Download CONCEPTS OF ELEMENTARY CYCLES

CONCEPTS OF ELEMENTARY CYCLES
Environmental interactions
 Every sphere of the environment interact with one
another for survival.
 Certain elements are required for the survival of all
living things e.g. C, N, O, P, etc
 Each has fixed quantity which are recycled continuously
within and among various components of the
environment
Cycles of matter
 Movement of matter through different spheres of the
environment (Biogeochemical cycles).
 Examples: carbon, nitrogen, sulphur, phosphorous
cycles etc
 Biogeochemical cycles can be endogenic or exogenic
Endogenic and Exogenic cycles
 Endogenic: These are cycles occurring at different sub
surface rocks below the earth’s surface (e.g.
phosphorous cycle)
 Exogenic: occurs on the surface of the earth (e.g. C,
N, S cycles)
THE CARBON CYCLE
 Carbon cycle shows the circulation of carbon in the environment
 Carbon is present in the atmosphere as gaseous CO2 in the atmosphere and HCO3and CO2(aq) in surface and ground water
 Occurs as minerals e.g. CaCO3 and MgCO3
 Fossil fuel (coal and petroleum).
 Photosynthesis (green plants) fix carbon as biological/organic carbon with the aid
of solar energy (carbohydrate – CH2O).
 During respiration, carbohydrate reacts with oxygen to produce CO2 which is
returned to the atmosphere and energy is released.
CH2O + O2 (g)
→
CO2(g) + H2O + Energy.
THE CARBON CYCLE…2
 Combustion of wood or fossil fuel releases CO2 into the
atmosphere.
2CH4 + 4O2 →
2CO2 + 4H2O
 Rock Formation Processes: C fixed in limestone, oil shale,
dolomite are released into the atmosphere as CO2 when
the minerals are broken down during weathering
 Micro organisms also play an important role in C – cycle.
CARBON CYCLE
Nitrogen Cycle
 Nitrogen: Essential component of protein.
 The atmosphere contains about 78% nitrogen
 Very stable but not available for use directly by most organisms in
the gaseous state.
 Can be converted to usable form through nitrogen fixation which
involves biochemical and atmospheric processes.
Nitrogen Cycle…2
 Biological fixation: This is made possible by nitrogen fixing
bacteria (Rhizobium) occurring freely in the soil or root of
leguminous plants
 These bacteria combine nitrogen gas with hydrogen to
produce ammonia.
N2(g) + 3H2(g) →
2NH3(g)
 The ammonia produced is further acted upon by another
specialized set of bacteria and converted to nitrates.
Nitrogen Cycle…3

Atmospheric Fixation: The electrical energy produced during lightening which occurs with
thunderstorm causes nitrogen to combine with atmospheric oxygen to produce nitrogen oxides
(NOx : NO, NO2 or N2O)
 When rain falls, No2 is dissolved to give nitric acid.
4NO2 +
2H2O + O2
→
4HNO3
 The nitrate obtained from both fixation processes are taken up by plant roots and transformed
into a number of organic compounds e.g. protein.
 The nitrogenous organic compound return to the soil when plants and animals die as NH4+
and nitrate.
3[CH2O] +
2N2 +
NH3 + 3/2O2
3H2O + 4H+
→
→ 3CO2 + 4NH4+
H + + NO2-
+ H2O
Nitrogen Cycle…4
 DENITRIFICATION: Bacteria e.g. bacillus metabolise
nitrogenous compounds with the assistance of nitrate
reductase enzyme to turn oxides of nitrogen to
nitrogen and water.
 occurs in deep sub strata of soil or areas of stagnant
water. This process depletes soil fertility.
2NO3- + 10e- + 12H+
→
N2 + 6H2O
OXYGEN CYCLE
 The atmosphere contains 20.8% oxygen.
 Oxygen occurs chemically bound to CO2, H2O and organic
matter.
 Becomes chemically bound by different energy yielding
processes like combustion and metabolic processes in
organisms.
 O2 is released during photosynthesis.
 It readily combines with and oxidizes other species e.g. C in
aerobic respiration or combustion of fossil fuels.
 Ozone, an isotope of oxygen in the atmosphere filters out
damaging UV radiation and protects life on earth.
Phosphorous Cycle
 An endogenous cycle
 Soil is the major contributor of phosphorous.
 Phosphorus is taken off from the soil in form of phosphate (sparingly
soluble mineral e.g. Ca3(PO4)2 ) from the root system of plants and
transported to the growing parts of the plants.
 This is then incorporated into a number of organic compounds e.g.
nucleic acids within the growing plants of the plant or animal.
 Mineralization of the biomass by microbial decay returns
phosphorous to salt solution from where it may precipitate as
mineral matter and taken up by plant roots again.
Phosphorous Cycle…2
 An endogenous cycle (Contd.)
 Large quantities of PO43- are extracted from PO43- minerals and bones to make
fertilizers and food additives.
 Phosphorous is a constituent of toxic compounds e.g. PH3– military nerve gas (tear
gas)
 Weathering of phosphorous containing rocks also release Phosphorous into the
environment.
 Dissolved P may sink and be deposited as sediment at the bottom of ocean and
transformed through geological processes to phosphate rocks.
SULPHUR CYCLE
 Involves several gaseous species, sparingly soluble minerals and
several species in solution (e.g. SO2, H2S)
 Others include SO42-, PbS, H2SO4 (acid rain) and biologically bound
sulphur in proteins.
Cysteine
HSCH2 – CH(NH2) – CO2H
Cystine
CH2S – S – CH2 CH(NH2)CO2H
Methionine
H2SCH2CH2 – CH(NH2) – CO2H