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Biochemical Cycles

The Water Cycle

Chemicals and nutrients are recycled in an ecosystem in biogeochemical cycles. This recycling process involves
both the biotic factors and the abiotic factors of the ecosystem. Through biogeochemical cycles, nutrients are
constantly being passed through living organisms to non-living matter and back again, over and over. These
recycled nutrients contain the elements carbon and nitrogen.

Water is obviously an extremely important aspect of every ecosystem. Life cannot exist without water. Many
organisms contain a large amount of water in their bodies, and many live in water, so the water cycle is
essential to life on earth. Water is cycled through the biotic and abiotic factors of an ecosystem, moving
between living things and non-living things, such as clouds, rivers, and oceans (Figure below).

Stages of Water Cycle

There are many processes involved in
the movement of water apart from the
major steps given in the above water
cycle diagram. Listed below are
different stages of the water cycle.

1. Evaporation
The sun is the ultimate source of
energy, and it powers most of the
evaporation that occurs on earth.
Evaporation generally happens when
water molecules at the surface of water
bodies become excited and rise into the
air. These molecules with the highest
kinetic energy accumulate into water
vapour clouds. Evaporation usually
takes place below the boiling point of
water. Another process called evapotranspiration occurs when evaporation occurs through the leaves of
plants. This process contributes to a large percentage of water in the atmosphere.

2. Sublimation
Sublimation occurs when snow or ice changes directly into water vapour without becoming water. It usually
occurs as a result of dry winds and low humidity. Sublimation can be observed on mountain peaks, where the
air pressure is quite low. The low air pressure helps to sublimate the snow into water vapour as less energy is
utilised in the process. Another example of sublimation is the phase where fog bellows from dry ice. On earth,
the primary source of sublimation is from the ice sheets covering the poles of the earth.

3. Condensation
The water vapour that accumulated in the atmosphere eventually cools down due to the low temperatures
found at high altitudes. These vapours become tiny droplets of water and ice, eventually coming together to
form clouds.
4. Precipitation
Above 0 degrees centigrade, the vapours will condense into water droplets. However, it cannot condense
without dust or other impurities. Hence, water vapours attach itself on to the particle’s surface. When enough
droplets merge, it falls out of the clouds and on to the ground below. This process is called precipitation (or
rainfall). In particularly cold weather or extremely low air pressure, the water droplets freeze and fall as snow
or hail.

5. Infiltration
Rainwater gets absorbed into the ground through the process of infiltration. The level of absorption varies
based on the material the water has seeped into. For instance, rocks will retain comparatively less water than
soil. Groundwater can either follows streams or rivers. But sometimes, it might just sink deeper, forming
aquifers.

6. Runoff
If the water from rainfall does not form aquifers, it follows gravity, often flowing down the sides of mountains
and hills; eventually forming rivers. This process is called runoff. In colder regions, icecaps form when the
amount of snowfall is faster than the rate of evaporation or sublimation. The biggest icecaps on earth are
found at the poles.
All the steps mentioned above occur cyclically with neither a fixed beginning nor an end.

The Carbon Cycle

Carbon is one of the most common elements found in living organisms. Chains of carbon molecules form the
backbones of many molecules, such as
carbohydrates, proteins, and lipids.
Carbon is constantly cycling between
living things and the atmosphere
(Figure below).

In the atmosphere, there is carbon

dioxide. Producers capture the carbon
dioxide and convert it to glucose
through the process of photosynthesis.
As consumers eat producers or other
consumers, they gain the carbon from
those organisms. Some of this carbon
is lost, however, through the process of
cellular respiration. That means when
our cells burn food for energy, carbon
dioxide is released. We, like all animals,
exhale this carbon dioxide and return it
back to the atmosphere. Also, carbon
dioxide is released to the atmosphere
as an organism dies and decomposes.

Formation of Fossil Fuels

Millions of years ago, there were so many dead plants and animals that they could not completely decompose
before they were buried. These plants and animals are organic matter, with lots of carbon. When organic
matter is under pressure for millions of years, it forms fossil fuels. Fossil fuels are coal, oil, and natural gas.
When humans dig up and use fossil fuels, we have an impact on the carbon cycle (Figure below).

The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis.
So, there is more carbon dioxide entering the atmosphere than is coming out of it. Carbon dioxide is known as
a greenhouse gas, since it lets in light energy but does not let heat escape, much like the panes of a
greenhouse. The increase of greenhouse gasses in the atmosphere is contributing to a global rise in Earth’s
temperature, known as global warming.

