The carbon cycle describes how carbon is exchanged between the atmosphere, organisms, oceans, and the Earth's crust. Carbon dioxide enters the atmosphere through volcanic emissions and respiration. Plants and algae absorb carbon dioxide through photosynthesis and incorporate carbon into organic molecules. Animals consume these organic molecules and release carbon dioxide during respiration. Decomposers break down dead organic matter, releasing carbon back into the atmosphere and oceans. Human activities like burning fossil fuels have increased atmospheric carbon dioxide levels, altering the natural carbon cycle.

1. Carbon Cycle

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3. Carbon Cycle Definition
 The carbon cycle is the cycle by which carbon moves through our Earth’s various
systems. The carbon cycle is influenced by living things, atmospheric changes, ocean
chemistry, and geologic activity are all part of this cycle. The levels of carbon are at an
all-time high, largely due to human activities.

4. Carbon Cycle Overview
 Carbon is an essential element for life as we know it because of its ability to form
multiple, stable bonds with other molecules. This is why nucleotides, amino acids,
sugars, and lipids all depend on carbon backbones: carbon provides a stable structure
that allows the chemistry of life to happen. Without carbon, none of these molecules
could exist and function in the ways that permit the chemistry of life to occur.

5. Carbon Cycle Steps
 Carbon in the Atmosphere
To become part of the carbon cycle, carbon atoms start out in a gaseous form. Carbon
dioxide gas – CO2 – can be produced by inorganic processes, or by the metabolisms of
living things.
Before Earth had life on it, carbon dioxide gas likely came from volcanic activity and
asteroid impacts. Today, carbon is also released into the atmosphere through the activities
of living things, such as the exhalations of animals, the actions of decomposer organisms,
and the burning of wood and fossil fuels by humans. However carbon dioxide gets into the
atmosphere, CO2 gas is the starting point of the carbon cycle.

6.  Producers Absorb Carbon
“Producers” – organisms that produce food from sunlight, such as plants – absorb
carbon dioxide from the atmosphere and use it to build sugars, lipids, proteins, and other
essential building blocks of life.
For plants, CO2 is absorbed through pores in their leaves called “stomata.” Carbon
dioxide enters the plant through the stomata and is incorporated into containing carbon
compounds with the help of energy from sunlight. Plants and other producer organisms
such as cyanobacteria are crucial to life on Earth because they can turn atmospheric carbon
into living matter.

7.  Producers are Eaten
“Consumers” are organisms that eat other living things. Animals are the most visible
type of consumer in our ecosystems, though many types of microbes also fall into this
category.
Consumers incorporate carbon compounds from plants and other food sources when
they eat them. They use some of these carbon compounds from food to build their own
bodies – but much of the food they eat is broken down to release energy, in a process that is
almost the reverse of what producers do.
While producers use energy from sunlight to make bonds between carbon atoms –
animals break these bonds to release the energy they contain, ultimately turning sugars,
lipids, and other carbon compounds into single-carbon units. These are ultimately released
into the atmosphere in the form of CO2.

8.  Decomposers Release Carbon
Plants and animals that die without being eaten by other animals are broken down by
other organisms, called “decomposers.” Decomposers include many bacteria and some
fungi. They usually only break down matter that is already dead, rather than catching and
eating a living animal or plant.
Just like animals, decomposers break down the chemical bonds in their food
molecules. They create many chemical products, including in some cases CO2.

9.  Human Activities
Recently, humans have made some big changes to the Earth’s carbon cycle. By
burning huge amounts of fossil fuels and cutting down roughly half of the Earth’s forests,
humans have decreased the Earth’s ability to take carbon out of the atmosphere, while
releasing large amounts of carbon into the atmosphere that had been stored in solid form as
plant matter and fossil fuels.
This means more carbon dioxide in Earth’s atmosphere – which is particularly
dangerous since carbon dioxide is a “greenhouse gas” that plays a role in regulating the
Earth’s temperature and weather patterns.

