Unit 2
Q1. Describe the following:
(a) Sulphur cycle
(b) Role of bacteria in nitrogen fixation
(a) Sulphur Cycle
The sulphur cycle is a biogeochemical cycle through which sulphur moves
between the atmosphere, lithosphere, hydrosphere, and biosphere. Sulphur is an
essential element for all living organisms as it is a component of amino acids like
cysteine and methionine, vitamins, and coenzymes.
Major steps of the sulphur cycle:
1. Weathering of Rocks:
Sulphur is mainly found in rocks and minerals (like pyrite - FeS2).
Through weathering, sulphur is released as sulphate (SO4^2−) into the soil
and water.
2. Absorption by Plants:
Plants absorb sulphur from the soil in the form of sulphate ions (SO4^2−).
It is then incorporated into organic molecules like proteins.
3. Consumption by Animals:
Animals obtain sulphur by consuming plant material.
It becomes part of their body proteins and other sulphur-containing
compounds.
4. Decomposition:
When plants and animals die, decomposers (bacteria and fungi) break
down the organic compounds, releasing sulphur as hydrogen sulphide
(H2S) or sulphate back into the soil.
Unit 2 1
, 5. Microbial Transformation:
Desulphurating bacteria convert organic sulphur into H2S.
Sulphur-oxidizing bacteria (like Thiobacillus) convert H2S or elemental
sulphur into sulphate (SO4^2−).
Anaerobic bacteria may reduce sulphates into H2S (sulphate reduction),
especially in swamps and sediments.
6. Atmospheric Phase:
H2S gas can be released into the atmosphere from natural sources
(volcanoes, decaying matter).
In the atmosphere, H2S is oxidized to sulphur dioxide (SO2) and eventually
to sulphur trioxide (SO3), which combines with water to form sulphuric
acid (H2SO4).
This returns to the earth via acid rain.
Equation Example:
SO2 + H2O → H2SO3 (sulphurous acid)
SO3 + H2O → H2SO4 (sulphuric acid)
(b) Role of Bacteria in Nitrogen Fixation
Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted
into a usable form like ammonia (NH3) by certain bacteria. Nitrogen is essential for
living organisms as it is a key component of proteins, nucleic acids, and
chlorophyll.
Types of Nitrogen-Fixing Bacteria:
1. Free-living (non-symbiotic) bacteria:
Examples: Azotobacter (aerobic), Clostridium (anaerobic), Cyanobacteria
(e.g., Anabaena, Nostoc).
These bacteria live freely in the soil and fix atmospheric nitrogen without a
host.
2. Symbiotic bacteria:
Unit 2 2
, Example: Rhizobium species.
These form symbiotic relationships with leguminous plants, living in root
nodules.
The plant provides carbohydrates and an anaerobic environment; bacteria
provide ammonia.
Process of Nitrogen Fixation:
Bacteria use the enzyme nitrogenase to convert atmospheric nitrogen (N2)
into ammonia (NH3).
Nitrogenase is very sensitive to oxygen and functions only under anaerobic
conditions.
Chemical Equation:
N2 + 8H+ + 8e− + 16ATP → 2NH3 + H2 + 16ADP + 16Pi
The ammonia is then converted to ammonium (NH4+) and absorbed by plants
to synthesize amino acids.
Importance:
Enriches soil fertility.
Reduces dependency on chemical fertilizers.
Essential for sustainable agriculture.
Conclusion:
Both the sulphur and nitrogen cycles are crucial for maintaining ecosystem
balance. Bacteria play a key role in recycling nutrients, especially in nitrogen
fixation, making essential elements available to plants and thus supporting the
entire food chain.
Q2. Write notes on:
(i) Phosphorus cycle
Unit 2 3
, (ii) Fungi as soil microflora
(i) Phosphorus Cycle
The phosphorus cycle is a biogeochemical cycle that describes the movement of
phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike the
nitrogen or carbon cycles, the phosphorus cycle does not involve a gaseous
phase under normal conditions. Phosphorus is a crucial element for living
organisms as it is a component of DNA, RNA, ATP, and phospholipids in cell
membranes.
Key Steps in the Phosphorus Cycle:
1. Weathering of Rocks:
Phosphorus is mostly found in phosphate rocks (e.g., apatite).
Through weathering, these rocks release inorganic phosphate ions
(PO4^3-) into the soil and water.
2. Absorption by Plants:
Plants absorb phosphate ions from the soil, which are then incorporated
into organic molecules such as nucleic acids and ATP.
3. Movement Through Food Chain:
When animals consume plants, phosphorus moves through the food chain.
It becomes part of animal tissues like bones and teeth (as calcium
phosphate).
4. Return to Soil:
Phosphorus returns to the soil through excretion (urine and feces) and
decomposition of plants and animals.
5. Sedimentation:
In aquatic systems, some phosphate settles and forms sedimentary rocks
over long periods.
This phosphorus can be uplifted to the surface by geological processes
like tectonic movements.
