This is the new updated edition of the first book in the bestselling Uncovering Student Ideas in Science series. Like the first edition of volume 1, this book helps pinpoint what your students know (or think they know) so you can monitor their learning and adjust your teaching accordingly. Loaded with classroom-friendly features you can use immediately, the book includes 25 “probes”—brief, easily administered formative assessments designed to understand your students’ thinking about 60 core science concepts.

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The purpose of this assessment probe is to elicit students’ ideas about the transformation of matter. The probe is designed to reveal whether students recognize that most of the matter that makes up the wood of a tree can be traced back to carbon dioxide in the air.

Familiar phenomenon

Transformation of matter, photosynthesis, carbon cycle

The best response is D: carbon dioxide. Plants take in carbon dioxide (a gas) through their leaves and water from the soil and use the energy from sunlight to rearrange the atoms into new substances—sugar and oxygen. This process happens inside the leaf of the plant. Sunlight provides the energy for this process to happen. Chlorophyll is a pigment found within the leaf cells that absorbs the energy from sunlight used for the reaction. After food (a sugar called glucose) is made in the leaf, it travels to other parts of a plant, where it is used for energy, tissue repair, and growth or stored for later use.

Most of the matter that makes up the structure of the tree can be traced back to the carbon and oxygen in carbon dioxide that was combined with hydrogen from water using energy from sunlight and transformed into a simple sugar (glucose) through photosynthesis. Although simplified for this explanation, a basic description of this reaction is as follows: A glucose molecule is made of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. When the plant makes a molecule of glucose, it gets 6 carbon atoms and 6 oxygen atoms from the carbon dioxide. Carbon dioxide doesn’t have any hydrogen in it, so the source of hydrogen is water. In order to get the hydrogen the plant needs to build a glucose molecule, it uses energy from the Sun to break water molecules apart, taking electrons and 12 hydrogen atoms from the 6 water molecules and releasing the 6 oxygen atoms. The 6 oxygen atoms from the carbon dioxide and the 6 oxygen atoms from the water are released into the air as diatomic molecules of oxygen (O2). The basic chemical equation is 6CO₂ + 6H₂O  C₆H₁₂O₆ + 6O₂. The electrons are used to produce high-energy molecules called ATP that are used to build the glucose molecule. To trace back the atoms in a single glucose molecule, 6 of the carbon atoms come from the carbon dioxide, 12 of the hydrogen atoms come from the water, and 6 of the oxygen atoms come from the carbon dioxide. With an estimated atomic mass of 12 for carbon, 1 for hydrogen, and 16 for oxygen, clearly the mass contributed by the carbon dioxide is much greater than the mass contributed by water. When wood is burned, carbon dioxide and water vapor are released back into the air. When the wood is completely burned, the remaining ashes consist of the small amount of inorganic material—the minerals taken in from the soil.

Elementary Students

In the elementary grades, students learn that plants need sunlight, water, and nutrients to grow and stay healthy. Upper elementary students learn that plants make their own food in a chemical process and get their material for growth from air and water, but not from soil. However, it is too abstract an idea for them to understand the details of the transformation of matter that takes place during photosynthesis and growth of a plant. Both younger and older students have difficulty accepting the idea that something as seemingly light as air could make up the bulk weight or mass of a tree, partly because students lack opportunities to recognize air is a substance that has weight (the term mass should wait until at least fifth grade). It is critical for upper elementary students to have opportunities to accept the idea that air is made up of particles of matter and that plants do not get the material they need for growth primarily from the soil.

By fifth grade, students develop a concept that food provides energy and a source of material for growth and repair. They begin to develop the idea that organisms cycle the materials they take from the environment. As organisms use food, their waste goes back into the environment in the form of solids, liquids, and gases. Understanding this cycle is a prerequisite for understanding the cycling of carbon dioxide and oxygen in middle school.

Middle School Students

In middle school, students learn about chemical reactions and the types of transformations of matter that occur during these reactions, although quantitative details of the chemical reactions can wait until high school. They transition from knowing plants need air to make food to knowing that it is the carbon dioxide in air that plants use to make an organic molecule called sugar and that another gas (oxygen) is released in the process. They use the idea of atoms to explain the rearrangement that happens when matter is transformed in a process like photosynthesis and that the process requires an input of energy. They learn that the sugar formed through photosynthesis can (1) be used immediately by the plant to provide the energy it needs to sustain life through processes such as respiration, (2) be stored for later use, or (3) be used for growth and repair. Although students can manipulate models to learn what happens during the transformation of carbon dioxide and water into sugar and oxygen, they may still have difficulty accepting the idea that a gas in the air contributes the most mass to the growth of a tree. It seems counterintuitive to students that most of the mass of the matter of a tree comes from carbon dioxide in the air. At this level, students learn about the role of photosynthesis at the organism level as well as its role in cycling matter through an ecosystem.

High School Students

In high school, students learn details about the chemical process of photosynthesis. Students’ increasing knowledge of chemistry, particularly carbon-based molecules, comes in handy when quantitatively reasoning through a problem such as this one by using molecular masses. They learn that simple sugars (such as the glucose molecule) produced through photosynthesis can be assembled into larger molecules such as cellulose, which makes up the wood of a tree. At this level, students connect processes like photosynthesis and respiration to the carbon cycle.

This probe can be used with students in grades 6–12. The sequoia tree was used as the subject of this probe because of its massive size, but a large familiar tree in your students’ environment may be substituted. You can also show students images of very large trees. Similar to the “seed and log” question in the Private Universe series (Harvard-Smithsonian Center for Astrophysics 1995), you might show a maple seed or acorn and a log cut from a tree and ask students where most of the “stuff” of the log came from as it grew from seed to seedling to large tree. The common word stuff can be used intentionally in this probe to explore students’ ideas without being hindered by their misunderstanding of the concept of matter or mass.

