Photosynthesis

Photosynthesis is how plants, algae, and some kinds of bacteria use sunlight to make their own food. The word comes from two Greek words. "Photo" means light, and "synthesis" means putting together. Photosynthesis is the main way energy gets into all the living things on Earth.

In plants, photosynthesis happens inside the leaves. Inside each leaf are tiny parts called chloroplasts. Chloroplasts hold a green chemical called chlorophyll. Chlorophyll's job is to catch energy from sunlight. It is also what makes plants green.

To make food, a plant needs three things:

• Sunlight, caught by chlorophyll in the leaves
• Water, pulled up from the soil through the roots
• Carbon dioxide, a gas the plant takes in from the air through tiny holes in its leaves called stomata

The plant uses sunlight to mix the water and carbon dioxide together. This makes a kind of sugar called glucose. Glucose is the plant's food. The plant uses some of it right away for energy. It stores the rest, or uses it to build new leaves, stems, flowers, and fruit.

Photosynthesis also makes oxygen. When the plant breaks water apart to use it, oxygen is left over. The plant releases the oxygen into the air. This is the oxygen that people and animals breathe. Almost all the oxygen on Earth was made by photosynthesis.

Plants are not the only living things that do photosynthesis. Algae in the ocean do it too, including the huge kelp forests under the sea. Some kinds of bacteria, called cyanobacteria, also do photosynthesis. In fact, cyanobacteria were the first living things on Earth to do it, starting about 2.7 billion years ago. The oxygen they made slowly changed Earth's air so that animals could breathe.

Plants are also where almost every food chain starts. A rabbit eats grass. A fox eats the rabbit. The energy in the fox's body came from the sun, through the grass, through the rabbit, and finally to the fox. Without photosynthesis, there would be no food for animals to eat and no oxygen to breathe.