Q. 44.0( 92 Votes )

Explain the following cycles in your own words with suitable diagrams.

A. Carbon cycle.

B. Nitrogen cycle.

C. Oxygen cycle.

Answer :

A. Carbon cycle:


i. A major part of the carbon dioxide from the atmosphere enters into the living organisms through photosynthesis.


ii. The green plants make the organic compounds (carbohydrates).


iii. These compounds are passed from green plants to the consumers (herbivores and carnivores).


iv. During respiration, plants and animals release carbon in the form of carbon dioxide back to the atmosphere.


v. The dead bodies of plants and animals as well as the body wastes, which are also rich in carbon compounds, are broken down by decomposers such as bacteria and fungi to release carbon dioxide.


vi. Carbon is also recycled during the burning of fossil fuels such as coal, petroleum products.



B. Nitrogen cycle:


i. Approximately 78% of air is nitrogen and it is a part of many molecules essential to life like proteins, nucleic acids (DNA and RNA), ATP, and some vitamins.


ii. Neither plants nor animals can use atmospheric nitrogen. Plants or animals use nitrogen when it is fixed by nitrogen-fixing bacteria present in the root nodules of pulses plants.


iii. These organisms convert fixed nitrogen into ammonia. After that other bacteria convert ammonia into nitrate, in a process called nitrification. It is two steps process. In the first step, Nitrosomonas convert ammonia into nitrite. In the second step, Nitrobacter converts nitrite into nitrate. This nitrate is then consumed by plants.


iv. Thus, nitrate is the usable form of nitrogen. Through the process of denitrification by denitrifying bacteria converts nitrates nitrogen gas which is released into the atmosphere. This complete the cycle.



C. Oxygen cycle:


i. Oxygen from the atmosphere is used up in three processes. These are combustion, respiration and in the formation of oxides of nitrogen.


ii. Oxygen required for respiration in plants and animals enters the body directly from the surrounding medium (air and water).


iii. Oxygen is returned to the atmosphere by photosynthesis.


During photosynthesis oxygen is released in the form of molecular oxygen as a by-product. Thus, the cycle is completed.



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