Teacher's Pages
Activity 15 - What happens to the food after it is made in
the leaf?
Activity 15 - Students Pages
Explain new ideas about photosynthesis
Apply ideas -- Teacher Coach
Purpose:
The purpose of this activity is twofold: a) To provide
another context in which students can
practice using the concept of photosynthesis to explain
phenomena, and b) To introduce some new ideas about photosynthesis.
Main Ideas:
1. Food travels from the cells in the leaf all over so that food
reaches each cell in the plant to
provide it with the food energy it needs to live and function.
2. The plant has structures such as veins to help food travel
through the plant.
3. Extra food that the cells do not immediately use is stored in
different parts of the plant --
seeds, fruits, vegetables.
Connections to Other Activities: The students started thinking
about the idea that the plant is made up of cells and that each cell needs food in the sand plant activity
(Activity Fourteen). In this lesson they continue thinking about that idea and how it might happen and how
the idea of photosynthesis is connected to the idea that the plant is made up of cells.
In future lessons, students will connect this idea of food
transport back to the idea introduced in Activity Six that the cotyledon contains stored food. Students will be
able to explain that the stored food was made during photosynthesis and traveled through tubes or veins in the
plant to be stored in the seed. This lesson also lays the groundwork for thinking about food chain
connections which are introduced (but not explored in depth in this unit) in Activity Eighteen (Snickers
Science).
Advance Preparation:
Purchase and cut up plant parts that you will test. Test the iodine solution: Make sure that the iodine solution you
are using will produce a visible result with the plant materials you are going to use. Sometimes
iodine solutions we have ordered from Biological Supply companies lose their effectiveness over
time. Make overhead transparencies of the strawberry plant, the bean
plant, the banana and potato plants pages in the Blackline Masters.
NOTE: The iodine solution is poisonous if ingested. The antidote
for accidental ingestion to provide milk or a starch solution made with cornstarch or flour and to contact a
physician. Therefore, it is important to have some milk available. You may want to do this lesson as a
demonstration if you believe that your students are not ready to handle this chemical safely.
Materials: Milk for accidental ingestion of iodine.
For each group of students:
Iodine solution -- Iodine tincture from the pharmacy can be used
effectively. It can be diluted with water and distributed in dropper bottles
Styrofoam plates or plastic trays
Foods to test as a model of the chemical test: sugar cubes,
bread, saltine, others of your choice
Plant parts to test: bean seeds, raw peanuts, potato, banana,
pear, celery, others
Lots of paper towels and/or sponges for clean up work
FRAME
"What do you think happens to the food after it is made in
the leaf?"
Listen for students making connections (or not) to the sand plant
activity.
Read or talk about the ideas here.
Finish reading and discussing student pages.
Additional Teacher Background Information about Plant
Biochemistry:
(Source: The Really Useful Science Book: A Framework of Knowledge
for Teachers, by Steve Farrow)
In photosynthesis, the energy from sunlight is transferred into
the chemical bonds of glucose molecules, which can be:
Converted to sucrose, for transport elsewhere in the plant
Converted to starch for storage
Converted to cellulose, for cells walls (growth and support)
Used in cell respiration in each cell in the plant (oxygen
combines with food to release energy for use by the cell)
Used in protein or fat synthesis
Type of Food
Examples
Functions
Carbohydrates
glucose; starch
energy supply; plant cell walls
Building blocks of other molecules
Fats
oils in plants
energy supply (double the "energy per gram" of carbohydrates)
Proteins, enzymes and hormones regulate processes such as
photosynthesis and cell
respiration; phototropism (movement toward light), geotropism (growth in response to gravity), budding, root
initiation, fruit development, leaf fall, cell elongation, cell division.
Individual Drawing
Have the students draw how they think food moves after it is made
in the leaf of the strawberry plant.
Move around and examine their drawings as they are working to
assess how many students are using ideas from the reading to guide their thinking. Do not correct
them. Instead, ask clarifying questions such as:
"Tell me what you are thinking about as you draw these
arrows? Where do you think the food is coming from? Where is it going to? Why?
Whole Class Discussion:
After students are finished, show an overhead transparency of the
strawberry plant and have a student come up and explain how he or she drew the arrows and why.
"Did anyone else come up with a different way of drawing the
arrows? Show us. Why did you draw them this way?"
NOTE: Students who are successfully making connections at this
point will play a key teaching role. It is likely that YOU will not have to model for students how to use
photosynthesis as part of your thinking in drawing the arrows, because some of your students will do it for
you.
