Grade by Grade
B. Life Systems · Growth and Changes in Plants

Photosynthesis: Plants Make Food from Sunlight

B2.5 A1.2About 35 minutes
Learning goals
  • I can explain that most plants get their energy directly from the Sun.
  • I can describe photosynthesis: plants take in carbon dioxide and water, use sunlight to make food, and give off oxygen.
  • I can explain why animals and people depend on photosynthesis.
  • I can do a fair test to show that plants need light to stay healthy.

Words to know

energy
What living things need to move, grow and do work. Food gives us energy.
photosynthesis
The way plants use sunlight to turn carbon dioxide and water into food (sugar). They give off oxygen.
carbon dioxide
A gas in the air. Plants take it in. Animals and people breathe it out.
oxygen
A gas in the air that animals and people need to breathe in. Plants give it off.
chlorophyll
The green material in leaves that catches sunlight.
sugar
The food a plant makes. It stores energy from the Sun.

Where does a plant's energy come from?

You get your energy from food. When you are hungry, you open the fridge. But a maple tree cannot open a fridge! It cannot walk to find food either.

So how does a tree get the energy to grow 30 metres tall? It gets it directly from the Sun. Most plants are like little solar-powered food factories.

Looking up through bright green maple leaves with sunlight shining through them against a blue sky.
Maple leaves spread out to catch as much sunlight as they can. Inside each leaf, light energy is used to make the tree's food. AI-generated image

Photosynthesis: making with light

The big word is photosynthesis (say: foh-toh-SIN-thuh-sis). It means "putting together with light".

Here is what happens inside a leaf:

  1. The leaf catches sunlight. The Sun's light gives the energy.
  2. Tiny openings in the leaf take in carbon dioxide from the air.
  3. The roots take in water, and the stem carries it up to the leaf.
  4. The leaf uses the light's energy to turn the carbon dioxide and water into sugar. Sugar is the plant's food.
  5. The leaf gives off oxygen into the air.
leaf makessugar (food)sunlightcarbon dioxide inwater from rootsoxygen out
Photosynthesis. Sunlight, carbon dioxide and water go into the leaf. The leaf makes sugar (food) and gives off oxygen.
Goes in (what the plant needs)Comes out (what the plant makes)
sunlight (energy)sugar (food for the plant)
carbon dioxide (from the air)oxygen (into the air)
water (from the roots)
The ingredients and the results of photosynthesis.

Why are leaves green?

Leaves have a green material called chlorophyll (say: KLOR-uh-fill). Chlorophyll catches sunlight. That is why leaves are usually the plant's food factories.

In the fall, Ontario's maple trees get ready for winter. The chlorophyll in their leaves breaks down, and other colours that were hiding, like yellow and orange, show through. The leaves stop making food and fall off. In spring, new green leaves grow and start making food again.

Why "most" plants?

Most plants make food with photosynthesis, but not all. In shady Ontario forests you might find ghost pipe, a small white plant with no green colour. It cannot make its own food from sunlight. It gets its food with the help of tiny living things called fungi in the soil.

Energy passes from plant to animal

The sugar a plant makes stores energy from the Sun. When an animal eats the plant, it gets that energy.

  • A deer eats leaves and twigs, so it gets energy that started in the Sun.
  • A wolf eats a deer. The wolf's energy also started in the Sun!
  • When you eat an apple, you get energy the apple tree caught from the Sun.

Also, every time you breathe in, you take in oxygen that plants gave off. Every time you breathe out, you give off carbon dioxide that plants can use. Plants and animals are a great team!

Worked example

Let's trace energy back to the Sun

Ava ate toast with strawberry jam for breakfast. Where did the energy in her breakfast come from at the very start?

  1. Toast is made from bread. Bread is made from flour, and flour comes from wheat seeds.
  2. Jam is made from strawberries.
  3. Wheat plants and strawberry plants made their food by photosynthesis.
  4. Photosynthesis uses energy from sunlight.

