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How Plants Gain Weight

Integrating modeling into a 5E learning cycle

We plant seeds in soil, so it’s reasonable to believe that plants get the materials they need for growth from the soil. But the truth is, soil contributes very little to the weight of a growing plant. Plants gain weight by turning air and water into food. In this article, we provide a unique approach to a classic activity in which fifth-grade students investigate plant growth by engaging in the science practice of modeling. Students develop an initial model of how plants grow, then evaluate and revise the model over time based on evidence from their own investigations. The revised model includes water and air (not soil) as the materials required for plant growth. Here we describe how modeling can easily be integrated into a 5E learning cycle. We also provide suggestions for how to use crosscutting concepts to support student sensemaking and model revision.

Modeling and the 5Es

Scientists plan and carry out investigations. They also develop and use models to visually represent their current understanding of a phenomenon. We have observed that when students engage in the practice of modeling, they deepen their understanding of both science and the nature of science (i.e., how new science knowledge is generated). As students develop a model to explain a phenomenon, they recognize gaps in their understanding and are motivated to test and revise their model over time based on evidence. An added benefit is that models make student thinking visible and can be used as a formative assessment to inform instruction.

We have found that opportunities for modeling can be easily integrated into a 5E learning sequence (see Table 1). In the Engage phase, students draw a model to describe their initial ideas about the cause of a phenomenon. In the Explore phase, students use the model to make predictions and test ideas through investigation. In the Explain phase, students reason together to interpret the evidence and make decisions about how to revise the model. In the Elaborate phase, students investigate new questions raised by the model and gather further evidence to build understanding. In the Evaluate phase, students draw a final model that explains the phenomenon under investigation based on the evidence they have gathered. By allowing multiple opportunities for students to articulate and revise their thinking over time, teachers can both support and assess student learning. The following is a classroom example of how we incorporated the modeling practice into a 5E learning sequence about plant growth.

Modeling Plant Growth

In fifth grade, students develop an understanding of the movement of matter in an ecosystem. The Next Generation Science Standards (5-LS2-1) ask students to develop a model to describe the movement of matter among plants, animals, decomposers, and the environment (NGSS Lead States 2013). Emphasis is on the idea that matter that is not food (air and water) is changed by plants into matter that is food. So, we began by investigating the movement of matter between a plant and the environment. The lessons described here are aligned to 5-LS1-1: Support an argument that plants get the materials they need for growth chiefly from air and water (NGSS Lead States 2013). Emphasis is on the idea that most plant matter does not come from the soil. We had mammoth sunflowers growing taller than students’ heads in the school garden. We decided to leverage students’ interest in the flowers to engage them in a discussion about plant growth.

Engage

Students were given a sunflower seed and asked to compare the weight of one seed to the weight of a giant sunflower we had harvested from the school garden. Students then recorded questions about the sunflowers in their science notebooks. They were curious. How do sunflowers grow so tall? Where had all the material come from to build such a heavy flower? Next, students gathered in small groups to develop a model of what causes a sunflower to gain weight as it grows. Six different models were generated by the student groups (see example in Figure 1). Students compared the different models to look for similarities and differences. Almost all the models included plant material as coming from soil and water. Some models included the Sun as a source of material, while others did not. When pressed, one student tried to argue that the Sun provided energy not materials to a growing plant. None of the initial models included air as a source of material for the growing sunflower.

Figure 1
A student’s initial model of what causes a sunflower to gain weight as it grows.

A student’s initial model of what causes a sunflower to gain weight as it grows.

Explore

Students then used the model to make a prediction. What would happen to the sunflower seed if it was not grown in soil? Would it survive? Is soil required for plant growth? Several students had previously grown a seed in a bag with only a wet paper towel, but most agreed that an adult plant would die without soil. One dissenter said she wasn’t sure about the soil. She commented, “Your parents ask you to water the plants, but never ask you to soil the plants.” She wondered why. Maybe plants could grow without soil as long as they had some alternative way to stay anchored to the ground. We then invited students to test that idea. We prompted students to plan and carry out an investigation to gather evidence that soil is (or is not) required for plant growth.

