The Oxygen Cycle

Oxygen is the gas you need to breathe to stay alive. The oxygen cycle describes how oxygen moves through nature. Plants make oxygen during photosynthesis. Animals breathe in oxygen and breathe out carbon dioxide. The carbon dioxide goes back to plants, and the cycle continues.

Making Oxygen

Plants, algae, and some bacteria produce oxygen during photosynthesis. They take in carbon dioxide and water and use sunlight to make food. Oxygen is released as a byproduct. About half of the world's oxygen comes from tiny ocean organisms called phytoplankton. The other half comes from trees and other land plants.

Using Oxygen

Animals breathe in oxygen and use it to turn food into energy. This process is called respiration. It produces carbon dioxide and water as byproducts. Oxygen is also used up when things burn or when metals rust. All of these processes return carbon dioxide to the air for plants to use again.

Fun Facts

• About 21 percent of the air is oxygen.
• The Amazon Rainforest produces about 6 percent of the world's oxygen.
• Oxygen was not always part of Earth's atmosphere. Bacteria started producing it about 2.4 billion years ago.

Did You Know?

When oxygen first appeared in Earth's atmosphere billions of years ago, it was actually toxic to most life forms at the time. The organisms that could use oxygen survived and evolved into all the air-breathing creatures alive today!