How do plants make their own food?

A food factory with no mouth

You have to eat to get energy. A gum tree never eats a thing, yet it can grow taller than a house. How? Plants make their own food inside their leaves, using light. The process is called photosynthesis, which means "putting together with light".

A plant needs three ingredients. It needs water, which its roots soak up from the soil. It needs carbon dioxide, a gas in the air, which drifts into the leaf through tiny holes called stomata, mostly on the underside of the leaf. And it needs sunlight to power the whole thing.

From those three ingredients the plant makes two things: sugar, which is its food, and oxygen, which it lets go into the air. That oxygen is the same gas you breathe to stay alive.

Catching the light

Inside the cells of a leaf are tiny green parts called chloroplasts. They are packed with a green substance called chlorophyll, and chlorophyll is very good at catching light.

Sunlight is a mix of all the colours of the rainbow. Chlorophyll soaks up the red light and the blue light, but it does not use the green. Green light bounces off the leaf or passes straight through it. That is why most leaves look green to us, and why the taro leaf in the photo glows green with the Sun behind it.

The light energy that chlorophyll catches is used to split water apart and join the pieces up with carbon dioxide. Scientists write the recipe like this: six carbon dioxide plus six water, with light energy, make one sugar (called glucose) plus six oxygen. The real process takes many small steps, but that is the sum of it.

What happens to the sugar

The plant uses glucose as its fuel. It breaks some of it down to power its cells, and it uses the energy to grow new leaves, longer stems, deeper roots, flowers, fruit and seeds.

When you eat an apple, a carrot or a slice of bread, you are eating energy that a plant captured from the Sun. When a kangaroo eats grass, or a shark eats a fish that ate smaller creatures that ate tiny ocean plants, the same thing is happening. Scientists say photosynthesis powers about 99 per cent of Earth's ecosystems.

Plants, algae (such as seaweed) and some kinds of bacteria, including cyanobacteria, can photosynthesise. Almost every other living thing, including you, depends on them for food.

The air you are breathing

Every time a plant makes sugar, it releases oxygen. Forests make a lot of it, but you might be surprised where so much of Earth's oxygen comes from. Scientists at NOAA, the United States ocean and weather agency, say about half of it comes from the ocean.

The ocean's oxygen is made by phytoplankton: tiny drifting plants, algae and bacteria that photosynthesise in the sunlit water near the surface. Like land plants, they have chlorophyll. Through photosynthesis, these tiny drifters soak up carbon dioxide on a scale that matches all the forests and other land plants put together.

So the next time you take a deep breath, thank a tree. Then thank the sea, which makes about half of the oxygen produced on Earth, though sea creatures use up about as much as it makes.

Why leaves turn yellow

Have you noticed some trees turning yellow and orange in autumn? Those colours were in the leaves all along. Leaves also contain yellow and orange pigments called carotenoids, but during spring and summer there is so much green chlorophyll that it hides them.

In autumn, the chlorophyll in those leaves breaks down and the green disappears. The yellows and oranges finally show through. Some trees also make new red and purple pigments called anthocyanins, which give maples and other trees their brightest colours.

Look for it in autumn on maples and other trees that drop their leaves for winter. What you are really seeing is a leaf's food factory closing down for the year.

📚 Words to know

Photosynthesis

The way plants, algae and some bacteria make sugar from carbon dioxide and water using light energy.

Chlorophyll

The green substance in leaves that catches light energy.

Chloroplast

A tiny green part inside a plant cell where photosynthesis happens.

Stomata

Tiny holes in a leaf that let carbon dioxide in and oxygen out. One on its own is called a stoma.

Glucose

A kind of sugar. It is the food that plants make for themselves.

Carbon dioxide

A gas in the air. Plants take it in, and you breathe it out.

🧪 Try this: hide a leaf from the Sun

1. Find a healthy plant outside, or a potted plant on a sunny windowsill. Ask before you use it. Choose one big leaf and leave it growing on the plant.
2. Cut a shape, like a star or your first initial, out of a piece of foil or thick black paper. Cover part of the leaf so your shape is in the dark, and clip or tape it on gently. Leave the rest of the leaf in the light.
3. After about a week, take the cover off. The covered part should look paler than the rest. Without light, the leaf could not keep up its green chlorophyll there.
4. Bonus: put a fresh green leaf in a clear glass of water and stand it in the sun. After an hour, look for tiny bubbles on the leaf. Some of that gas is oxygen made by photosynthesis, but some is just air coming out of the water as it warms. For a fair test, put an identical leaf in a glass in the shade and compare.