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                        ACTIVITY 4: What is Food for Plants?

                                                            Activity 4 - Students Pages

(It may be helpful to view the student page alongside this teacher page.)

Purposes:

To elicit students' definitions of food.
To introduce a scientific definition of food that focuses on material that is energy-rich.
To contrast everyday definitions of food with a definition that focuses on energy-rich materials.
To use a scientific definition of food to explain why juice is a food but water is not.

OVERALL: Developing a shared understanding of the central question: What is food for plants? What do we mean by "food"?

Materials:

A bottle of juice
A pitcher or bottle of water
A bag of sugar

A Possible Teacher Narrative:

FRAME

Read or talk about the first paragraph on the student's pages of this activity.

ACTIVITY

Have students write down their own definitions of food.

Hold up a bottle of juice, a bottle or pitcher of water, and a bag of sugar.

"Do you think juice is a food? Water? Sugar? Write down your responses."

Read or talk about the two paragraphs at the bottom of page 7.

Common Student Responses

Food is anything you eat, anything you take into your body. So juice, water, and sugar are all foods.
Food is stuff you chew, so liquids like juice and water are not food -- they are drinks.
Food is something that helps you live and grow. So juice, water, and sugar are all foods.

Although it is true that food is something that helps you live and grow, students using this definition of food tend to consider anything that helps you live and grow as food. Many students, for example, consider water and vitamins to be food just like sugar is food. If they continue to use such a definition of food, they will fail to recognize the critical distinction between energy-rich food made by plants during photosynthesis and non-energy supplying materials plants take in from the environment (water, carbon dioxide, minerals).

Establish the problem and Elicit Students' Ideas

Explain Definition of Food

Scientists' Conceptions

In contrast to students' definitions of food, scientists often (not always!) define food in terms of a specific function: It provides the energy that living things need to live and grow. By this definition, water is not a food because it does not provide a rich source of energy for living things. Water is needed for life, but it is not energy-supplying food. Both juice and sugar are food because they provide energy.

Students should not be expected to understand a scientific conception of food at this point. The concept is presented to them on the student's pages of this activity. These questions are designed to help students begin to question their beliefs that water is food for plants. Students will be asked to refer back to this definition frequently in this unit.

ACTIVITY

Consider a way of defining food that focuses on energy: Have students read the first four paragraphs on the student's pages of this activity OR introduce this definition of food in some other way.

After discussing the new definition, have students CHANGE their written definitions of food.

Role play: Have one or more students act out what they would feel like if all they had to "eat" was water.
Emphasize the definition of food as energy-providing material. Emphasize that water does not provide energy to us.

Additional Teacher Background Resources

You might want to consider using a more complicated definition of food that includes nutrients as well as energy at some point in this unit. However, we recommend leaving out the issue of nutrients at this point in order to get students focused on the energy issue, which is the critical issue in photosynthesis. We suggest you might introduce this definition later in the unit (see Activity 10) or in follow-up unit that focuses on matter cycling in ecosystems.

For reference to ways of defining and introducing students to a definition of food that includes both nutrients AND energy, see Kathleen Hogan's Eco-Inquiry Unit (1995, Kendall Hunt Publishers, Dubuque, Iowa).

In those materials, the students are given the following definition of food (p. 125):

Food is a substance that gives both nutrients and energy to a living thing. A NUTRIENT is a
mineral that organisms need to stay healthy, but nutrients do not provide energy that living
things need to be active. So for something to be food, it has to have ENERGY, expressed as
CALORIES, in addition to nutrients.

REFLECT AND CONNECT

Have students respond to the questions on p. 9.

Use a discussion of these questions to remind students about the central question of this unit: How do plants get their food?

"We need to have a shared understanding of the word "food" in order to investigate this question together."

Common Student Responses and Suggested Teacher Responses:

1. Why couldn't you live on water alone?

Because water does not have
energy.
Because your body needs a
balanced diet.
You can live on water alone, at
least for a pretty long time.
Because you said so.
Because of the skit Bobby acted
out.
This question is an application question-students have been told a new
definition of food. Now they need chances to work with the idea. They will
get many such chances in this unit, but this first application question is
heavily scaffolded, reminding the students to use the energy-providing
definition of food.

Most students will dutifully follow directions and use the scientific definition
of food provided on the previous page. They will state that water does not
have energy, but at this point that will not mean much to them. Do not be
fooled into believing that they really understand it at this point. An
understanding of energy will evolve as students use it in a variety of contexts
during this and other science units.

2. Why couldn't you live on dirt?

Because dirt must not have energy in it that we can use to live and grow.
Because our digestive systems are not made to handle dirt -- but worms' bodies are.
Because dirt is not food for humans.
Because dirt is food for plants but it is not food for humans. This is another heavily scaffolded application question. It challenges the student to use the new definition of food in talking about dirt .

This question is laying groundwork to later challenge students' conceptions that soil is food for plants. It is intended to lead students to the conclusion that dirt is not a food, that it does not provide living things with energy. Ideally, this would challenge their common conception that soil is food for plants. However, most students will assume that even though soil is NOT
food for humans, it IS food for plants. The last response if not uncommon --plants and people are different is the most common explanation.