The Oxygen Cycle — Passage and Quiz

Oxygen is a gas that makes up about 21% of the air we breathe. It's essential for most life on Earth. The oxygen cycle shows how this important element moves through the air, water, and living things.

The main source of oxygen on Earth is plants. Through a process called photosynthesis, plants use sunlight to turn carbon dioxide and water into food. Oxygen is released as a byproduct. A single large tree can produce enough oxygen in a year to keep a family of four breathing!

Animals, including humans, need oxygen to live. When we breathe in, our lungs take in oxygen from the air. Our bodies use this oxygen to turn food into energy. When we breathe out, we release carbon dioxide.

Oxygen is also found in water. Fish and other water animals use their gills to take oxygen from the water. Some of this oxygen comes from plants growing in the water, like algae.

When plants and animals die, decomposers break them down. This process uses up oxygen. Forest fires and the burning of fossil fuels also use up oxygen and release carbon dioxide.

The oxygen cycle is closely linked to the carbon cycle. As plants produce oxygen, they take in carbon dioxide. As animals use oxygen, they release carbon dioxide. This balance helps keep our air breathable and our planet healthy.