The energy came from the Sun. The wheat and strawberry plants caught sunlight and stored the energy in their food, and Ava ate it.

the Sunwheat andstrawberry plantstoast andstrawberry jamAvaeats breakfastenergy moves along the arrows
Ava's breakfast energy started in the Sun. Plants caught it by photosynthesis and stored it in their food.
Worked example

Let's explain an observation

A family left a wooden board on their lawn for two weeks. When they lifted it, the grass underneath was pale yellow and weak. The grass around it was green. Why?

  1. What was different for the grass under the board? It had no light.
  2. Without light, the grass could not do photosynthesis, so it could not make food.
  3. It used up its stored food and stopped making green chlorophyll, so it turned pale.

The grass under the board was missing light, so it could not make food by photosynthesis. Plants need sunlight to stay healthy. (If the board is removed, the grass usually turns green again.)

Just for fun

What if you could do photosynthesis? Lunchtime would mean standing in the sunshine for a while. But on a cloudy day, everyone would be hungry!

Hands-on activity

Light or no light? A fair test

You need: 2 small plants of the same kind and size (bean seedlings from your bean diary, or 2 pots of grass seed), the same pots and soil, a measuring cup, a ruler and a notebook.

  1. Question: Do plants need light to stay healthy?
  2. Predict: Write what you think will happen.
  3. Put Plant A on a sunny windowsill. Put Plant B in a dark cupboard in the same room (so the temperature is about the same).
  4. Give both plants the same amount of water (for example 30 mL) on the same days.
  5. Every 2 days for 2 weeks, record each plant's colour and height in cm. Draw them.
  6. Conclude: What happened? Did your results match your prediction? Explain using the word photosynthesis.

What to look for: Plant B usually turns pale yellow and gets weak or floppy (it may grow tall and thin as it reaches for light). Plant A stays green and sturdy. Bring Plant B back into the light afterward and watch it recover.

Grown-up note: Wash hands after handling soil.

Your turn

1

Where do most plants get their energy?

2

What does a plant take in from the air for photosynthesis?

3

What gas does a plant give off during photosynthesis?

4

What food does a plant make?

5

What is the green material in leaves that catches sunlight?

6

Name the three things a plant needs for photosynthesis.

7

Why do animals need plants to do photosynthesis?

More practice: warm-up

1

True or false: Plants make their own food.

2

Which part of the plant does most of the photosynthesis?

3

Which part brings water to the leaves?

4

What gas do you breathe in?

5

What gas do you breathe out?

More practice: on your own

1

Why do maple leaves change colour and fall in autumn?

2

A plant is kept in a dark closet for two weeks. What will probably happen? Why?

3

Explain how energy gets from the Sun to a moose.

4

What does the word photosynthesis mean?

5

Ghost pipe is white and has no chlorophyll. Can it make food from sunlight? How does it get food?

Challenge

1

A room has lots of house plants and some fish in a tank with water plants. Explain how the plants and animals help each other with gases.

2

Your lunch is a cheese sandwich. Trace the energy in the cheese back to the Sun.

3

Why do you think trees in a thick forest grow very tall, thin trunks before they spread out their branches?

Check your understanding

Quiz

1.

Where do most plants get their energy?

2.

In photosynthesis, a plant takes in...

3.

In photosynthesis, a plant gives off...

4.

What does chlorophyll do?

5.

A plant left in a dark closet for two weeks turns pale and weak. Why?

6.

A fox eats a rabbit that ate clover. Where did the fox's energy start?

For parents and teachers

Big idea (B2.5): Most plants get energy directly from the Sun through photosynthesis: they absorb carbon dioxide (and water) and, using light energy captured by chlorophyll, make sugar and release oxygen. All the energy in our food can be traced back to the Sun through plants. The light/dark fair test practises scientific experimentation (A1.2).

Simplifications we used: We do not mention that plants also use oxygen to break down their sugar (respiration), day and night. If your child asks, "Do plants breathe?", you can say, "Yes, plants use some oxygen too, but in the light they make much more than they use."

Common difficulties:

  • Thinking plants get their food from the soil. Repeat: soil gives water and nutrients; the food (sugar) is made in the leaves.
  • Mixing up which gas goes in and which comes out. Memory trick: "Plants breathe in what we breathe out."

How to help: At meals, play "Back to the Sun": pick a food and trace it back step by step. During the fair test, help your child keep the water and temperature the same for both plants.