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 product of photosynthesis. The probe is designed to reveal (1) whether students recognize that the food a plant makes is the food it uses and (2) whether students have a biological concept of food.

Justified list

Photosynthesis, food, sugar, glucose

The best response is sugar. It is the only thing on the list that is the food a plant uses. However, several of the things on this list meet students’ common-sense view of food as things a plant takes in from its environment or needs to live.

Plants make their own food in the form of a simple sugar (glucose), which is a carbohydrate. This simple sugar can be transformed into other sugars, such as fructose and sucrose, or stored for later use in the form of starch. Plants differ significantly from animals in that they are able to manufacture their own food through a process called photosynthesis, using energy from sunlight and matter from carbon dioxide and water that plants take in from their environment to produce sugar and give off oxygen. The food a plant uses is the food it makes through photosynthesis using the inputs of sunlight, carbon dioxide, and water. Plants do not acquire food from their environment; they make it.

Part of the confusion among students (and adults) is due to how we define the word food colloquially and how we commonly use the words food and nutrients interchangeably. To be biologically defined as food, a substance must provide two things: (1) the energy an organism needs to sustain life and (2) the organic matter that provides the building blocks (atoms and molecules) for growth and repair. Foods contain carbon-based molecules such as carbohydrates, proteins, and lipids (fats).

Foods, in a colloquial sense, are often defined as nutrients taken in by organisms (such as animals eating or plants absorbing nutrients from the soil). Nutrients are also needed by living organisms to carry out their life processes, but not all nutrients are considered food. Nutrients can be organic or inorganic and are needed to carry out metabolic processes. Not all nutrients provide energy, a requirement to be considered food in a biological sense. Examples of inorganic nutrients essential to metabolic processes that do not provide energy are vitamins, minerals, and water. Plants take in minerals and water from the environment. All foods can be considered nutrients, but not all nutrients are considered food.

The “plant food” commonly sold in plant stores is an example of the colloquial, or everyday, use of word food. This “plant food” that comes in a can or jar is not food in a biological sense. It provides a source of inorganic nutrients needed by a plant that may not be present in the soil. Likewise, soil is not food but rather a source of plant nutrients such as minerals and water.

Other things on the list that are not food for the plant are sunlight, leaves, chlorophyll, carbon dioxide, and oxygen. Sunlight is the form of energy used by the plant during photosynthesis, but it does not contain the matter that provides the building blocks needed to grow or repair plant structures. Leaves are the plant structures where photosynthesis takes place. Leaves take in carbon dioxide and give off oxygen. They may be food for animals that eat leaves, but they are not food for a plant. Chlorophyll is a substance contained in the plant’s chloroplasts that is involved in photosynthesis. It traps the light needed for photosynthesis.

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 can make their own food and get their material (food) for growth from air and from 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 to make food. By fifth grade, students develop a concept that food provides energy and a source of material for growth and repair.

Middle School Students

In middle school, students transition from knowing plants need sunlight, air, and water to knowing how they use those elements. They learn that a plant takes in carbon dioxide in air and water from its environment and then rearranges the atoms to make an organic molecule called sugar. They learn that sunlight provides the energy for this process called photosynthesis and that another gas (oxygen) is released in the process. They learn that the sugar formed through photosynthesis can be used immediately by the plant to provide the energy it needs to sustain life through processes such as respiration, can be used for growth and repair, or can be stored for later use. They learn that plants take in nutrients from the soil, such as minerals and water.

High School Students

In high school, students learn details about the chemical process of photosynthesis. They learn that simple sugars (such as the glucose molecule) produced through photosynthesis can be transformed into other sugars or assembled into larger molecules. They learn how the sugars produced during photosynthesis fuel cellular respiration, releasing energy for the plant.

This probe can be used with students in grades 6–12. It can be used earlier if students are familiar with sugar being the product of photosynthesis. This probe works well with the interactive card sort strategy (Keeley 2016). Print answer choices on cards and have pairs or small groups of students sort them into three columns: (1) things that are food for a plant, (2) things that are not food for a plant, and (3) things we don’t agree on or are not sure about yet. As students sort, they discuss their ideas about “food” and justify their reasoning and eventually come up with criteria for what is considered to be the food a plant uses. As the formative assessor, you can see at a glance whether there is only one card in the food column.