To support the planning process, we provided a variety of materials including paper towels, cotton balls, small rocks, sand, and garden soil. We also provided students with a graphic organizer (see Fair Test Organizer in Online Resources). Working in small groups, students identified what they were going to change (the independent variable), what they were going to measure (the dependent variable), and what they were going to keep the same (the variables to control). Each student group reviewed their research plan with us to get feedback before planting their seeds. They set up their experiments and washed their hands with soap after touching the soil. After several weeks of observing and measuring the growth of their plants, students agreed that the plants growing in the cotton balls were just as healthy as the plants growing in the soil (see Figure 2). Students concluded that soil is not required for plant growth.

Figure 2
Students plan and conduct an investigation to gather evidence that plants can grow in the absence of soil. The pea plants growing in cotton balls are just as tall and healthy as the plants in the soil.

Students plan and conduct an investigation to gather evidence that plants can grow in the absence of soil. The pea plants growing in cotton balls are just as tall and healthy as the plants in the soil.

Students then watched a brief video excerpt describing an experiment conducted by Jan Baptist van Helmont in the early 1600s (see Online Resource Veritasium, 0:45-1:17). The same experiment is described in an online lesson from Mystery Science, called What Do Plants Eat? (Online Resources). In this experiment, the scientist grew a willow tree in a pot for five years. He then measured the weight of the tree and the weight of the soil. He found that the tree had gained 160 pounds, while the weight of the soil was unchanged. The scientist concluded that soil does not provide the materials for a growing plant.

Explain

We asked students to consider where the 160 pounds of materials had come from to build the tree. Most students reasoned that water must be providing the materials. Although one student suggested that air might be important since trees give us air to breathe. This was a good time to revisit our initial model of plant growth. We discussed what ideas needed to be added, changed, or removed from our model based on the new evidence. Students agreed that soil should be removed from the model as a source of material for a growing plant. What should be added or changed? We asked students to pause and generate new questions about the model. Students recorded questions in science notebooks then shared them with a partner. The questions included, “Does the water transform into leaves, stems, and flowers?” “How does the Sun help plants grow?” and “Do plants need air to survive?” We decided to start by investigating air as a possible source of material for a growing plant.

Elaborate

We reasoned that for air to be matter and a source of material for a growing plant, it would have to weigh something. Students were asked to identify evidence from their everyday experiences to support the argument that air is matter, meaning that air has weight and takes up space. Each student group came up with their own way to support the claim. Using a digital scale, students showed that a football loses weight when deflated, a puffy bag of chips loses weight after it is opened, and a soda can loses weight after it is crushed. We also demonstrated how balloons—filled with air—weigh more than empty balloons (see Balancing Balloons video in Online Resources). The demonstrations provided evidence that air has weight, making air a possible source of material for a growing plant.

Next, we wanted students to figure out how a plant might take in air from the surrounding environment. Students can observe firsthand the stomata on leaves using a microscope and prepared slides (see Figure 3; Online Resources). These microscopic openings look like mouths and provide compelling evidence to students that plants need air to survive.

Figure 3
Electron microscope image of a leaf. The plant uses small openings (stomata) to exchange gases with the environment.

Electron microscope image of a leaf. The plant uses small openings (stomata) to exchange gases with the environment.

Students can also read several texts to obtain information about the structure and function of leaves, stems, and roots. We recommend short books from the series A Closer Look at Plants by Annette Whipple, as well as Plants Need Sunlight by Jennifer Colby. Digital versions of these books are available for free in the Epic! Digital Library (Online Resources). From the reading, students learn that roots absorb water, and leaves have stomata that exchange gases with the environment. Emphasize matter that is not food (air and water) is changed by plants into matter that is food. Plants gain weight by “eating the food” and transforming the food into roots, stems, leaves, and flowers.

Evaluate

With this information, our students were prepared to draw a final model that explains how plants get the materials they need for growth. In a class discussion, we reviewed the evidence we had gathered from our investigations. We generated a list of components that should be included in the revised model. We reminded students to include components that are too small to see, such as stomata on leaves and particles of matter in the air. We demonstrated how to draw “circles of magnification” to include these small-scale components. As we watched students draw, we were happy to see that all students now included air as an important component of their model system (see example in Figure 4).

