Oxygen cycle

Oxygen is one of the most common elements on Earth. It moves in a giant circle called the oxygen cycle.

This cycle moves oxygen through the air, water, and living things. It also moves through the lithosphere, which is the Earth's crust. Oxygen is found in many different forms. It can be in molecules like water or carbon dioxide. It also exists as a gas called diatomic oxygen. This gas is what we usually mean when we say oxygen.

The cycle is very important for all life. It keeps oxygen moving so it stays available for everyone.

There are many ways oxygen is made and used. One big way is through photosynthesis. This is a process where plants and tiny sea life make food. They use water and carbon dioxide to create sugars. During this, they release free oxygen into the air and water.

Another way oxygen is made is called photolysis. This happens when high-energy light from the sun hits the atmosphere. The light breaks apart water and nitrous oxide. This leaves oxygen behind while other parts escape into space. On the other side, oxygen is used up through respiration. This is how animals and bacteria breathe to live. Decay also uses up oxygen as things break down.

Scientists have studied how much oxygen is in different places. Most of the Earth's oxygen is actually hidden in the ground. The lithosphere holds about 99.5% of the total mass of oxygen. It stays inside minerals like silica and various oxides. The atmosphere, water, and living things hold very little. Together, they hold less than 0.05% of the total mass.

In the air, oxygen makes up about 21% by volume. In the hydrosphere, or water, it makes up about 33%. The biosphere, where living things are, holds about 22%. These numbers show how much oxygen is spread out.

History tells us how the world changed because of oxygen. Long ago, a major event happened called the Great Oxygenation Event. This was when bacteria began using oxygenic photosynthesis. This change helped allow bigger, more complex life to grow.

In 1986, scientists discovered a tiny sea bug called Prochlorococcus. This tiny organism is a huge part of the cycle. It can account for up to half of the photosynthesis in open oceans. In fact, ocean life is responsible for more than half of all oxygen production. This shows how much the sea helps the whole planet.

We can see the oxygen cycle working in our daily lives. When you breathe, you are part of the cycle. You use oxygen through respiration and release carbon dioxide. The plants around you are working to replace that oxygen.

Oxygen also helps create the ozone layer in the sky. Ozone is a type of oxygen called O3. This layer is very important because it protects us. It absorbs harmful ultraviolet radiation from the sun. Without this cycle and the ozone, life would be much harder. The cycle keeps our world in a steady balance.