Carbon Cycle on Land

Carbon in the atmosphere is present in the form of carbon dioxide. Carbon enters the atmosphere through
natural processes such as respiration and industrial applications such as burning fossil fuels. The process
of photosynthesis involves the absorption of CO2 by plants to produce carbohydrates. The equation is as
follows:
CO2 + H2O + energy → (CH2O)n +O2
Carbon compounds are passed along the food chain from the producers to consumers. The majority of the
carbon exists in the body in the form of carbon dioxide through respiration. The role of decomposers is to eat
the dead organism and return the carbon from their body back into the atmosphere. The equation for this
process is:
(CH2O)n +O2 → CO2 + H2O

Oceanic Carbon Cycle

This is essentially a carbon cycle but in the sea. Ecologically, oceans take in more carbon than it gives out.
Hence, it is called a “carbon sink.” Marine animals convert carbon to calcium carbonate and this forms the raw
building materials require to create hard shells, similar to the ones found in clams and oysters.
When organisms with calcium carbonate shells die, their body decomposes, leaving behind their hard shells.
These accumulate on the seafloor and are eventually broken down by the waves and compacted under
enormous pressure, forming limestone.
When these limestone rocks are exposed to air, they get weathered and the carbon is released back into the
atmosphere as carbon dioxide.

Importance of Carbon Cycle

Even though carbon dioxide is found in small traces in the atmosphere, it plays a vital role in balancing the
energy and traps the long-wave radiations from the sun. Therefore, it acts like a blanket over the planet. If the
carbon cycle is disturbed it will result in serious consequences such as climatic changes and global warming.
Carbon is an integral component of every life form on earth. From proteins and lipids to even our DNA.
Furthermore, all known life on earth is based on carbon. Hence, the carbon cycle, along with the nitrogen
cycle and oxygen cycle, plays a vital role in the existence of life on earth.

Key Points on Carbon Cycle

• Carbon cycle explains the movement of carbon between the earth’s biosphere, geosphere, hydrosphere
and atmosphere.
• Carbon is an important element of life.
 • Carbon dioxide in the atmosphere is taken up by green plants and other photosynthetic organisms and
is converted into organic molecules that travel through the food chain. Carbon atoms are then released
as carbon dioxide when organisms respire.
• The formation of fossil fuels and sedimentary rocks contributes to the carbon cycle for very long
periods.
• The carbon cycle is associated with the availability of other compounds as well.

Nitrogen Cycle
Nitrogen Cycle Definition
“Nitrogen Cycle is a biogeochemical process which transforms the inert nitrogen present in the
atmosphere to a more usable form for living organisms.”
Furthermore, nitrogen is a
key nutrient element for
plants. However, the
abundant nitrogen in the
atmosphere cannot be used
directly by plants or animals.
Read on to explore how
the Nitrogen cycle makes
usable nitrogen available to
plants and other living
organisms.

What is the Nitrogen

Cycle?
Nitrogen Cycle is a
biogeochemical process
through which nitrogen is
converted into many forms,
consecutively passing from
the atmosphere to the soil to
organism and back into the
atmosphere.
It involves several processes
such as nitrogen fixation,
nitrification, denitrification,
decay and putrefaction.
Nitrogen gas exists in both
organic and inorganic forms. Organic nitrogen exists in living organisms, and they get passed through the food
chain by the consumption of other living organisms.
Inorganic forms of nitrogen are found in abundance in the atmosphere. This nitrogen is made available to
plants by symbiotic bacteria which can convert the inert nitrogen into a usable form – such as nitrites and
nitrates.
Nitrogen undergoes various types of transformation to maintain a balance in the ecosystem. Furthermore, this
process extends to various biomes, with the marine nitrogen cycle being one of the most complicated
biogeochemical cycles.
Nitrogen Cycle Explained – Stages of Nitrogen Cycle
Process of the Nitrogen Cycle consists of the following steps – Nitrogen fixation, Nitrification, Assimilation,
Ammonification and Denitrification. These processes take place in several stages and are explained below:

Nitrogen Fixation Process

It is the initial step of the nitrogen cycle. Here, Atmospheric nitrogen (N2) which is primarily available in an
inert form, is converted into the usable form -ammonia (NH3).
During the process of Nitrogen fixation, the inert form of nitrogen gas is deposited into soils from the
atmosphere and surface waters, mainly through precipitation.
The entire process of Nitrogen fixation is completed by symbiotic bacteria, which are known as
Diazotrophs. Azotobacter and Rhizobium also have a major role in this process. These bacteria consist of a
nitrogenase enzyme, which has the capability to combine gaseous nitrogen with hydrogen to form ammonia.
Nitrogen fixation can occur either by atmospheric fixation- which involves lightening, or industrial fixation by
manufacturing ammonia under high temperature and pressure conditions. This can also be fixed through man-
made processes, primarily industrial processes that create ammonia and nitrogen-rich fertilisers.
Types of Nitrogen Fixation

