Photosynthesis: how plants eat

You need to eat to have energy, but what do plants eat? They have no mouth and they do not go to the supermarket, yet they grow and become huge. The secret is called photosynthesis, and it is one of the most amazing things in nature: plants make their own food using sunlight.

The three ingredients

To do photosynthesis, a plant needs three things. The first is sunlight, which its leaves catch. The second is water, which the roots absorb from the soil and which travels up the stem to the leaves. The third is carbon dioxide, a gas in the air that enters through tiny holes in the leaves called stomata.

The green kitchen: chloroplasts

Inside the leaves there are thousands of tiny “kitchens” called chloroplasts. They contain chlorophyll, a green substance that traps sunlight. That is why leaves are green. Using the energy of light, the plant mixes water and carbon dioxide and makes sugar, which it uses to grow and to make flowers and fruit.

The gift: oxygen

While making its sugar, the plant releases oxygen into the air. This is the gas that people and animals breathe. Without plants, algae and other living things that do photosynthesis, there would not be enough oxygen to live. That is why forests are called the “green lungs” of the planet, although a large share of the oxygen is also made by algae in the sea.

And at night?

Without light, photosynthesis stops. Plants stay alive and use some of the sugar they stored during the day, just as you use the energy from breakfast later in the afternoon.

Experiment: plants look for light

Plant some beans in two cups with damp cotton. Put one cup on a sunny windowsill and the other in a dark cupboard. After a few days you will see that the plant on the window is green and strong, while the one in the cupboard is pale and weak. Without light there is no photosynthesis, and without photosynthesis the plant cannot feed itself well. If you turn the cup on the window, you will also see the stem bend toward the light.

Why it matters to us

Almost all the food on the planet begins with a plant that did photosynthesis: bread, fruit, vegetables and even meat, because animals eat plants. Looking after forests, parks and gardens means looking after our food and the air we breathe.

💡 Algae in the sea make a big share of the planet's oxygen.