10.  Carbon Cycle Examples
The carbon cycle consists of many parallel systems which can either absorb or release
carbon. Together, these systems work to keep Earth’s carbon cycle – and subsequently its
climate and biosphere – relatively stable. Below are some examples of parts of Earth’s
ecosystems that can absorb carbon, turn carbon into living matter, or release carbon back
into the atmosphere.

11.  Atmosphere
One major repository of carbon is the carbon dioxide in the Earth’s atmosphere.
Carbon forms a stable, gaseous molecule in combination with two atoms of oxygen. In
nature, this gas is released by volcanic activity, and by the respiration of animals who affix
carbon molecules from the food they eat to molecules of oxygen before exhaling it.
Carbon dioxide can be removed from the atmosphere by plants, which take the
atmospheric carbon and turn it into sugars, proteins, lipids, and other essential molecules
for life. It can also be removed from the atmosphere by absorption into the ocean, whose
water molecules can bond with carbon dioxide to form carbonic acid.

12. Lithosphere
The Earth’s crust – called the “lithosphere” from the Greek word “litho” for “stone”
and “sphere” for globe – can also release carbon dioxide into Earth’s atmosphere. This gas
can be created by chemical reactions in the Earth’s crust and mantel.
Volcanic activity can result in natural releases of carbon dioxide. Some scientists
believe that widespread volcanic activity may be to blame for the warming of the Earth that
caused the Permian extinction.
While the Earth’s crust can add carbon to the atmosphere, it can also remove it.
Movements of the Earth’s crust can bury carbon-containing chemicals such as dead plants
and animals deep underground, where their carbon cannot escape back into the atmosphere.

13.  Biosphere
Among living things, some remove carbon from the atmosphere, while others release
it back. The most noticeable participants in this system are plants and animals.
Plants remove carbon from the atmosphere. They don’t do this as a charitable act;
atmospheric carbon is actually the “food” which plants use to make sugars, proteins, lipids,
and other essential molecules for life. Plants use the energy of sunlight, harvested through
photosynthesis, to build these organic compounds out of carbon dioxide and other trace
elements. Indeed, the term “photosynthesis” comes from the Greek words “photo” for
“light” and “synthesis” for “to put together.”

14.  Oceans
The Earth’s oceans have the ability to both absorb and release carbon dioxide. When
carbon dioxide from the atmosphere comes into contact with ocean water, it can react with
the water molecules to form carbonic acid – a dissolved liquid form of carbon.
When there is more carbonic acid in the ocean compared to carbon dioxide in the
atmosphere, some carbonic acid may be released into the atmosphere as carbon dioxide. On
the other hand, when there is more carbon dioxide in the atmosphere, more carbon dioxide
will be converted to carbonic acid, and ocean acidity levels will rise.

15. The carbon cycle, under normal circumstances, works to ensure the stability of variables
such as the Earth’s atmosphere, the acidity of the ocean, and the availability of carbon for use by
living things. Each of its components is of crucial importance to the health of all living things –
especially humans, who rely on many food crops and animals to feed our large population.
Carbon dioxide in the atmosphere prevents the sun’s heat from escaping into space, very
much like the glass walls of a greenhouse. This isn’t always a bad thing – some carbon dioxide in
the atmosphere is good for keeping the Earth warm and its temperature stable.
But Earth has experienced catastrophic warming cycles in the past, such as the Permian
extinction, which is thought to have been caused by a drastic increase in the atmosphere’s level
of greenhouse gases. No one is sure what caused the change that brought about the Permian
extinction. But, greenhouse gases may have been added to an atmosphere by an asteroid impact,
volcanic activity, or even massive forest fires.
Whatever the cause, during this warming episode temperatures rose drastically. Much of the
Earth became desert, and over 90% of all species living at that time went extinct. This is a good
example of what can happen if our planet’s essential cycles experience a big change.