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

a. Carbon cycle.

b. Nitrogen cycle.

c. Oxygen cycle.

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Solution

a. Carbon cycle

Carbon enters life forms through the process of photosynthesis. During photosynthesis, carbon dioxide and water combine to produce glucose and oxygen. This changes the atmospheric carbon into glucose molecules.

Glucose, which is a source of food, is utilized by organisms to produce energy during respiration. During this process, glucose is broken down in the presence/absence of oxygen to produce carbon dioxide.

Another process that releases carbon dioxide is combustion or burning. Many substances release carbon dioxide on burning. Vehicular emissions, industrial fumes and the gases released during the process of cooking are some instances of release of carbon dioxide through combustion.

Thus, carbon is cycled repeatedly through different forms by the various physical and biological activities.

b. Nitrogen cycle

The nitrogen cycle involves conversion of nitrogen from its elemental form in the atmosphere into simpler molecules in the soil and water. These further get converted into more complex molecules in living beings. Finally, the complex molecules are reduced and released back into the atmosphere as simple nitrogen molecules.

Fixing of atmospheric nitrogen involve the following steps:
1. Nitrogen fixation: It is the process wherein atmospheric nitrogen is converted into water-soluble nitrates. This step is performed by organisms like Rhizobium, Azotobacter and blue-green algae.

N₂ NO₃⁻ or NH₄

2. Nitrification: It is the process in which ammonia is first converted into nitrites and then into nitrates. This step is performed by nitrifying bacteria like Nitrosomonas and Nitrobacter.
NH₃ NO₂⁻ NO₃⁻
3. Assimilation: In this step, NH₃ and NO₃ are incorporated into the biological tissues.

4. Ammonification: It is the process in which complex, dead organic matter is decomposed into ammonia. This step is performed by organisms like decay bacteria and fungi. Organic nitrogen compounds NH₄⁺

5. Denitrification: It is the process wherein the nitrates present in the soil are reduced to release nitrogen back into the atmosphere.

NO₃⁻ N₂

c. Oxygen cycle

Oxygen is an important component of life. We cannot survive without oxygen. It comprises about 21% of atmospheric air. It is also present in dissolved form in water bodies and helps in the survival of aquatic life. In combined form, it is found both in Earth’s crust and in the air. In the air, it occurs as carbon dioxide. In the crust, it is present as oxides of most metals and silicon and also as carbonate, sulphate, nitrate and other minerals.

Oxygen is also a part of several essential bio-molecules such as carbohydrates, proteins, nucleic acids and fats.
There are processes that utilize atmospheric oxygen. At the same time, the balance of oxygen in the atmosphere is also maintained.

Oxygen is utilized during respiration, combustion and formation of oxides of nitrogen.

Oxygen is returned to the atmosphere in only one major way, and that is photosynthesis.

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