• The commonly held idea that plants take their food in from the environment, rather than making it internally, is highly resistant to change. Even when taught how plants make food by photosynthesis, students still hold on to the notion that food is taken in from the outside (AAAS 2009).
• Before using computer simulations, some students thought that water is the source of energy for plants (Çepni, Tas, and Köse 2006).
• Much of the research on students’ ideas about food for plants was conducted in the 1980s and still applies to students’ ideas today. Universally, the most persistent notion is that plants take their food from the environment, particularly the soil. Students also believe that plants have multiple sources of food and that carbon dioxide or even sunlight is food for a plant. Typically, students do not consider starch as food for plants. Their reasoning is that starch is something plants make, not something they eat. The everyday reference to fertilizers as “plant food” may promote the idea of fertilizer as being food for plants (Driver et al. 1994).
• In a study by Tamir (1989), some students thought sunlight, associated with energy, was the food for plants. Many students also considered minerals taken in from the soil as food or believed that minerals had a direct role in photosynthesis.
• In a study by Wandersee (1983) that surveyed 1,405 students ages 10–19 about the product of photosynthesis, most students selected proteins, relating them to food for growth rather than energy. Some students in this study also mentioned plants getting vitamins from the soil.
• Understanding that the food plants make is very different from other nutrients, such as water and minerals, may be a prerequisite for understanding the idea that plants make their food rather than acquire it from the environment (Roth, Smith, and Anderson 1983).

Aaron, R., B. Hug, and R. G. Duncan. 2017. Core idea LS1: From molecules to organisms: Structures and processes. In Disciplinary core ideas: Reshaping teaching and learning, ed. R. G. Duncan, J. Krajcik, and A. E. Rivet, 123–144. Arlington, VA: NSTA Press.

Bradley, S., S. Rybczynski, and D. Herrington. 2016. Food and energy for all: Turning a demonstration into an inquiry activity. Science Scope 40 (4): 49–56.

Keeley, P. 2012. Food for plants: A bridging concept. Science and Children 49 (8): 26–29.

Keeley, P. 2014. Food for plants: A bridging concept. In What are they thinking? Promoting elementary learning through formative assessment, P. Keeley, 113–120. Arlington, VA: NSTA Press.

NGSS Archived Webinar: Ecosystems, Interactions, Energy, and Dynamics, https://common.nsta. org/resource/?id=10.2505/9/WSNGSS14_Feb11.

Weinburgh, M. 2004. Teaching photosynthesis: More than a lecture but less than a lab. Science Scope 27 (9): 15–17.

• A similar probe, “Food for Corn,” in Uncovering Student Ideas in Life Science, Volume 1, can be used with this probe or as a follow-up after students have had an opportunity to discuss and revise their initial ideas (Keeley 2011b). If you are puzzled as to why students select chlorophyll, consider using the probe “Chlorophyll,” which is also in that book (Keeley 2011a).
• The advantage to using this probe in a talk format is that students discuss their ideas about food and what food is, which is an important concept that is often missing in science lessons.
• Take the time to elicit students’ definitions of the word food; many students use this word in a way that is not consistent with its biological meaning. Have students identify and discuss the difference between the everyday meaning and use of the word food and its scientific meaning and use. Contrasting the two and providing examples may help them see the difference and understand how everyday use of words affects our thinking about phenomena.
• Before introducing a chemical equation for photosynthesis, first help students understand that “an element, carbon (which is solid in its pure form), is present in carbon dioxide (which is a colorless gas in the air) and that this gas is converted by a green plant into sugar (a solid, but in solution) when hydrogen (a gas) from water (a liquid) is added using energy from sunlight which is consequently converted to chemical energy” (Driver et al. 1994, p. 30).
• High school students can often define photosynthesis and provide the equation, but questions that ask them to apply a basic understanding are often not asked of students. Ask questions that encourage students to use their ideas about photosynthesis to explain the food-, growth-, and energy-related needs of plants.
• Show a container of plant food (which is a mixture mainly of nitrogen, potassium, and phosphorus) and a container of vitamins for humans. Build an analogy between the two to show that their purpose is to provide essential inorganic nutrients, not provide energy. For example, vitamins would not provide you with a source of energy unless they were sugar coated.