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Oxygen cycle isdefined as the biogeochemical cycle by which oxygen is exchanged between the biosphere, lithosphere, atmosphere and hydrosphere components of the earth. It is the sequence in which oxygen from the atmosphere is used by living organisms through respiration and eventually released back into the atmosphere through photosynthesis. Photosynthesis is the main driving factor of the oxygen cycle, and without it, the oxygen cycle will cease to exist. The main way oxygen is lost from the atmosphere is through respiration and decomposition of dead plant and animal matter in which animal life consumes oxygen and releases carbondioxide in the process. Oxygen is second only to nitrogen in abundance among uncombined elements in the atmosphere. During the cycling of oxygen in the ecosystem, plants and animals use oxygen in the atmosphere to respire and they return the oxygen back to the atmosphere and water as carbon dioxide (CO2). CO2 is then taken up by algae and terrestrial green plants and later converted into carbohydrates during the process of photosynthesis (Figure 1).


Figure 1. Oxygen cycle.

Oxygen is critical to life because most living organisms cannot survive without a source of oxygen. All living organisms take in oxygen from the atmosphere through the process of respiration and releases carbondioxide which is taken up or utilized by plants to produce food. Plants carryout this process through photosynthesis and oxygen is released in the process. Oxygen is ubiquitous and very reactive in nature. It reacts so strongly with other elements in chemical reactions that significant amounts of energy is released in the process. But this energy released is not burned up because it is used by plants during photosynthesis. Oxygen, like carbon and hydrogen, is a basic element of life. In the form of ozone (O3), oxygen provides protection of life on earth by filtering out the sun’s ultraviolet (UV) rays as they enter the stratosphere. This helps to prevent unnecessary warming of the earth’s climate. The ozone layer absorbs harmful UV radiations from the atmospheres. Oxygen is also needed for the decomposition of organic waste in the ecosystem. Organic waste materials from living organisms are “biodegradable” in nature; and these materials are usually broken down by aerobic bacteria that convert these organic matters into stable inorganic materials.     

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