The final models were used as an assessment of student learning. We considered students proficient if their model included (1) the Sun as a source of energy (not materials), (2) inputs of air and water through the leaves and roots, respectively, and (3) a description of how these materials are needed to make food for the growing plant. Note that the assessment did not require students to define the unseen particles in the air (see Assessment Boundary for NGSS Standard 5-PS1-1). To conclude the lesson, we asked students to reflect on how their model of plant growth had changed over time. Like scientists, we had developed a model, used the model to make predictions, and revised the model based on evidence we had gathered from our own investigations. When we take time to explicitly teach and reflect on the role of models in science, we support our students’ developing understanding of how new scientific knowledge is generated.

Value of Crosscutting Concepts

Developing models to explain observable phenomena is challenging. We have found crosscutting concepts to be helpful in supporting student sensemaking and the modeling practice. Crosscutting concepts can provide a frame of reference for developing a model and can prompt model revision. In this lesson, we used the crosscutting concept of energy and matter to frame our thinking about plant growth. We were careful to use the language of the crosscutting concepts to frame our questions and class discussions (see STEM Teaching Tool #41 for sentence frames in Online Resources). For example, an initial student question about the mammoth sunflowers was, “Why are sunflowers so big?” Together we identified energy and matter as relevant to understanding plant growth. We reviewed the crosscutting concept, Energy and matter move into, out of, and within a system (NRC 2012). Thinking about a sunflower as a living system, students were then prompted to rewrite a question about the sunflowers using the word energy or matter. The revised student questions then included, “How does a sunflower use energy from the Sun to grow?” “How does matter move into and out of a sunflower?” We commended the students for thinking and writing like scientists, using crosscutting concepts to make their questions sound more scientific. These questions provided the impetus for developing our initial model of plant growth.

We turned to the crosscutting concepts again during the process of model revision. That is, we used crosscutting concepts as strategies for sensemaking. In this lesson, students got stuck when they found that soil doesn’t contribute to the weight of a plant. Crosscutting concepts came to the rescue. We prompted students to consider whether something important was missing from our model of the system, identifying air as a relevant component. We had to change scale, imagining air as tiny particles too small to see. We looked to the structure of a leaf for clues about its function, leading to the discovery of stomata. These thinking strategies—listing parts of a system, changing scale, and closely observing a structure to infer its function—helped our students productively participate in science. It was the application of crosscutting concepts that moved our thinking forward and motivated model revision. Also, the quality of students’ final models was improved as students considered how to visually represent the movement of matter and energy in a living system on two different scales (both observable and too small to see). In conclusion, we have found that as students apply crosscutting concepts and move through the process of developing, using, and revising a model, they deepen their understanding of science and the scientific process.

Figure 4
A student’s final model explaining that air and water (not soil) are the materials transformed into food for the growing plant.

A student’s final model explaining that air and water (not soil) are the materials transformed into food for the growing plant.

Online Resources

Digital books from Epic! Digital Library: https://www.getepic.com

Fair Test Organizer from Scholastic: https://images.scholastic.co.uk/assets/a/d0/7f/fair-test-organiser-1116647.pdf

Lesson from Mystery Science: What do Plants Eat? https://mysteryscience.com/ecosystems/mystery-2/matter-cycle-food-chain/94?r=63732397

Plant anatomy microscope slide set: https://www.carolina.com/plant-microscope-slides/plant-anatomy-identification-student-microscope-slide-set/293225.pr?intid=jl_pdp&jl_ctx=on_site

STEM Teaching Tool #41: Prompts for Integrating Crosscutting Concepts into Assessment and Instruction: http://stemteachingtools.org/brief/41

Video from Veritasium: Where Do Trees Get their Mass? https://www.youtube.com/watch?v=2KZb2_vcNTg

YouTube Video: Balancing Balloons (Air Has Weight) https://www.youtube.com/watch?v=o5LT_wfI98w


April Mitchell (a02289733@usu.edu) is a doctoral student at Utah State University in Logan, Utah. Cayme Olsen is a fifth-grade teacher at Shadow Valley Elementary in Ogden, Utah. Kimberly Lott is a professor of science education at Utah State University.

References

National Research Council (NRC). 2012. A framework for K–12 science education: Practices, crosscutting concepts, and core ideas. Washington DC: National Academies Press.

NGSS Lead States. 2013. Next Generation Science Standards: For states, by states. Washington, DC: National Academies Press.

Biology Life Science NGSS Elementary

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