Photosynthesis Explained

Plants make their own food

Animals have to find food and eat it. Plants do something amazing instead: they make their own food out of sunlight, air and water. This process is called photosynthesis — from the Greek words for "light" (photo) and "putting together" (synthesis).

Photosynthesis is one of the most important chemical reactions on Earth. Almost all life depends on it, either directly or indirectly.

The ingredients

To make food, a plant needs three things:

• Light energy — usually from the Sun.
• Water — taken up by the roots from the soil.
• Carbon dioxide — a gas taken from the air through tiny holes in the leaves called stomata.

If you have read about the parts of a plant, you already know that roots collect water and leaves collect light. Now you can see why those jobs matter so much.

The role of chlorophyll

Light energy cannot just be absorbed by anything. Plant cells contain tiny green structures called chloroplasts, and inside them is a green pigment called chlorophyll.

Chlorophyll absorbs red and blue light from sunlight but reflects green light — which is exactly why leaves look green to our eyes! The captured light energy powers the whole reaction.

What photosynthesis produces

Using light energy, the plant combines carbon dioxide and water to make two things:

• Glucose — a sugar that stores energy. The plant uses glucose to grow, or stores it (for example as starch in a potato).
• Oxygen — released into the air as a by-product.

We can write the whole process as a simple word equation:

carbon dioxide + water → glucose + oxygen (in the presence of light energy and chlorophyll)

So when you breathe in fresh air, much of that oxygen was made by plants, algae and tiny ocean organisms doing photosynthesis.

Why leaves are the perfect food factory

Look closely at a leaf and you will see it is well designed for the job:

• It is flat and wide to catch as much light as possible.
• It is thin, so gases can move in and out easily.
• It has veins that carry water in and sugars out.
• Its underside has stomata that open to let carbon dioxide in.

Why photosynthesis matters

Photosynthesis is the start of almost every food chain. Plants are called producers because they produce their own food. Animals that eat plants get that stored energy, and animals that eat those animals get it too. Without photosynthesis, food chains would collapse. You can explore this further in food chains and ecosystems.

Photosynthesis also keeps our air breathable by releasing oxygen and removing carbon dioxide.

Try it yourself: see photosynthesis at work

You can show that a leaf releases oxygen using just a glass and some water.

1. Find a leafy water plant (pondweed like Elodea works well, often sold for aquariums).
2. Place a few sprigs in a clear glass of water.
3. Put the glass in bright sunlight or under a bright lamp.
4. Watch the surface of the leaves for several minutes.

You should see tiny bubbles forming on the leaves and rising up. Those bubbles are oxygen — proof that photosynthesis is happening! Try moving the glass into the shade and you will see the bubbles slow down or stop, because less light means less photosynthesis.