Plants | Lesson 3 - Investigating Plants

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Students conduct an investigation about plants in the light and dark and about mass changes. The focus of this lesson is to make observations and prepare data to be used as evidence for their explanations in Lessons 4 and 5.

Guiding Question

What happens when plants are left in the light and in the dark, and where does a plant’s mass come from?

Activities in this Lesson

Note: There are two different pathways to choose from in Lesson 3. Please see the Plants Unit Front Matter and/or the Background Information section below for clarification in making this instructional decision.

Activity 3.1: Predictions and Planning about Radish Plants Growing (50 min)
Activity 3.2: (GL or PT): Observing Plants’ Mass Changes, Part 1 (30 min)
Activity 3.3: Observing Plants in the Light and Dark (60 min)
Activity 3.4: (GL or PT): Observing Plants’ Mass Changes, Part 2 (45 min)
Activity 3.5: Evidence-Based Arguments About Plants (50 min)

Unit Map

Plants Lesson 3 Unit Map

Target Performances

Lesson 3 – Investigating Growing Radish Plants (students as investigators and questioners)
Activity	Target Performance
Activity 3.1: Predictions and Planning about Radish Plants Growing	Students develop hypotheses about how matter moves and changes and how energy changes when radishes move and grow and make predictions about how they can use their investigation tools—digital balances and BTB—to detect movements and changes in matter.
Activity 3.2 (PT or GL): Observing Plants’ Mass Changes, Part 1	Students harvest their radish plants and dry down the plants and the paper towel or gel in preparation for Activity 3.4.
Activity 3.3: Observing Plants in the Light and Dark	Students observe how plants affect BTB in the light and dark, identify patterns in data, and reach consensus with other groups about their results.
Activity 3.4 (PT or GL): Observing Plants’ Mass Changes, Part 2	Students measure the dry weight of harvested plants and of paper towels or gel, identify patterns in data, and reach consensus with other groups about their results.
Activity 3.5: Evidence-Based Arguments about Plants	Students (a) use data from their investigations to develop evidence-based arguments about how matter moves and changes and how energy changes when plants grow, move, and function; and (b) identify unanswered questions about matter movement and matter change that the data are insufficient to address.

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-LS1-7. 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 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. 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.

Three-dimensional Learning Progression

This lesson provides students with experiences to observe and to collect data which will support them in their next lesson when they construct their explanations about photosynthesis and cellular respiration. They will observe that plants “breathe” (i.e., exchange gases with air) differently in the light and in the dark.

This lesson includes two PEO (Predict-Explain-Observe) Inquiry Activity Sequences with the final E (Explain) occurring in the next two lessons. 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?)

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. This Activity will ask students to harvest their plants to prepare for future dry massing.
Bromothymol blue (BTB). This is a liquid 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.

Key Ideas and Practices for Each Activity

Activity 3.1 is the Predictions and Planning Phase of the instructional model (beginning the climb up the triangle). During this phase, students record their predictions and express ideas about what happens to plants when they are in the light and the dark as well as where a plant’s mass comes from as it grows. They use the Predictions and Planning Tool to do this.

Activity 3.2 will set up the Observations Phase of the instructional model for Activity 3.4. During this activity, the students will harvest their radish plants. If you are following the 2-turtle Gel Protocol, students will collect wet mass in 3.2 and set up to collect dry mass later in Activity 3.4. If you are following the 1-turtle Paper Towel Protocol, students will collect dry mass only in Activity 3.2. There are again two protocol pathways centered around the growing plants investigation for Lesson 3. These correspond to the 2-turtle (Gel Protocol) and 1-turtle (Paper Towel Protocol) that you chose in the Pre-Lesson. You will need to follow the same pathway you chose initially in order to remain consistent with the data collected from the beginning of the unit with the data students will collect here. See the Student Challenges and Teacher Choices in the Plants Unit document as a reminder.

