Question

Natural reservoir of phosphorus is
A. Sea water
B. Animal bones
C. Sedimentary rocks
D. Fossils

Answer 1

Phosphorous is a major constituent of nucleic acid, cell membrane, cellular energy transfer system, bones and teeth in living organisms. It is present in the plants and animals of different trophic levels in ecosystems through food chains.

Complete answer:
Phosphorus has a natural reservoir in the rocks as phosphates.
Phosphorus present in the insoluble form in soil is converted into soluble form by chemical secretions of micro-organisms and plant roots.
The dissolved phosphate is absorbed by plants and is used to build organic compounds like phospholipids, nucleotides, nucleic acids, etc.
The phosphates in the rocks dissolve in soil solution during the weathering of rocks and plants absorb this from the soil. Other animals and herbivores then obtain it from the plants.
The phosphorus present in the plants and animals in organic form is returned to the soil in inorganic form through decomposition of excreta of animals and dead remains of plants and animals by microorganisms, to be reused by plants.
The phosphorus present in bones and teeth of animals is resistant to decay and thus remains out of the cycle for a long time.
Sea water, animal bones and fossils are the reservoirs for the carbon cycle. Carbon is the constituent of all organic compounds of the cell such as proteins, carbohydrates, etc. It is present in the atmosphere as $C{O_2}$, in the hydrosphere as dissolved $C{O_2}$ and in the lithosphere as fossil fuels or carbonates and graphite in rocks.

Note: There is no respiratory release of phosphorus into the atmosphere. Also the atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs. The gaseous exchange of phosphorus between organisms and the environment is negligible.