Ecosystems — BIOLOGY STD 12 Science — Question

Gaseous Cycle:-

In this reservoir is the atmosphere and nutrient occurs in either gaseous or vapour form. These cycles are comparatively faster than others, e.g carbon cycle and nitrogen cycle.

Carbon cycle: Carbon constitutes 49% of dry weight of organisms and it is next only to water. About 71% carbon is found dissolved in oceans. This oceanic reservoir regulates the amount of carbon dioxide in the atmosphere. The fossil fuels also represent a reservoir of carbon. Therefore, carbon cycling occurs through atmosphere, ocean and through living and dead organisms. Approximately, 4 × 10kg of carbon is fixed in the biosphere through photosynthesis annually.

Carbon fixation or carbon assimilation: refers to the conversion process of inorganic carbon (carbon dioxide) into organic compound. Carbon dioxide is returned to the atmosphere via respiratory activities of producers and consumers. Decomposers also contribute substantially to CO₂ pool by their processing of waste materials and dead organic matter of land or oceans.

Sedimentary Cycle:-

In this reservoir is the earth's crust. Nutrient occurs in non-gaseous form. These cycles are comparatively slower than gaseous cycles, e.g. phosphorus cycle and sulphur cycle.

Phosphorus Cycle: Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems. Animal shells, bones and teeth all are made up of phosphorus only.

The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates. When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the plant roots and finally animals and herbivores obtain phosphorus from those plants. The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.