(a) Oxygen cycle: Oxygen form about 21% of all the atmospheric gases. It is also present in dissolved form in water bodies and helps in use survival of aquatic life. Oxygen is also present in combined forms, such as carbon dioxide (CO2) and water (H2O). It is also present in the combined form in the earth’s crust as the oxides of metals and silicon. Oxygen is an essential component of most biological molecules like proteins, carbohydrates, nucleic acids and fats. All living organisms, plants, animals and decomposers, take oxygen from their surroundings and utilise it for respiration. Oxygen also enters the plants as CO2 during photosynthesis, and into all the organisms in the form of water. Oxygen is utilised in decay and decomposition of dead organisms, and also in the burning of fuels, such as wood, coal and petroleum. Here, again, the by-products, CO2 and H2O are released into the atmosphere.
The green plants, in turn, utilise carbon dioxide and water to prepare carbohydrates through photosynthesis, releasing oxygen as a by-product. In fact, green plants are a major source of oxygen in the atmosphere.
(b) Replenishment: Photosynthesis is the source of replenishment of atmospheric oxygen. During photosynthesis, carbon dioxide is consumed along with water and oxygen gas is liberated. Oxygen gas is liberated during photosynthesis, comes from water. Thus, oxygen utilized in the form of H2O and CO2 is replenished back into the earth’s atmosphere.
