- Science
- Classes 6-8
- 4 min read
How plants make food: photosynthesis
Find out how green leaves use sunlight, water and carbon dioxide to make food and give out oxygen: photosynthesis.
Let us begin
The big idea
Animals must find their food, but green plants make their own. They do this in their leaves, using sunlight. This process is called photosynthesis, and it also gives us the oxygen we breathe.
Photosynthesis means making food using light. It happens mainly in the leaves. The word comes from photo (light) and synthesis (putting together).
A plant needs three things: sunlight, water and carbon dioxide. The roots take in water from the soil, and it travels up the stem to the leaves. Carbon dioxide enters from the air through tiny holes on the leaf called stomata.
Leaves contain a green substance called chlorophyll. It traps the energy of sunlight. This energy joins water and carbon dioxide together to make glucose, a kind of sugar, and oxygen. The oxygen goes out into the air through the stomata.
In words: carbon dioxide + water → glucose + oxygen, in the presence of sunlight and chlorophyll.
Plants use some glucose for energy and to grow. The rest is stored as starch, in roots, seeds and fruits, like in potato and rice. Photosynthesis is important because nearly all food chains start with plants, and plants also add oxygen to the air.
Worked examples
See how it works
Chemical equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with sunlight and chlorophyll). Check the atoms. Left: C = 6, H = 12, O = 12 + 6 = 18. Right: C = 6, H = 12, O = 6 + 12 = 18. Both sides match, so it is balanced.
What does a plant need?
A plant kept in a dark cupboard for many days cannot make food well, because there is no light. Its leaves turn pale and it grows weak.
Where does each thing enter?
Water comes in through the roots. Carbon dioxide comes in through the stomata in the leaves. Sunlight falls on the leaves. Oxygen leaves through the stomata.
Try it at home
Seedlings in light and in the dark
You will need
- Moong (green gram) seeds, soaked overnight
- Two small cups with wet cotton wool or wet paper towel
- A bright windowsill
- A dark cupboard
- Water
What to do
- Put soaked seeds on the wet cotton in each cup.
- Place one cup on a sunny windowsill and the other inside a dark cupboard.
- Sprinkle a little water on both every day. Keep the cotton just wet, not soaking.
- After 5 to 7 days, take the dark-cupboard cup out and compare the two sets of seedlings.
- Which seedlings look greener and which look pale and thin? Write down why.
Ask a grown-up: Please help to keep the cotton damp, and wash the cups after the activity.
Fun fact
Plants and the green algae in oceans together make much of the oxygen in the air we breathe.
Practice sheet
Check what you learned
Practice sheet: How plants make food: photosynthesis (Classes 6-8, Science)
- What does the word photosynthesis mean?
- Name the three things a plant needs for photosynthesis.
- What is the green substance in leaves called?
- Which gas do plants take in for photosynthesis?
- Which gas do plants give out during photosynthesis?
- What are the tiny holes on a leaf called?
- Which part of the plant takes in water from the soil?
- Write the word equation of photosynthesis.
- Count the oxygen atoms on each side of 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Why do plants matter for all food chains?
Answer key
Answers: How plants make food: photosynthesis
- Making food using light.
- Sunlight, water and carbon dioxide (with chlorophyll in the leaf).
- Chlorophyll.
- Carbon dioxide.
- Oxygen.
- Stomata.
- The roots.
- Carbon dioxide + water → glucose + oxygen (in sunlight, with chlorophyll).
- Left: 6 × 2 + 6 × 1 = 18. Right: 6 + 6 × 2 = 18. Both sides have 18.
- Plants make food, so they are the producers on which all other living things depend.