Oxygen cycle

The air has things we need.

Plants make the air we breathe. They use water and air to make food. This also makes the air.

Tiny life in the sea helps too. Most of this air comes from life. Some comes from light in the sky.

Animals and tiny bugs use the air. They breathe it to live. This uses up some of the air.

Most of the Earth's oxygen is in the ground. It is in the rocks and dirt. The air keeps us going.

Oxygen moves all around our planet. This movement is called the oxygen cycle.

Oxygen is found in the air, water, and living things. Most oxygen is actually in the ground. It stays in rocks and the Earth's crust. The air we breathe has 21% oxygen.

Life helps make the oxygen we need. Plants on land use a way called photosynthesis. They take in water and carbon dioxide. This makes food and lets out oxygen. Tiny life in the sea helps too. A small sea bug called Prochlorococcus makes much of it. In fact, ocean life makes over half of Earth's oxygen.

Light from the sun also helps. High energy light can break apart water in the air. This is called photolysis. It leaves extra oxygen behind.

Oxygen is used up in other ways. Animals use it for respiration. This is how they breathe to live. Tiny bugs also use oxygen when things decay. This takes oxygen out of the air. The cycle keeps things moving so life can continue.

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.

The oxygen cycle is a complex biogeochemical cycle. This cycle describes how oxygen atoms move through Earth's different systems. These systems are the atmosphere, the biosphere, the hydrosphere, and the lithosphere. Oxygen changes its state during these movements. It moves between different oxidation states in ions, oxides, and molecules. This happens through chemical processes called redox reactions. These reactions occur both within and between the various reservoirs of the planet.

Oxygen is one of the most common elements on Earth. However, most of it is not in the air we breathe. The largest reservoir is actually the lithosphere, which is the Earth's crust and mantle. The lithosphere contains about 99.5% of the Earth's total mass of oxygen. Most of this oxygen is found in silicate and oxide minerals. In contrast, the atmosphere, hydrosphere, and biosphere hold less than 0.05% of the total mass. Within the atmosphere, oxygen makes up 21% by volume. In the hydrosphere, it accounts for 33%. The biosphere contains about 22% oxygen by volume.

There are several ways that free oxygen, or O2, is produced. The primary biological source is oxygenic photosynthesis. During this process, organisms use carbon dioxide and water to create sugars. This reaction releases free oxygen as a byproduct. Photosynthesizing organisms include land plants and marine phytoplankton. A tiny marine bacterium called Prochlorococcus is very important here. Discovered in 1986, it can account for half of the photosynthesis in open oceans. In fact, ocean life produces more than half of all oxygen on Earth.

Oxygen is also produced through an abiotic process called photolysis. This is a non-biological process driven by light. High-energy ultraviolet radiation from the sun hits the atmosphere. This radiation breaks down water and nitrous oxide into separate atoms. The hydrogen and nitrogen atoms escape into space. This leaves the oxygen behind in the atmosphere. This process adds a small but steady amount of oxygen to the planet.

While oxygen is produced, it is also consumed by various sinks. The main biological sink is through respiration and decay. During aerobic respiration, animals and bacteria consume oxygen. They release carbon dioxide as a result. Decay also uses oxygen as organic matter breaks down. Other losses occur through microbial oxidation and chemical weathering. Even human activities act as a sink. The combustion of fossil fuels is an anthropogenic, or human-caused, source of oxygen loss.

Scientists track these movements using measurements of capacity and flux. Flux refers to the amount of oxygen moving in or out of a reservoir each year. For example, photosynthesis on land adds about 16,500 units of 10^10 kg of O2 annually. Ocean photosynthesis adds about 13,500 units. On the loss side, aerobic respiration removes about 23,000 units. These massive numbers show how active the cycle is. The cycle stays in a delicate balance of gains and losses.

The history of oxygen changed the course of life on Earth. Long ago, the Great Oxygenation Event occurred. This was when bacteria evolved oxygenic photosynthesis. This change created the conditions needed for complex eukaryotic metabolism. This means it allowed for more advanced life forms to develop. Oxygen also leads to the formation of the ozone layer. Ozone is a molecule made of three oxygen atoms, known as O3. This layer sits in the stratosphere. It is vital because it absorbs harmful ultraviolet radiation from the sun.

The oxygen cycle connects many different scientific fields. It links biology, through photosynthesis, with geology, through the movement of minerals in the lithosphere. It also involves chemistry through the redox reactions that move atoms. Understanding this cycle helps us see how the atmosphere, water, and living things depend on each other. It shows how a single element can sustain the entire planet's complexity.