Teacher's Pages
ACTIVITY 10 - Counting Calories - Measuring Food Energy
ACTIVITY 10 - STUDENT'S PAGE
(It may be helpful to view the student page alongside this teacher
page.)
Purpose:
This activity is designed to challenge students' beliefs
that plants take in food from the soil -- that the fertilizers and minerals plants take in through their roots
(and called "plant food" at the stores) is their energy-providing food. Using an analogy to vitamin pills for
people, the argument made here is that plants need minerals to be healthy but that minerals do not provide the
energy they need to be active and grow.
Materials:
For teacher demonstration:
Peanuts, fertilizer sticks, and vitamin pill (sugar free) to burn
Matches
Tongs to hold food items
Cup of water
Bag of potato chips
Overhead transparency of a potato chip bag label
For each student:
Data chart
3 x 5 post-it note
For each student group:
One label from a vitamin pills container
NOTE: Do not use children's chewable vitamins because they
contain sugar and will complicate the argument that we are trying to build here
that vitamins and minerals do not have calories.
One label from some kind of plant food (it is helpful to have
each group have a different kind of plant food label to examine).
One label from some kind of minerals (it is helpful to have each
group have a different kind of label to examine -- zinc, iron, etc.).
One label from a bottle of water.
A variety of other labels from foods that contain calories -- try
to include at least one drink:
juice, cereal, candy bars, peanuts, rice, etc. Each group can
have a different set of foods to
examine.
Optional: Diet Coke with caffeine (this has no calories but
students associate it with giving you
"energy" -- it can provoke interesting discussion but
will lead you into a new issue about caffeine).
Advance Preparation: Gather foods , vitamins, and plant foods
with labels. For vitamin pills and minerals, make Xerox copies of labels so that you do not distribute bottles
of pills to students (this is also a more economical approach since the vitamins are quite expensive).
FRAME
Read from "Is plant food...? to What do you think?".
Have students write and talk about their ideas about the hypothesis: Plant food or minerals and fertilizers are food
for plants.
ACTIVITY
Read the definition of calories. This is a key term in this
lesson, because we are going to use calories to determine whether different substances have food energy in them.
Teacher Demonstration:
Before burning the peanut, ask students if they think a peanut
has calories in it? Do they think it has food energy in it? So do they think it will burn? For how long?
After the burning, emphasize that the food energy in the peanut
was burned leaving black ashes that are the minerals that were in the peanut (they did not burn because
they do not contain energy!!).
"Does this help you think in a new way about what aerobics
instructors mean when they say, "Go for the burn?" (burning up food and releasing energy at a faster
rate).
Being Sensitive to Diverse Learners
The introduction of calories into the discussion is sure to raise
issues about diet and fatness in students' minds. Students of this age, especially girls, are acutely
sensitive to issues about their weight. They are bombarded with messages from advertising and entertainment, as
well as from family and peers, about what their bodies should look like -- THIN!!! Therefore, these
students are likely very aware of the need to "count calories" when dieting to lose weight.
What implications does this have for the science lesson? Ask
students if they have heard about calories before. Have them describe what they know about calories. Listen
carefully to their responses. If there is teasing going on about weight and fat, this might be a good time
to take an important detour in the lesson and talk about issues of weight perception and the importance
placed on thinness in our society. Students could do a study of advertisements to monitor messages that the
media sends us about weight.
Additional Background for Teachers
Why do plants need water? Plants need water in order to
synthesize food, as a medium for metabolic reactions and internal transport, and for support (this can be
seen when a plant begins to wilt if it is not watered regularly). Most of this water comes from the soil, and
reaches the plant tissues through the root system. A small amount is absorbed directly from the leaves.
Why do plants need nutrients (minerals from the soil)? Iron and
magnesium salts are used in the
production of chlorophyll, nitrates and sulfates are used in
protein and DNA production, phosphates are used in 'energy carrier' molecules (ATP), and calcium is laid
down in the middle layer between plant cell walls.
Why do plants need food? Plants need food to be active, to live
and grow, to support life processes and chemical reactions.
[These notes adapted from Steve Farrow's The Really Useful
Science Book: A Framework of Knowledge for Teachers, Falmer Press, 1996].
FRAME
Checking for understanding of calories:
Look at a food label together -- for example, hold up a bag of
potato chips.
"Do these chips contain energy I can use? How can I find
out?"
If students suggest to burn the chip, burn it and observe.
"What if I am at the store and want to know if something has
calories in it. The store manager would not be happy if I opened the bag and started burning chips! How else can
I find out?"
