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Natural Resources

Tutormate > CBSE Syllabus-Class 9th Biology > Natural Resources

9 Oxygen Cycle

Oxygen plays an essential part of the life existence of the planet. It is found both in the earth’s crust and also in the atmosphere as carbon dioxide. Oxygen is an important element of the biomolecules like proteins, nucleic acids, etc. It makes around 65 percent of the total mass of the body around 30 percent of the earth and 21 percent of the atmosphere.


  • A large amount of oxygen is stored in the earth’s crust in the form of oxides but we cannot breathe this oxygen as it is in the combined state.
  • Phytoplankton is one of the biggest sources of oxygen that live near the ocean.
  • Fishes also breathe oxygen even under water; their gills extract oxygen from water.
  • Oxygen is also produced when the sunlight reacts with water vapor in the atmosphere.


Oxygen from the atmosphere is used up in the following four processes:

  • Combustion: It is also one of the most important processes which are just like burning for which oxygen, fuel, and heat is required. When anything burns it uses the oxygen and replaces it with carbon dioxide in case of hydrocarbons.
  • Respiration: It is the metabolic processes in which we inhale oxygen and exhale carbon dioxide. All living species, from a single bacteria, plants, and animals use oxygen for breathing. Oxygen enters the living world through respiration; it oxidizes the food materials and produces energy and carbon dioxide:
C6H12O6  + 6O2  6CO2 + 6H2O + Energy(Glucose)
  • In the formation of oxides of nitrogen or Rusting: It is the formation of oxides which is also called oxidation. Rusting is the best example of oxidation.  In this process, metals like iron or alloy rust when they are exposed to moisture and oxygen for a long period of time. New compounds of oxides are formed by the combination of oxygen with the metal.
  • Decomposition: When an animal dies, it decomposes and it uses all the oxygen and releases carbon dioxide.

Production of oxygen:

Oxygen is mainly created by plants by the process of photosynthesis.

  • Photosynthesis is a biological process by which all green plants synthesize their food in the presence of sunlight.
  • The carbon dioxide (CO2), sunlight and water is utilised by the plants to produce food materials during the process of photosynthesis.
6CO2 + 6H2O +Light Energy   C6H12O6 +6O2(Carbon   (water)                                    (Glucose)    (oxygen) dioxide)
  • The consequent free oxygen may now be used again in respiration or it may return to environment as molecular oxygen of atmosphere.


Oxygen cycle is a biological process which helps in maintaining the oxygen level by moving through three main regions of the earth which are Atmosphere, Lithosphere, and Biosphere. It is the biogeochemical cycle which explains the movement of oxygen gas within the atmosphere, the ecosystem biosphere, and the lithosphere.

Oxygen cycle along with carbon cycle and nitrogen cycle plays an important role in the existence of life on earth.


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