Question
Sun November 27, 2011 By:
 

please explain the nitrogen,carbon and oxygen cycles in written form

Expert Reply
Fri December 02, 2011
Nitrogen cycle:

Nitrogen is a vital element found in all living organisms. It is a part of proteins, nucleic acids, some vitamins as well as alkaloids and urea. Though nitrogen gas makes up 78% of our atmosphere, most life forms cannot use atmospheric nitrogen directly.

However a few nitrogen fixing bacteria can fix atmospheric nitrogen into usable forms like nitrates and nitrites. These bacteria may be free living (eg. Azotobacter) or found mainly in root nodules of leguminous plants (eg. Rhizobium).

The high temperatures and pressures created in the air during lightning convert nitrogen into oxides of nitrogen, which then dissolve in water to give nitric and nitrous acids and fall on land along with rain. These can then be utilised by the living organisms.

Plants utilise nitrates and nitrites to produce amino acids, which are then used to form proteins. In addition, they also form nucleic acids and vitamins using the nitrates and nitrites. These complex compounds are then passed on to the consumers when they eat the plants.

When plants and animals die, their proteins are decomposed by certain microorganisms to ammonia. This process is called ammonification and is carried out by ammonifying bacteria like Bacillus vulgaris.

Certain soil bacteria oxidise the ammonia into nitrites and then into nitrates. This process is called nitrification and is carried out by nitrifying bacteria like Nitrosomonas and Nitrobacter.

Another type of anaerobic bacteria converts the nitrates and nitrites into elemental nitrogen in a process called denitrification. The bacteria carrying out this process is called denitrifying bacteria (eg. Pseudomonas species). This nitrogen gas is released back to atmosphere.
Thus the cycle in which nitrogen passes from its elemental form in the atmosphere into simple molecules in the soil and water, then to more complex molecules in living beings and back again to the simple atmospheric nitrogen molecule is called nitrogen cycle.
Carbon cycle:
Carbon is found elemental form as well as combined form on the Earth.
In combined state it is present as carbon dioxide in the atmosphere, as carbonate and hydrogen-carbonate salts in various minerals and as life molecules.

It is through photosynthesis (carried out by life-forms containing chlorophyll) that carbon gets incorporated into life-forms.  Photosynthesis converts carbon dioxide (from the atmosphere or dissolved in water) to glucose in the presence of sunlight by all life-forms that contain chlorophyll. These glucose molecules are either converted into other substances or used to provide energy to the organism for carrying out its life processes or building other molecules.

Energy is provided through the process of respiration, which converts glucose back into carbon dioxide. In addition, combustion of fuels to provide energy for various needs like heating, cooking, transportation and industrial processes also releases carbon dioxide to atmosphere. Today the percentage of carbon dioxide in the atmosphere has doubled since the industrial revolution. Thus carbon is cycled repeatedly through different forms by the various physical and biological activities.

Oxygen cycle -
Oxygen exists abundantly on Earth in elemental form as well as in combined form. In combined form it is present as oxides, carbonates, sulphates, nitrates in Earth’s crust, as carbon dioxide in air and as part of vital life molecules.

Oxygen from the atmosphere is used up in three processes, i.e. combustion, respiration and in the formation of nitrogen oxides. However, oxygen is returned back to the atmosphere in only one major process, that is, photosynthesis.

Oxygen from atmosphere is used by most living organisms during respiration, due to which carbon dioxide is released to atmosphere. Plants use carbon dioxide during photosynthesis and release oxygen to the atmosphere. Oxygen is essential for complete combustion. Oxygen is also utilized for the formation of ozone layer.
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