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Biogeochemical Cycles

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Biogeochemical Cycles - Lesson Summary

Biogeochemical cycles
The cycling of chemicals between the biological and the geological world is called biogeochemical cycle.

The biotic and abiotic components of the biosphere constantly interact through biogeochemical cycles. During these interactions, there is a transfer of nutrients between living organisms and the non-living environment.
The four important biogeochemical cycles are water cycle, nitrogen cycle, carbon cycle and oxygen cycle.

Water cycle
The water cycle involves various steps like evaporation, transpiration, condensation and precipitation.
    •  When the water bodies are heated during the day, water enters the atmosphere as water vapour by the process of evaporation.
    •  There is another way in which water evaporates into the atmosphere. This happens through transpiration.
    •  The water vapours in the atmosphere changes to water droplets and collects to form clouds. This process is called condensation.
    •  Air currents move the clouds formed by condensation and carry them over the land, where they break into rain, snow or fog. This is called precipitation.

Nitrogen cycle
The sequence in which nitrogen passes from the atmosphere to the soil and organisms, and then is eventually released back into the atmosphere, is called the nitrogen cycle.
    •  Nitrogen makes up 78 percent of the earth’s atmosphere. The percentage of nitrogen in the atmosphere is maintained by nitrogen cycle.
    •  Nitrogen is an essential constituent of proteins, nucleic acids like DNA and RNA, vitamins, and chlorophyll.
    •  Plants and animals cannot utilise atmospheric nitrogen readily. It has to be fixed by some organisms called as nitrogen fixers.
    •  Nitrogen-fixing bacteria like Rhizobium live in symbiotic association in the root nodules of certain leguminous plants.. These bacteria convert atmospheric nitrogen into ammonia, which is utilised readily by plants.
    •  Nitrogen-fixing bacteria along with free living bacteria in the soil achieve 90 percent of nitrogen fixation.
    •  Lightning plays an important role in nitrogen fixation. When lightning occurs, the high temperature and pressure convert nitrogen and water into nitrates and nitrites.
    •  Nitrates and nitrites dissolve in water and are readily used by aquatic plants and animals.
    •  Ammonification is the process by which soil bacteria decompose dead organic matter and release ammonia into the soil.
    •  Nitrification is the process by which ammonia is converted into nitrites and nitrates.
    •  Denitrification is the process by which nitrates are converted into atmospheric nitrogen.

Carbon cycle
Carbon is cycled repeatedly through different forms by the various physical and biological activities constituting the carbon cycle.
Carbon cycle maintains the balance of the element carbon in the atmosphere. Carbon is found in various forms on the Earth. 
*Diamond and graphite found in the soil are made up of an element called carbon. 
*Carbon is present in the atmosphere as carbon dioxide. 
*Carbon can also occur as carbonates and bicarbonate salts in minerals. The endoskeletons and exoskeletons of various aquatic animals are also formed from carbonate salts.
*Carbon is an essential part of nutrients like carbohydrates, fats, proteins, nucleic acids and vitamins.
Carbon cycle maintains the amount of carbon in the atmosphere. The carbon cycle starts in plants. 
Step - 1
Plants, use carbon dioxide in the atmosphere, convert it into glucose in the presence of sunlight by the process of photosynthesis. Plants and animals break these carbohydrates for energy and release carbon dioxide through respiration.
Step - 2
When the plants and animals die, fungi and bacteria decompose the dead remains.  This releases the carbon in the remains as carbon dioxide. 
Step - 3
Some plants and animals which get burried in the soil under certain temperature and pressure over millions of years get transformed into fossil fuels. Coal and petroleum are some of the fossil fuels. On burning these fuels, carbon dioxide is released into the atmosphere. 

Oxygen cycle
The sequence in which oxygen from the atmosphere is used by organisms and eventually released back into the atmosphere through photosynthesis is called as oxygen cycle.
    •  Oxygen makes up 21 percent of the air. It is an essential constituent of carbohydrates, proteins, fats and nucleic acids. 
    •  Oxygen is found in air, in combined form as carbon dioxide, and in the earth’s crust as carbonates, sulphates and nitrates. 
    •  Plants and animals use atmospheric oxygen during respiration and release the same during photosynthesis. 
    •  Fossil fuels require oxygen for combustion. 
    •  The ozone layer is present in stratosphere, one of the layers of the atmosphere. Each molecule of ozone is made up of three oxygen atoms. The ozone layer prevents harmful radiations from reaching the earth’s surface, where they might damage life forms.

Solar radiation reaching the earth is reflected back into the atmosphere. Much of the heat is lost to the environment.  But some of the energy is trapped by the atmosphere thereby causing Greenhouse effect.


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