 O2 + 6H2O → 6 CO2 + 12 H2 O

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

In Activity 4.3, we use a standard but simplified formula for the overall chemical change occurring in photosynthesis:

6 CO2 + 6 H2O → C6H12O6 + 6 O 2

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

6 CO2 + 12 H2O → C6H12O6 + 6 O 2 + 6 H2O

Thus all of the oxygen atoms in O2 (bolded in the equation above) come from H2O, while the oxygen atoms in CO2 all go to glucose or water. Although we ask students to identify C-C and C-H bonds as high in energy, it is important to recognize that releasing most of that energy requires a reaction with oxygen. It is more accurate to say that the chemical system of glucose and oxygen has more potential energy than the chemical system of carbon dioxide and 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.

Our research has consistently shown 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 (C-C and C-H bonds). The products of cellular respiration have only lower-energy C=O and H-O bonds, so the energy released by the oxidation reaction is available for cell movement and function. 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 leaves the plant as heat.

A note on cellular respiration. Students usually do not think about plants doing cellular respiration. They learn that plants do photosynthesis, and often cellular respiration is overlooked. Students may not even wonder how seeds actually sprout when they have no leaves, no chlorophyll, and no way to photosynthesize. Fully grown plants also undergo cellular respiration on a continuous basis. It is easier to detect this process in plants during the night, as well as in winter months, when plants are not also photosynthesizing. During cellular respiration, plants take organic materials and oxidize them, which releases energy and gives off inorganic carbon dioxide and water as wastes. Many students also incorrectly see cellular respiration as the way plants convert food or stored biomass (fat, starch) into energy for movement, cell functions, and growth. Students need to develop an explanation of cellular respiration that conserved both matter and energy, and makes the connection between atomic-molecular transformations and macroscopic observations.

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 diphoate) 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.

Key Carbon-Transforming Processes: Photosynthesis & Cellular Respiration

Unit Map

Plants Lesson 4 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