1. Atmospheric fixation: A natural phenomenon where the energy of lightning breaks the nitrogen into
nitrogen oxides, which are then used by plants.
2. Industrial nitrogen fixation: It is a man-made alternative that aids in nitrogen fixation by the use of
ammonia. Ammonia is produced by the direct combination of nitrogen and hydrogen. Later, it is
converted into various fertilisers such as urea.
3. Biological nitrogen fixation: We already know that nitrogen is not used directly from the air by
plants and animals. Bacteria like Rhizobium and blue-green algae transform the unusable form of
nitrogen into other compounds that are more readily usable. These nitrogen compounds get fixed in
the soil by these microbes.

Nitrification
In this process, the ammonia is converted into nitrate by the presence of bacteria in the soil. Nitrites are
formed by the oxidation of ammonia with the help of Nitrosomonas bacteria species. Later, the produced
nitrites are converted into nitrates by Nitrobacter. This conversion is very important as ammonia gas is toxic
for plants.
The reaction involved in the process of Nitrification is as follows:
2NH3 + 3O2 → 2NO2– + 2H+ + 2H2O
2NO2– + O2 → 2NO3–

Assimilation
Primary producers – plants take in the nitrogen compounds from the soil with the help of their roots, which are
available in the form of ammonia, nitrite ions, nitrate ions or ammonium ions and are used in the formation of
the plant and animal proteins. This way, it enters the food web when the primary consumers eat the plants.
Ammonification
When plants or animals die, the nitrogen present in the organic matter is released back into the soil. The
decomposers, namely bacteria or fungi present in the soil, convert the organic matter back into ammonium.
This process of decomposition produces ammonia, which is further used for other biological processes.

Denitrification
Denitrification is the process in which the nitrogen compounds make their way back into the atmosphere
by converting nitrate (NO3-) into gaseous nitrogen (N). This process of the nitrogen cycle is the final stage
and occurs in the absence of oxygen. Denitrification is carried out by the denitrifying bacterial
species- Clostridium and Pseudomonas, which will process nitrate to gain oxygen and gives out free nitrogen
gas as a byproduct.

Nitrogen Cycle in Marine Ecosystem

The process of the nitrogen cycle occurs in the same manner in the marine ecosystem as in the terrestrial
ecosystem. The only difference is that it is carried out by marine bacteria.
The nitrogen-containing compounds fall into the ocean as sediments get compressed over long periods and
form sedimentary rock. Due to the geological uplift, these sedimentary rocks move to land. Initially, it was not
known that these nitrogen-containing sedimentary rocks are an essential source of nitrogen. But, recent
research have proved that the nitrogen from these rocks is released into the plants due to the weathering of
rocks.

Importance of Nitrogen Cycle

The importance of the nitrogen cycle are as follows:

1. Helps plants to synthesise chlorophyll from the nitrogen compounds.

2. Helps in converting inert nitrogen gas into a usable form for the plants through the biochemical
process.
3. In the process of ammonification, the bacteria help in decomposing the animal and plant matter, which
indirectly helps to clean up the environment.
4. Nitrates and nitrites are released into the soil, which helps in enriching the soil with the necessary
nutrients required for cultivation.
5. Nitrogen is an integral component of the cell and it forms many crucial compounds and important
biomolecules.

Nitrogen is also cycled by human activities such as the combustion of fuels and the use of nitrogen fertilisers.
These processes increase the levels of nitrogen-containing compounds in the atmosphere. The fertilisers
containing nitrogen are washed away in lakes, rivers and result in eutrophication.

Conclusion

• Nitrogen is abundant in the atmosphere, but it is unusable to plants or animals unless it is converted
into nitrogen compounds.
• Nitrogen-fixing bacteria play a crucial role in fixing atmospheric nitrogen into nitrogen compounds that
can be used by plants.
• The plants absorb the usable nitrogen compounds from the soil through their roots. Then, these
nitrogen compounds are used for the production of proteins and other compounds in the plant cell.
 • Animals assimilate nitrogen by consuming these plants or other animals that contain nitrogen. Humans
consume proteins from these plants and animals. The nitrogen then assimilates into our body system.
• During the final stages of the nitrogen cycle, bacteria and fungi help decompose organic matter, where
the nitrogenous compounds get dissolved into the soil which is again used by the plants.
• Some bacteria then convert these nitrogenous compounds in the soil and turn it into nitrogen gas.
Eventually, it goes back to the atmosphere.
• These sets of processes repeat continuously and thus maintain the percentage of nitrogen in the
atmosphere.

Phosphorus Cycle
Phosphorus Cycle Definition
“Phosphorus cycle is a biogeochemical process that involves the movement of phosphorus
through the lithosphere, hydrosphere and biosphere.”