• Studies have shown that middle and high school students tend to think that matter is converted into energy and energy is converted back into matter. They describe this “cycling” of matter and energy as energy and matter being recycled through soil nutrients (Jin and Anderson 2012).
• Studies have shown that high school and undergraduate students often struggle to understand the cycling and transformation of carbon. They cannot always account for the movement of carbon between photosynthesis and respiration, and some students think that photosynthesis requires human input of carbon dioxide (Brown and Schwartz 2009; Wilson et al. 2017).
• Students have a difficult time imagining plants as chemical systems. In particular, middle school students think organisms and materials in the environment are very different types of matter. For example, some students think plants are made of leaves, stems, and roots and the nonliving environment is made of water, soil, and air. Students see these substances as fundamentally different and not transformable into each other (AAAS 2009).
• The question in this probe is based on a similar question used in the Private Universe series, where Harvard graduates were shown a seed and a log and asked where most of the mass of the log came from. Very few mentioned carbon dioxide. The most common responses were that it came from sunlight, the soil, or water (Harvard-Smithsonian Center for Astrophysics 1995).
• In a study of 759 15-year-old students who had studied photosynthesis, only 8% could relate photosynthesis to plant growth by describing how a tree makes tissue from the things it takes in from the environment. Only 3 students out of 759 said that tree tissue is made from carbon dioxide and water using light energy. Other studies have also found that students have a difficult time accepting that weight increase and growth in plants is attributed to the incorporation of matter from a gas (Driver et al. 1994).
• Barker and Carr (1989) found that many children regarded sunlight as one of the reactants in photosynthesis, along with carbon dioxide and water. Some students consider light to be made of molecules, thus contributing to the matter that makes up a plant.
• In Wandersee’s study (1983) of 1,405 students ages 10–19, many thought that the soil in a plant pot would lose weight as the plant grows because the plant uses the soil for food.

American Association for the Advancement of Science (AAAS)/Project 2061. 2017. Toward high school biology: Understanding growth in living things. Arlington, VA: NSTA Press.

Long, C. 2014. Matter and energy in organisms and ecosystems. In Hard-to-teach biology concepts: Designing instruction aligned to the NGSS, 2nd ed., S. Koba and A. Tweed, 171–188. Arlington, VA: NSTA Press.

Penniman, L. 2011. How much carbon is in the forest? The Science Teacher 78 (1): 56–60.

Petrosina, A., M. Mann, and S. Jenevein. 2018. Where does a tree get its mass? Science Scope 41 (9): 55–61.

Taylor, M., K. Cohen, R. K. Esch, and P. S. Smith. 2012. Investigating students’ ideas about the flow of matter and energy in living systems. Science Scope 35 (8): 36–36.

Thompson, S. 2014. Historical plant studies: Tools for enhancing students’ understanding of photosynthesis. Science Scope 37 (6): 43–53.

• Before students can accept the idea that the mass of a plant comes mostly from the carbon dioxide in the air, they have to accept air as matter that has mass (or weight, for students who struggle with the concept of mass). Students need multiple opportunities to discover that gases have significant mass.
• To understand the chemical change that happens during photosynthesis, students need to trace atoms, not the observable materials such as soil, air, and water (Anderson and Doherty 2017).
• Students who understand that cells are about 70% water tend to pick that as an answer. High school chemistry students can use the equation for photosynthesis and molecular masses to show that even though water is taken in and transformed along with carbon dioxide, the carbon dioxide molecules contribute significantly more mass to the sugar than water molecules.
• Manipulating physical models of molecules may help middle school and high school students see what happens to the atoms in the carbon dioxide and water molecules as they are rearranged to form glucose and oxygen.
• Photosynthesis is a complex reaction that is frequently treated in high school as an equation to be memorized along with dark and light reactions. Students often have little opportunity to learn how the process contributes to the growth and energy needs of a plant.
• Ask students how the idea that plants get their material for growth from the soil can be challenged with evidence from everyday phenomena. For example, if a tree got its mass from soil, why is there not a hole around the tree? How do plants grow hydroponically when they are not in soil?
• If students fail to recognize that carbon dioxide as a gas has weight, show students dry ice and explain that it is a solid form of carbon dioxide. Have students put on protective gloves and hold a piece of dry ice to sense the “felt weight.” (Safety note: Students should touch dry ice only with heavy protective gloves.)
• Use Jean Baptist van Helmont’s experiment from the 1600s as a context to learn how scientists of the past explored the question of where plants get the materials they need to grow. The prevailing view at that time was that plants grew by taking material out of the soil. Have students evaluate the results of his experiment.
• Following discussion of the probe, have students examine the global significance (especially global warming and climate change) of trees being made mostly of carbon. What are the global effects of large-scale deforestation? What happens when trees are burned for clearing large tracts of land? What is released into the atmosphere and what effect does that have? Why is it important to plant trees?
• Share a quote from Nobel laureate Dr. Richard Feynman’s speech that he gave at the 14th annual National Science Teachers Association convention in New York City, which so beautifully captures the essence of this probe: “The world looks different after learning science. For example, trees are made of air, primarily. When they are burned, they go back to air, and in the flaming heat is released the flaming heat of the Sun, which was bound in to convert the air into tree. And in the ash is the small remnant of the part which did not come from air, that came from the solid earth, instead” (Feynman 1969). You can view a classic video of the late Dr. Feynman describing this phenomenon at www.youtube.com/watch?v=P1ww1IXRfTA.