"Let's keep thinking about these different ideas as we do
some more experimenting with plants."
Anticipated Student Responses
Category 1: Some students may still be convinced food is taken
into the plant from the soil, and they will draw arrows moving into the roots and traveling up the plant..
These students may draw arrows to all parts of the plant and in their explanations show understanding of the
idea that plants are made of cells and that all cells need food.
Category 2: Some students may pick up on the idea that food made
in the leaves can be stored in different parts of the plant. They will draw arrows from the leaves to the
strawberry.
Category 3: Students who are connecting with the idea that the
plant is made of cells and that all cells need to get food will draw arrows from the leaf to all parts of
the plant.
ACTIVITY
Note: In this activity, the students test for starch rather than
sugar. The reason for this is simply that starch is a much easier substance for us to test. However, it introduces
a new complication to the explanation of photosynthesis -- it requires that we talk to students about how
plants can change the sugar it makes into different substances such as starch (fat, proteins also).
Use this section to explain the new idea that plants can not only
make sugar, they can also change that sugar into other forms of food such as starch.
Have students predict whether bread and sugar cubes are food.
Then model the iodine test, placing several drops on bread and
sugar cubes.
Explain that the bread has starch and that is why the bread
turned black wherever the iodine touched it.
"Do sugar cubes have starch in them?"
Clarify, if necessary, that both sugar and starch are
energy-containing food.
Model the use of the iodine with saltines.
CAUTIONS: Communicate to students the importance of careful use
of the iodine. It is a
poison and cannot be ingested. It will also stain clothing and
skin.
Use this section to introduce students to the experimental
procedures and to get them making
predictions.
Before you allow the student groups to start testing materials,
you might want to post and go over special rules for this experiment such as:
Food materials must stay on the trays or styrofoam plates.
Iodine is only to be used on plant materials that are on the test
trays/plates.
Procedures for taking turns using the iodine within the group.
Some helpful suggestions to give the students before they start
to work:
"Some of the materials you test may take longer than the
bread and saltine for the reaction to take place. Allow the iodine to sit on the plant part you are testing
for several minutes before you decide whether or not it has starch in it."
"Try testing and observing different parts of the seeds
(peanut shell, seed coat, embryo, cotyledon)"
Note: A careful test of the seeds will usually reveal starch in
the cotyledon (where the food is stored) and no starch in the embryo.
REFLECT AND CONNECT
Have the students work on the "Making Sense of the
Experiment" questions (#'s 4, 5, 6, & 7) in their groups.
EMPHASIZE
The importance of putting heads on their arrows to
indicate direction of movement (otherwise, you will see lines everywhere on the drawings and have no idea
where the student started her or his lines).
When students are ready, organize them for a whole class
discussion.
Have overhead transparencies ready of the various plants (bean
plant, banana plant, potato plant, celery plant).
"Let's compare results of our experiments. Which plant parts
did you find contained starch?"
"Where do you think that starch came from?
"Let's look at questions 2 and 3 on p. 59...." Listen
for continuing confusions, misconceptions, as well as for the desired student responses.
Ideal Student Responses:
2. Cells in the seed cannot make their own food, because plants
need carbon dioxide,
water, and light to make food and the cells in the seed cannot
get any light so they cannot
make their food.
3. Food stored in the peanut plant was made (in the form of
sugar) in the leaves during
photosynthesis. It then traveled through the tubes in the plant
to all parts of the plant
including the seed where it was stored in the form of starch.
Have the students take the lead in discussing questions 4 - 6 by
having them take turns being the teacher at the overhead projector. They will draw their arrows and
explain their reasoning to the class.
Ask the class to do coach their peer teachers: "Does anyone
have any ideas that could be added? Or are their some things you think need to be clarified or changed in
Rebecca's explanation?
Sometimes the celery testing works out really well and you can
see dark stains as shown in this drawing -- highlighting for students the veins that carry the food
throughout the plant. It also highlight that the celery stem is a food for us -- it does contain food material.
If your celery works out well, use this page to discuss the tubes
that carry food.
Additional Teacher Background Information
Source:
There are two kinds of transport tubes in plants -- xylem and
phloem. In xylem, water and dissolved minerals taken from the soil are transported upward. In phloem,
the products of photosynthesis are distributed from the leaves to the main stem, then up to the
growing tips and down to the roots. How phloem actually transports food is something of a mystery. In a
phloem tissue system, it is not uncommon to observe different substances moving in different directions --
at different rates!