Student's Pages
Activity 11
"How Plants Use Sunlight to Make Their Own Food"
Activity 11 - Teachers Page
Is Sunlight Food for Plants?
The grass seeds in the light continued to grow after the food in
their cotyledons ran out. The plants need light to live and grow. They will die if they do not have light. Does this mean
that sunlight is their food?
To find out if light is food for plants, you need to know more
about plants than we can observe in this experiment. A good explanation of why the plants in the light live and why the
plants in the dark die must tell us more than what happened. It must tell us more than "plants need light." We want to
know why the plants need light. What do they do with the light?
Is it
their food?
Scientists have developed good explanations for these
observations. They have done many complicated experiments, and they have done lots of thinking and imagining about what goes
on inside of plants. What happens to light when it goes into the leaves of plants? Scientists have found that the
sun is not eaten or digested by the plant. It is not food for plants.
BUT sunlight does have something to do with food for plants.
Scientists have found that plants are able to do something amazing with the energy that comes from the sun. No humans or
animals are able to do what plants can do. Neither worms, fish, birds, monkeys, nor people can do what plants can do
with the sun's light energy.
Plants can use the light from the sun to make their own food
inside their leaves. They do not have to go out and catch or buy their food like people do. Read the next section to find out
how plants make food inside their leaves.
Plants are the only living things that can make their own
energy-containing food out of water and air. Water and air??!
How can plants turn water and air into food that they eat, food
that we eat?
The secret is the sunlight. Green plants have a special substance
in their leaves that makes them green. This substance is special because it can trap the sun's light energy. It is
called chlorophyll. After the sunlight is trapped by the
chlorophyll, the plant can do something quite amazing -- it can use that
energy to transform air and water into energy-containing sugars.
To make their own energy-containing food, plants need three
things: water, sunlight, and carbon dioxide from the air. As you read, label the picture to show where each of these three
things enters the plant. Then think about what happens to each of these three things when the plants turns them into food.
The sunlight is soaked into the leaves. A green substance in the
leaf cells (called chlorophyll) is able to trap the sun's energy in the leaf. Plants take in water from the soil. The water
travels from the roots up tubes inside the plant. The water then goes up the stem. Finally, the water reaches the cells in
the leaf. Carbon dioxide goes into the leaves through tiny holes in the leaf (they are so tiny that you need a microscope to
be able to see them!). When sunlight, water, and carbon dioxide are in the leaf cells, the plant can begin making food in
the leaves.
The leaf cells use energy from the sun to mix water and air
together to make a totally new material out of them -- food! This food is not water. It is not air. It is made out of water and
air, but it is a new material. It contains energy that living
things can use to live and grow. The food it makes is in the form of
sugar.
The way that plants make food is called photosynthesis.
"Photo" means light and "synthesis" meaning
putting together. Photosynthesis is using light energy to put together air and
water to make food energy.
ONLY plants are able to use sunlight to make their own food
inside their bodies. Imagine if people could make food that way! All we would have to do when we were hungry is to get plenty
of air and water and to stand in the sun. No more trips to the grocery store or to McDonald's!
The ONLY way plants can get food is to make it for themselves.
They do not get food from the soil, or from water, or from the grocery store. They get all the food they need by making it
themselves.
REFLECT AND CONNECT:
Using the Idea of Photosynthesis?
1. Read again your beginning ideas about how plants get their
food.
Do your beginning ideas say the same thing that you just read
about?
How were your ideas different from what you just read about?
Now try using the idea of photosynthesis to explain how plants
get their food:
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2. Think about our grass plant experiment. Now try using the idea
of plants making their own food to explain why the plants lived in the light but died in the dark.
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3. Read a description of photosynthesis from another book.
Tell one new idea you got about photosynthesis from reading this
explanation.
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Of the two explanations you read, which one makes more sense to
you? Why?
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4. Use the idea of photosynthesis to explain why the soil in Van
Helmont's bucket did not change even though the tree gained 164 pounds. How did the tree gain all that
weight?
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Photosynthesis and our Bean Experiment
We just said that plants can only get their food by making it
themselves. But you may remember another way we thought that plants can get food. We said that young plants (embryos) can
get food from their cotyledons (seeds). Isn't that a different way that plants can get food? No, not really! How do
you think the food got into the seed?
The food in the seed did not come from the soil or the water or
the air. The food in the seed's cotyledon was MADE by the parent plant by combining water and air using sun energy. The
parent plant can make more food than it needs. Some of the extra food is stored in the seed. So when the seed falls off
the parent plant, it has some food ready for the new embryo (baby plant).
REFLECT AND CONNECT:
Putting Together Two Big Words -- The Cotyledon and
Photosynthesis
1. What story does this cartoon tell about how plants get their
food?
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2. Explain why the seed needs food in the cotyledon to start
growing. Why can't the embryo in the seed just make its own food?
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Did that question stump you? If so, think about the three things
a plants needs to make food. Does the seed in the soil have all three of these things?
The Amazing Leaf's Secret
The green leaf is the only thing in the world that can make food
out of air and water. Scientists have tried to discover and copy the leaf's secret. They have tried to make food in a
laboratory using water and air and light. But the leaf is very complicated. People have not been able to steal the leaf's
secret. People have built bakeries that can take food made by the plant (like wheat) and CHANGE it into many different, fancy
kinds of food -- bread, doughnuts, chocolate chip cookies.
But no human bakery can make food out of carbon dioxide and
water! We have not found out the secret of how to take sunlight energy and change it into food energy. ONLY the plants
have the secret. Without plants, there would be no food in the world.
Check your Understanding
What things go into the leaf?
Can it provide energy to living things (in its current form)?
Is it food for the plant?
Is it needed to make food?
Putting it Altogether -- Food for Plants
We have seen that food for plants that has energy in it does not
come from the soil, or from the water, or from fertilizer, or from minerals. Plants do not take in food from their environment.
Instead, plants have an amazing ability to use energy from the
sun to change water and air into food energy. As long as plants have light, air, and water, they can make their own food
inside their leaves. This complicated way of making food is called photosynthesis. If plants do not have light, they cannot
make food and will die of starvation. They cannot make food
in the dark. They can only get food by making it in their leaves
in the light.
Now try using these ideas to explain the following situations:
1. When a plant is dry and wilted, what do you do to help it?
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Does this mean that the water is food for the plant?___________
Explain.
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2. Cows make milk that we drink. And milk contains food energy
that we can use to live and grow. But we said that only plants can make food!
Can a cow make food out of air and water like plants? _________
Where does the cow get the food energy it needs to make milk?
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The first step in making milk is for cows to eat plants!