Biogeochemical Cycles

key notes :

Biogeochemical Cycles

There is a constant interaction between biotic and abiotic components of a biosphere and it is a dynamic and stable system. There is a transfer of matter and energy between the different components of the biosphere.

Water Cycle

  • The whole process in which water evaporates and falls on the land as rain and later flows back into the sea via rivers is known as the water cycle. 
  • All of the water that falls on the land does not immediately flow back into the sea. Some of it seeps into the soil and becomes part of the underground reservoir of freshwater.
    Some of this underground water finds its way to the surface through springs or this is brought out to the surface for our use through wells or tube wells. 
  • Water is used by terrestrial animals and plants for various life processes.
The Water Cycle

As water flows through rocks containing soluble minerals, some of them get dissolved in water. Thus, rivers carry many nutrients from the land to the sea, and these are used by marine organisms.

Nitrogen Cycle

  • Nitrogen gas makes up 78% of our atmosphere. Nitrogen is an essential nutrient for all life forms. 
  • However, except for a few forms of bacteria, life-forms are not able to convert inert nitrogen from the air into nitrates and nitrites which could be taken up and used to make molecules like proteins, nucleic acids and vitamins etc. 
  • The nitrogen-fixing bacteria are found in the roots of legumes in special structures called root nodules. 
  • Another way nitrogen can be converted into nitrites and nitrates is by lightning when nitrogen and oxygen in the atmosphere under the influence of high temperature and pressure are converted into oxides of nitrogen. These oxides dissolve in water to give nitrous and nitric acids and fall on land along with rain.
  • The whole process in which elemental nitrogen of the atmosphere passes into the soil and water in the form of simple molecules which get converted to more complex molecules in a living being and go back to the atmosphere as simple nitrogen molecules are called the nitrogen cycle.
The Nitrogen Cycle

Processes involved in the Nitrogen Cycle:
(i) Ammonification: It is the process of decomposition of dead organic matter into ammonia. This is done by micro-organisms living in the soil e.g., decay bacteria and fungi.
(ii) Nitrification: It is the process of conversion of ammonia into nitrites and then into nitrates. This is done by nitrifying bacteria Nitrosomonas and Nitrobacter.
(iii) Denitrification: It is the process of reducing nitrates present in the soil to release nitrogen gas back into the atmosphere with the help of the bacteria Pseudomonas.

Try yourself:The process of nitrogen-fixation by bacteria does not take place in the presence of

  • a.molecular form of hydrogen
  • b.elemental form of oxygen
  • c.water
  • d.elemental form of nitrogen

Carbon Cycle

  • All life forms are based on carbon-containing molecules like proteins, carbohydrates, fats, etc. 
  • The endoskeletons and exoskeletons of various animals are also formed from carbonate salts.
  • Carbon gets included into the life-forms through the basic process of photosynthesis that takes place in the presence of sunlight by all life form that contains chlorophyll. 
  • In this process, the carbon dioxide from the atmosphere or dissolved in water gets converted into glucose molecules. These glucose molecules are used to provide energy for the synthesis of other biologically important molecules.
  • During respiration, glucose is oxidised to carbon dioxide again which goes to the atmosphere. 
  • Carbon dioxide is also added to the atmosphere by the combustion of fossil fuels. 
  • Carbon is thus cycled repeatedly through different forms by various physical and biological activities

Greenhouse Effect

  • It is an effect occurring in the atmosphere because of the presence of certain gases like carbon dioxide that absorb infrared (heat) radiations, thereby increasing the temperature worldwide.
  • A greenhouse is a structure to protect the plants from the cold atmosphere. The plants are kept in an enclosure. Heat rays from the Sun enter the greenhouse and are trapped there. 
  • Heat does not escape easily keeping the temperature warm inside. 
  • The carbon dioxide layer around the earth performs the same function. It traps the heat of the Sun and then does not allow it to escape. This results in raising the average temperature, that is, it results in warming of the atmosphere. 
  • Global warming that is taking place these days is the result of the greenhouse effect. There is a need to check the level of carbon dioxide in the atmosphere.

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