Activity 3.3 is the first part of the Observations Phase of the instructional model (going up the triangle). During this phase, the students conduct the investigation for plants in the light and in the dark, record data, and try to identify patterns in their data and observations. The important practices students focus on in this activity are 1) making measurements and observations, 2) recording their data and evidence, and 3) reaching consensus about patterns in results. They use the Observations Worksheet and Class Results Poster to do this.

Activity 3.4 is the second part of the Observations Phase of the instructional model (going up the triangle). During this phase, the students conduct the investigation for plants growing by harvesting and drying their plants. The important practices students focus on in this activity are 1) making measurements and observation, 2) recording their data and evidence, and 3) reaching consensus about patterns in results. They use the Observations Worksheet and Class Results Poster to do this.

Activity 3.5 is the Evidence-Based Arguments Phase of the instructional model (going up the triangle). During this phase, the students review the data and observations from their investigations of plants and develop arguments for what happened during the investigations. In this phase, they also identify unanswered questions: at this point they have collected data and observations about macroscopic scale changes (BTB color changes, mass changes), but they do not have an argument for what is happening at the atomic-molecular scale. They use the Evidence-Based Arguments Tool to record their arguments at this phase.

Plants are composed of materials that they get from air and soil minerals. Given the range of experiences young children may have with plants, it is interesting that most develop the same story about plant growth—that small seeds are planted in soil and, given water, grow into mature plants over the course of weeks and months. Sunlight is also necessary for plants to grow. It is no wonder that most students believe most plant mass comes from soil and water since these are the visible inputs they see given to plants.

Students are not completely wrong about soil and water. Much of a plant’s total wet mass is actually water. This water contributes to short-term mass gain, but most water does not contribute to long-term building of the large organic molecules that are plant dry mass. The dry mass is carbon-based substances. This carbon does not come from soil or water, but rather from carbon dioxide taken in from the air. Scientists have traced specific carbon atoms (Carbon-14) from glucose back to CO₂. Scientists have also shown that most oxygen in glucose comes originally from CO₂. The O₂ plants give off comes mostly from water. Water does contribute some to biomass, through hydrogen atoms, which comprises a very small percentage of plant mass, but most of the atoms from water eventually leave the plant. Soil minerals—like nitrogen from ammonia—add to plant biomass (about 2% of dry mass) when incorporated into proteins inside the plant.

Air—CO₂ in air contributes carbon (45% of dry mass) and oxygen (45% of dry mass)
Soil
Minerals—can potentially contribute nitrogen, phosphorous, calcium, magnesium, etc. (totaling about 4% of dry mass)
Water—hydrogen atoms from water are about 6% of the plant’s dry mass.

There are two pathways from which to choose when teaching this lesson, depending on which turtle trail you chose in the Pre-Lesson, as well as whether or not your students’ plants seem ready at this point. See the Plants Unit Front Matter and the Student Challenges and Teacher Choices in the Plants Unit for more details.

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 “weigh” 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: Photosynthesis and Cellular Respiration

Content Boundaries and Extensions

Talk and Writing

At this stage in the unit, the students will complete the inquiry sequence for Plant Investigations—they will go up the triangle. This means they will go through the Predictions Phase, the Observations Phase, and the Evidence-Based Arguments Phase. The tables below show specific talk and writing goals for these phases of the unit.

Talk and Writing Goals for the Predictions Phase	Teacher Talk Strategies That Support This Goal	Curriculum Components That Support This Goal
Treat this as elicitation and brainstorming (like the Expressing Ideas Phase), but with more directed questioning.	Now that we have set up the investigation, we want to predict what we think will happen to matter and energy.	Three Questions Handout Predictions and Planning Tool
Elicit a range of student ideas. Press for details. Encourage students to examine, compare, and contrast their ideas with the ideas of other students.	Who can add to that? What do you mean by _____? Say more. So I think you said _____. Is that right? Who has a different idea? How are those ideas similar/different? Who can rephrase ________’s idea?	Investigation Video (selected sections)
Encourage students to provide evidence that supports their predictions.	How do you know that? What have you seen in the world that makes you think that?
Have students document their ideas to revisit later.	Let’s record our ideas so we can come back to them and see how our ideas change.	Predictions and Planning Tool