What is the oxygen cycle?

Oxygen cycle

The oxygen cycle is the circulation of the element in the biosphere. It is characterized by a series of stages and reactions in which living beings intervene, mainly plants. It is a vital biogeochemical cycle.

What is the oxygen cycle?

The oxygen is the third most abundant element in the universe and the most plentiful in the Earth ‘s crust. It is essential for life. Plus its high chemical reactivity means that it has to be constantly replenished by living beings. The stages and reactions that oxygen undergoes in the biosphere is known as the oxygen cycle.

  • What is the oxygen cycle?
  • features
  • Stages of the cycle
  • Oxygen cycle in the human body
  • Oxygen cycle in plants
  • Oxygen cycle disturbances
  • Consequences
  • Importance

What is the oxygen cycle?

The oxygen cycle refers to the circulation, transformation and resupply of the element among living beings on planet Earth. Humans and other animals breathe in diatomic oxygen (O2) – produced by plants – and exhale carbon dioxide (CO2). Carbon dioxide gas that plants use in the photosynthesis process for the production of energy and oxygen.

Only photosynthesizing organisms : plants, algae and cyanobacteria, have the ability to produce oxygen. They use energy from sunlight, water, and CO2. They release a part of carbon dioxide through cellular respiration and produce glucose and oxygen from photosynthesis.

The oxygen cycle is associated with other life cycles , such as the water and carbon cycles. When exhaling diatomic oxygen, humans also expel water vapor (H2O). The CO2 produced by living beings nourishes the troposphere, it is a carbon dioxide, composed of one carbon atom and two oxygen. And when humans or other heterotrophic beings die, they decompose and release small amounts of O2.


The oxygen cycle is a biogeochemical cycle , vital like the water, carbon and nitrogen cycles. It is characterized by biological, geological, chemical and physical processes that develop continuously from each other.

The atmosphere is its largest stage, it is composed of diatomic oxygen, also called dioxygen; H2O, carbon dioxide and ozone (O3). Oxygen also constitutes almost 90% of the hydrosphere. It makes up the earth’s core, the soil, ice masses, and living organisms.

The cycle begins with the photosynthesis process and the transformation of inorganic substances into organic matter, essential energy for all living beings.

Stages of the cycle

The oxygen cycle is made up of at least four main stages or phases:

  • Atmospheric phase: Oxygen makes up 21% of the air in the atmosphere, it is present in the form of gases whose amounts vary according to height. Ozone is its allotropic form, it is made up of molecules with three oxygen atoms, it is located at high altitude and protects the biosphere from ultraviolet rays. Dioxygen, a gas without color, odor, and taste, is produced at normal temperature and pressure. Carbon dioxide and water vapor or H2O are derived from the respiration of living things.
  • Photosynthesis phase: It is the starting point of the cycle. Thanks to its chlorophyll function, plants, algae and cyanobacteria absorb carbon dioxide from the air. In the chemical process, they synthesize CO2 and convert it into carbohydrates or carbohydrates, part of the body’s structure and the main source of energy. Carbohydrates undergo various reactions. The oxidation process that photosynthesizers use to transform inorganic substances into organic matter is a biochemical reaction .

Chloroplasts, organelles contained in the cytoplasm of eukaryotic cells, harbor stroma, composed of enzymes responsible for the transformation of inorganic matter.

When the primary electron donor is water, its combination with carbon dioxide to form glucose produces a surplus of oxygen, which is released into the atmosphere. This process is known as oxygenic photosynthesis. There is an anoxygenic photosynthesis, whose electron donor is hydrogen sulfide, not related to the oxygen cycle, because its product is sulfur.

  • Respiration phase : During the respiration process there is an exchange of gases between living beings and the atmospheric environment. The body breathes in oxygen, carries air to the lungs, and exhales (exhales) carbon dioxide.

In man and other mammals , the respiratory system is made up of the nostrils that connect with the larynx , trachea, and finally the lungs. It is in the lungs, in their alveoli, where the exchange of gases with the blood takes place. Oxygen spreads through the circulatory system and gas exchange occurs between blood and tissues, reaching cells throughout the body. The resulting carbon dioxide travels in reverse, passes from the cells to the blood, the lungs and is expelled from the body.

