Biology ECOSYSTEM

Nutrients Cycle

Organisms are built up on chemical substances. They require certain chemicals like N2, O2, H2, P, C, etc. continuously for their survival. These chemicals enter the organisms from the environment and come out after undergoing changes or without changes. Thus these elements tend to circulate in a characteristic path from the environment to the organism and back to the environment. This cyclical path of the elements from the abiotic system to the biotic system and back is called biogeochemical cycles (Bio = living organism; Geo = water, air, earth). As these chemicals form the components of food, these cycles are also called nutrient cycles.

# (1) Phases of biogeochemical cycles : Each biogeochemical cycle has two phases, namely the biotic phase (organic phase) and the abiotic phase.
(i) Biotic phase : It refers to the flow of chemicals in the living organisms through food chain.
(ii) Abiotic phase : It refers to the distribution and flow of chemicals in the non-living environment.

# (2) Types of biogeochemical cycles : The biogeochemical cycles are classified into two types, namely gaseous cycles and sedimentary cycles.
(i) Gaseous cycles : In gaseous cycles the main reservoirs of chemicals are the atmosphere and ocean. e.g. Carbon cycle, N2 cycle, O2 cycle etc.
(ii) Sedimentary cycle : In sedimentary cycles the main reservoirs are soil and rocks. e.g., Sulphur cycle, phosphorus cycle, etc.

# (3) Important biogeochemical cycles
(i) Carbon cycle
(ii) Nitrogen cycle
(iii) Oxygen cycle (iv) Phosphorus cycle
(v) Sulphur cycle

Carbon Cycle

The cycling of carbon between biotic and abiotic systems is called carbon cycle. It is a gaseous cycle. The main source of carbon is the carbon dioxide (CO2). CO2 is present in the air and water. Air is the main reservoir. CO2 content of air is 0.03%. Its amount remains constant.

# (a) Flow of Carbon into the biotic system : Carbon flows into the biotic system in two ways :
 Photosynthesis : Carbon enters the biotic system through photosynthesis. In photosynthesis green plants utilize CO2 and incorporate the carbon of CO2 in glucose. Glucose is used for the synthesis of other types of carbohydrates, proteins and lipids. These compounds, containing carbon, are stored up in the plant tissues. When plants are eaten up by herbivores, the carbon flows into the body of herbivorous animals through food chain. When herbivores are eaten by carnivores, the carbon enters the body of carnivorus animals.
[6CO2 + 6H2O ---> C6H12O6 + 6O2.]

 Formation of shell : The CO2 dissolved in sea water is utillized by the marine animals like protozoans, corals, molluscs, algae, etc., for the construction of shell. In these animals CO2 is converted into calcium carbonate (CaCO3) which is used for the construction of shells.
CO2 + H2O --> H2CO3 (Carbonic acid)
H2CO3 --> H+ + HCO3 (Bicarbonate)
HCO3 + Ca+ --> H+ + CaCO3 (Calcium carbonate)

(b) Flow of Carbon into the abiotic system : The carbon of the biotic system flows into the abiotic system in five ways :

 Respiration : Plants and animals release CO2 by respiration (biological oxidation).
C6H12O6 --> CO2 + H2O + Energy

 Decomposition : When plants and animals die, the dead bodies are decomposed into CO2 by decomposers like bacteria, algae, etc.

 Shells : After the death of marine animals, CaCO3 stored in the shells is either deposited as sedimentary rocks or dissolved in water to release CO2 by the reversion of the above said reactions.

 Coal : A certain proportion of carbon from plants is deposited as coal. Carbon from coal returns to air in the form of CO2 through combustion and weathering.

 Forest fire : Combustion of wood in the forest, releases carbon from plants in the form of CO2.

Phosphorus cycle

The cycling of phosphorus between biotic and abiotic system is called phosphorus cycle. It is a sedimentary cycle. Phosphorus is an important mineral nutrient. The main source of phosphorus is rocks. Through erosion and weathering phosphorus is made available in the soil. Plants absorb ionic phosphate through roots. In plants it is incorporated into the protoplasmic components like DNA, RNA, AMP, ADP, ATP, GDP, GTP, NADP, phospholipids etc. from plants, it passes into herbivores and animals, the organic molecules containing phosphate are decomposed and phosphate is liberated as inorganic ion phosphate. It is again used by plants.
The excess of phosphate in the bodies of animals is excreted out through faces. The bird guano (excreta) contains a large amount of phosphate. Phosphate is also released to the soil through the combustion of forest trees and grasses. A large amount of phosphate is lost in the sea by sedimentation. A certain amount of phosphorus gets locked in bones and teeth.