It might be helpful at this point to show an overhead
transparency of the nutritional label for the potation chips. Help the students read the label and look for whether or
not it contains calories.
"Today you are going to look at labels to see if they
contain calories -- do they contain food energy? Will they burn, showing they have calories? WE ARE ESPECIALLY
INTERESTED IN LOOKING AT PLANT FOOD THAT YOU GET FROM THE STORE. Does plant food contain energy
that plants can use to live and grow? So today we are testing the hypothesis that plants get
their food from the minerals and fertilizers in the soil.
Predictions
Use this section to make some predictions about vitamin pills and
plant food sticks.
ACTIVITY
Go over the directions here.
Each group will have a different set of materials to examine, but
each group should have at least one vitamin pill label, one plant food label, one label from a bottle
of water, one label from some kind of minerals, and a variety of calorie-containing foods.
Students should work in groups, but it is preferable to have EACH
student fill out a data chart.
ACTIVITY
As students work, circulate and help them read the labels.
Students sometimes have difficulty reading the plant food labels because they do not look like the food labels
we are used to. Help them see that nowhere on the label are there any calories or sugars listed.
Point out the list of minerals that are on the label, noting that these must not contain calories since there
are no calories noted on the labels.
Some students (and teachers!) may wonder whether manufacturers
are required to list calories for plant foods since they are not intended for human consumption. Great
question! I am still exploring the answer to this one.
If you include Diet Coke (or another brand), students may wonder
about the lack of calories in this "food," because they associate the drink with giving you energy (the
caffeine). Ask them if they think the label is wrong to say there are no calories? How do they explain that
there are no calories listed?
REFLECT AND CONNECT
1. Have students look at their data charts to write down items
they found that are NOT food by our scientific definition.
Anticipated responses: vitamin pills, plant food, fertilizer
stakes, water, minerals, Diet Coke
2. Have students predict whether vitamin pills and plant food
sticks will burn. Ask for their reasons.
Desired response: Since they do not contain calories (food
energy), they will not burn.
3. Burn the vitamin bills and plant food sticks. Have students
write down their explanations of what happened. Why didn't they burn?
4. Distribute post-it notes. Have each student write about some
evidence to either support or challenge the hypothesis that minerals and plant food are food for plants. Have
students place their post-its on the class data chart. Discuss the evidence that they cited.
Anticipated Responses:
CHALLENGING THE HYPOTHESIS:
They are not food because they do not contain any calories, which
tells you they have
no food energy. They do not have energy in them but they are important to plants
because my Mom and I did an experiment at home, and the ones that got
fertilizers grew better than the
ones that didn't. In the Van Helmont experiment, the tree gained a lot of weight
but only took in a few minerals from the soil. So the minerals could not be their food.
SUPPORTING THE HYPOTHESIS
In the Van Helmont experiment, the tree took some minerals from
the soil so this
must be one source of their food. They do not have energy in them but they are important to plants
because my Mom and I did an experiment at home, and the ones that got
fertilizers grew better than the ones that didn't.
Why would they call fertilizers "Plant Food" if they
were not food for the plants?
My family always fertilizes the plants, and it helps them grow
better so it must be
food.
5. Have students draw something they learned today -- either a
diagram, a picture, a concept map, or some other visual representation of their learning. This more
open-ended response is intended to help students reflect and synthesize and to help the teacher
understand the various ways in which students understood this lesson.
REFLECT AND CONNECT--SYNTHESIZING THE RESULTS
The Grass Experiment
Van Helmont's Experiment
The Food Analysis Project
NOTE:
BEFORE STARTING THIS TASK, MAKE SURE YOU HAVE RETURNED TO THE
GRASS PLANT EXPERIMENT
AND ANALYZED THE RESULTS. (See Activity 8)
FRAME
Read and discussion the three questions here. Use these to review
the three experiments.
"Now let's put these three experiments together and see what
we can figure out about our hypotheses about how plants get their food [refer to class data chart
listing the various hypotheses]."
ACTIVITY
Read and discuss this section together.
REFLECT AND CONNECT
The purpose of these questions is to challenge students to think
across all the experiments they have done and to consider whether they are convinced by the arguments
that say that water, soil, and minerals are not food for plants. It is not enough for students to
memorize the argument given in the text on p. 41.
These arguments must make sense to the students. If they do not
make sense, it is important for the teacher to elicit this from the students at this point.
Whether or not students are convinced about water, soil, and
minerals, you should proceed at this point to introduce the idea of photosynthesis. Hopefully, students will be
at least uncertain about the role of water, soil, and minerals and ready to hear a new way of making sense of
the data they have been exploring.