  • Return phase: This is the name given to the return of oxygen to the atmosphere. It returns as a residue from the respiration of living beings (carbon dioxide). Expiration or expulsion of air from the lungs occurs when the chest, ribs and diaphragm relax when they return to their natural state. The cavity torá cycad capacity decreases, intrapulmonary pressure increases and far from atmospheric pressure.

Oxygen cycle in the human body

Oxygen enters the human body through the nasal cavities. It passes through the larynx, pharynx and trachea, while the size of the thorax increases and the lungs let air in to equalize the pressure . In the process, the diaphragm (contracts and falls) and other muscles intervene, which even modify the position of the bones, specifically the ribs.

In the lungs, gases cross the wall of the surrounding blood capillaries and alveoli. A small amount of oxygen is dissolved in the blood, but the larger portion is mixed with hemoglobin to be transported and released in the tissues. Oxygen passes from the blood to the cells through the cell membrane. In this process, the cells reduce the amount of oxygen, they absorb it for their metabolic functions .

Carbon dioxide and water vapor are released from tissue cells and will be expelled during the passive phase of respiration. The wastes pass through the cell membrane into the blood, from the venous capillaries to the vena cavae, the heart and the lungs.

Unlike oxygen, a small fraction of carbon dioxide combines with hemoglobin to form deoxyhemoglobin, another dissolves in plasma, and more than 70% is transported in the form of carbon dioxide (carbon dioxide + water). Respiration is the process of transforming oxygen into carbon dioxide.

The transfer of oxygen in the human body is vital, it generates energy at the cellular level . Not surprisingly, the element is listed as one of the four most essential for life.

Oxygen cycle in plants

The oxygen cycle in plants is key in the global process. Plants, algae, and bacteria produce oxygen that accumulates in the atmosphere and is used by living things for their vital functions.

Through the process of photosynthesis, they transform the energy from sunlight into chemical energy. They use water and carbon dioxide to – through cellular respiration and fermentation – create molecules of glucose and other sugars. These molecules supply cells with immediate energy . When they incorporate inorganic carbon (carbon dioxide) into organic molecules, they fix carbon that contributes to the formation of other necessary molecules in the body.

In addition to fixing carbon and introducing energy into ecosystems, photosynthesis releases oxygen as a by-product, affecting the composition of the Earth’s atmosphere . The whole process happens in the leaves, at least in most green plants.

Carbon dioxide enters through the stomata or pores of the leaves, and is irrigated to the mesophyll, the intermediate tissue. The mesophyll cells are composed of chloroplasts and these, in turn, by structures called thylakoids.

Chlorophyll is in the thylakoid membranes, which is responsible for absorbing sunlight. Light energy to be converted into chemical energy thanks to the formation of ATP and NADPH . The first, an energy reserve molecule . The second, a reduced electron carrier. In this light-dependent process, the water molecules turn into oxygen gas.

The production of glucose does not need direct light, it takes place in the stroma. It is part of the Calvin cycle.

Oxygen cycle disturbances

Like other biogeochemical cycles, the oxygen cycle is prone to alterations, especially caused by human action. The ozone layer is deteriorated by the exposure of nitrogen oxides, used in fertilizers. Also the use of halons, methyl chloroform and chlorofluorocarbons , among other polluting chemical compounds.

Man is also responsible for the destruction of extensive plant areas, sources of oxygen.


The use of harmful chemical compounds deteriorates the ozone layer , bringing with it increasingly pronounced global warming. Along with deforestation and other reckless actions of man, the decomposition of the layer points towards:

  • The accumulation of greenhouse gases , produced by industries, vehicles and the burning of fossil fuels .
  • The disappearance of photosynthetic organisms , which implies the depletion of O2 sources and the extinction of marine species.
  • Loss of biodiversity , destruction of glaciers, droughts, floods and other ecological disasters.

The increase in the population demands a greater consumption of oxygen, how could it be accessed in toxic conditions.


Life on Earth depends on the balance of the oxygen cycle. In its natural form the element is highly reactive, oxidizing, and can be toxic. The participation of living beings is required for the circulation, transformation and constant supply of oxygen to the biosphere .

The cycle provides energy to organisms, which through cellular metabolisms take advantage of the energy for their vital functions. Oxygen makes up all organic molecules and also water . If this is depleted, there would be no supply of water and other vital resources.

The alteration of the oxygen cycle would cause damage to the different ecosystems .

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