6. Human Impact:
Unit 2 4
Q1. Describe the following:
(a) Sulphur cycle
(b) Role of bacteria in nitrogen fixation
(a) Sulphur Cycle
The sulphur cycle is a biogeochemical cycle through which sulphur moves
between the atmosphere, lithosphere, hydrosphere, and biosphere. Sulphur is an
essential element for all living organisms as it is a component of amino acids like
cysteine and methionine, vitamins, and coenzymes.
Major steps of the sulphur cycle:
1. Weathering of Rocks:
Sulphur is mainly found in rocks and minerals (like pyrite - FeS2).
Through weathering, sulphur is released as sulphate (SO4^2−) into the soil
and water.
2. Absorption by Plants:
Plants absorb sulphur from the soil in the form of sulphate ions (SO4^2−).
It is then incorporated into organic molecules like proteins.
3. Consumption by Animals:
Animals obtain sulphur by consuming plant material.
It becomes part of their body proteins and other sulphur-containing
compounds.
4. Decomposition:
When plants and animals die, decomposers (bacteria and fungi) break
down the organic compounds, releasing sulphur as hydrogen sulphide
(H2S) or sulphate back into the soil.
Unit 2 1
, 5. Microbial Transformation:
Desulphurating bacteria convert organic sulphur into H2S.
Sulphur-oxidizing bacteria (like Thiobacillus) convert H2S or elemental
sulphur into sulphate (SO4^2−).
Anaerobic bacteria may reduce sulphates into H2S (sulphate reduction),
especially in swamps and sediments.
6. Atmospheric Phase:
H2S gas can be released into the atmosphere from natural sources
(volcanoes, decaying matter).
In the atmosphere, H2S is oxidized to sulphur dioxide (SO2) and eventually
to sulphur trioxide (SO3), which combines with water to form sulphuric
acid (H2SO4).
This returns to the earth via acid rain.
Equation Example:
SO2 + H2O → H2SO3 (sulphurous acid)
SO3 + H2O → H2SO4 (sulphuric acid)
(b) Role of Bacteria in Nitrogen Fixation
Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted
into a usable form like ammonia (NH3) by certain bacteria. Nitrogen is essential for
living organisms as it is a key component of proteins, nucleic acids, and
chlorophyll.
Types of Nitrogen-Fixing Bacteria:
1. Free-living (non-symbiotic) bacteria:
Examples: Azotobacter (aerobic), Clostridium (anaerobic), Cyanobacteria
(e.g., Anabaena, Nostoc).
These bacteria live freely in the soil and fix atmospheric nitrogen without a
host.
2. Symbiotic bacteria:
Unit 2 2
, Example: Rhizobium species.
These form symbiotic relationships with leguminous plants, living in root
nodules.
The plant provides carbohydrates and an anaerobic environment; bacteria
provide ammonia.
Process of Nitrogen Fixation:
Bacteria use the enzyme nitrogenase to convert atmospheric nitrogen (N2)
into ammonia (NH3).
Nitrogenase is very sensitive to oxygen and functions only under anaerobic
conditions.
Chemical Equation:
N2 + 8H+ + 8e− + 16ATP → 2NH3 + H2 + 16ADP + 16Pi
The ammonia is then converted to ammonium (NH4+) and absorbed by plants
to synthesize amino acids.
Importance:
Enriches soil fertility.
Reduces dependency on chemical fertilizers.
Essential for sustainable agriculture.
Conclusion:
Both the sulphur and nitrogen cycles are crucial for maintaining ecosystem
balance. Bacteria play a key role in recycling nutrients, especially in nitrogen
fixation, making essential elements available to plants and thus supporting the
entire food chain.
Q2. Write notes on:
(i) Phosphorus cycle
Unit 2 3
, (ii) Fungi as soil microflora
(i) Phosphorus Cycle
The phosphorus cycle is a biogeochemical cycle that describes the movement of
phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike the
nitrogen or carbon cycles, the phosphorus cycle does not involve a gaseous
phase under normal conditions. Phosphorus is a crucial element for living
organisms as it is a component of DNA, RNA, ATP, and phospholipids in cell
membranes.
Key Steps in the Phosphorus Cycle:
1. Weathering of Rocks:
Phosphorus is mostly found in phosphate rocks (e.g., apatite).
Through weathering, these rocks release inorganic phosphate ions
(PO4^3-) into the soil and water.
2. Absorption by Plants:
Plants absorb phosphate ions from the soil, which are then incorporated
into organic molecules such as nucleic acids and ATP.
3. Movement Through Food Chain:
When animals consume plants, phosphorus moves through the food chain.
It becomes part of animal tissues like bones and teeth (as calcium
phosphate).
4. Return to Soil:
Phosphorus returns to the soil through excretion (urine and feces) and
decomposition of plants and animals.
5. Sedimentation:
In aquatic systems, some phosphate settles and forms sedimentary rocks
over long periods.
This phosphorus can be uplifted to the surface by geological processes
like tectonic movements.
6. Human Impact:
Unit 2 4