What is Phosphorus Cycle?

Phosphorus is an important element for all living organisms. It forms a significant part of the structural
framework of DNA and RNA. They are also an important component of ATP. Humans contain 80% of
phosphorus in teeth and bones.
Phosphorus cycle is a very slow
process. Various weather processes
help to wash the phosphorus present in
the rocks into the soil. Phosphorus is
absorbed by the organic matter in the
soil which is used for various biological
processes.
Since phosphorus and phosphorus-
containing compounds are present only
on land, atmosphere plays no
significant role in the phosphorus cycle.
Let us have a brief look at the
phosphorus cycle, its steps and the
human impact on phosphorus cycle.

Steps of Phosphorus Cycle

Following are the important steps of phosphorus cycle:

1. Weathering
2. Absorption by Plants
3. Absorption by Animals
4. Return to the Environment through Decomposition
Weathering
Phosphorus is found in the rocks in abundance. That is why the phosphorus cycle starts in the earth’s crust.
The phosphate salts are broken down from the rocks. These salts are washed away into the ground where
they mix in the soil.

Absorption by Plants
The phosphate salts dissolved in water are absorbed by the plants. However, the amount of phosphorus
present in the soil is very less. That is why the farmers apply phosphate fertilizers on agricultural land.
The aquatic plants absorb inorganic phosphorus from lower layers of water bodies. Since phosphate salts do
not dissolve in water properly, they affect plant growth in aquatic ecosystems.

Absorption by Animals
The animals absorb phosphorus from the plants or by consuming plant-eating animals. The rate of the
phosphorus cycle is faster in plants and animals when compared to rocks.

Return of Phosphorus Back to the Ecosystem

When the plants and animals die they are decomposed by microorganisms During this process, the organic
form of phosphorus is converted into the inorganic form, which is recycled to soil and water.
Soil and water will end up in sediments and rocks, which will again release phosphorus by weathering. Thus,
the phosphorus cycle starts over.

Human Impact on Phosphorus Cycle

A number of human activities, use of fertilizers, artificial eutrophication, etc. has a great impact on the
phosphorus cycle.
The phosphorus fertilizers increase the level of phosphorus in the soil. Overuse of these fertilizers reduces the
fertility of the soil and is also harmful to the microorganisms present in the soil. When these are washed away
into the nearby water bodies, they are hazardous to aquatic life.
During the shipping of food from farms to cities, the amount of phosphorus that is washed away in water
causes eutrophication. This leads to the growth of algae. These form algal blooms or die, which is toxic to the
aquatic ecosystem.

Sulphur Cycle
Sulphur is one of the most abundant elements on the earth. It is a yellow, brittle, tasteless, odourless non-
metal. Sulphur is present in all kinds of proteins. Plants directly absorb sulphur-containing amino acids such as
methionine, cystine, and cysteine.
Sulphur is released into the atmosphere by the burning of fossil fuels, volcanic activities, and decomposition of
organic molecules.
On land, sulphur is stored in underground rocks and minerals. It is released by precipitation, weathering of
rocks and geothermal vents.
The process of sulphur cycle is explained below:

• The sulphur is released by the weathering of rocks.

 • Sulphur comes in contact with air and is converted into sulphates.
• Sulphates are taken up by plants and microbes and are converted into organic forms.
• The organic form of sulphur is then consumed by the animals through their food and thus sulphur
moves in the food chain.
• When the animals die, some of the sulphur is released by decomposition while some enter the tissues
of microbes.
• There are several natural sources such as volcanic eruptions, evaporation of water, and breakdown of
organic matter in swamps, that release sulphur directly into the atmosphere. This sulphur falls on earth
with rainfall.

Steps of Sulphur Cycle

Following are the important steps of the sulphur cycle:

Decomposition of Organic Compounds

Protein degradation releases amino acids that contain sulphur. Sulphates are reduced to H2S by the action of
Desulfotomaculum bacteria.

Oxidation of Hydrogen Sulphide to Elemental Sulphur

Hydrogen sulphide oxidises to produce elemental sulphur. Certain photosynthetic bacteria from the families
Chlorobiaceae and Chromatiaceae initiate the oxidation process.

Oxidation of Elemental Sulphur

Elemental sulphur present in the soil cannot be utilized directly by the plants. Therefore, it is converted into
sulphates by chemolithotrophic bacteria.

Reduction of Sulphates
Sulphates are reduced to hydrogen sulphide by Desulfovibrio desulfuricans. This occurs in two steps:

• Firstly, the sulphates are converted to sulphites utilizing ATP.

• Secondly, the reduction of sulphite to hydrogen sulphide.