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The carbon cycle
Key words
atmosphere
carbon
carbon cycle
chlorophyll
glucose
a
b
photosynthesis
respiration
The carbon cycle
Carbon is the fourth most abundant
element in the Universe.
● The total amount of carbon on planet
Earth is fixed. The same carbon atoms
have been used in countless other
molecules since Earth began. The
carbon cycle is the complex set of
processes through which all carbon
atoms rotate.
● Carbon exists in Earth’s atmosphere
primarily as carbon dioxide.
● All green plants contain chlorophyll, a
pigment that gives them their
characteristic color. During
photosynthesis, chlorophyll traps
energy from sunlight and uses it to
convert carbon dioxide and water into
glucose and oxygen.
● Carbon is transferred from green
plants to animals when animals eat
plants or other animals.
● All animals and plants need energy to
drive their various metabolic
processes. This energy is provided by
respiration. During this process,
glucose reacts with oxygen to form
carbon dioxide and water. These waste
products are subsequently released
into the atmosphere. In essence,
respiration is the opposite process to
photosynthesis.
● When plants and animals die, their
bodies decompose. In the presence of
air, the carbon they contain becomes
carbon dioxide, which is released into
the atmosphere.
● When plants and animals decay in the
absence of air, carbon cannot be
converted into carbon dioxide.
Instead, it remains and forms fossil
fuels such as coal, crude oil, and
natural gas.
● When fossil fuels are burned, the
carbon they contain becomes carbon
dioxide and is released into the
atmosphere.
© Diagram Visual Information Ltd.
●
Respiration
Photosynthesis
glucose + oxygen→ carbon +water+energy
dioxide
sunlight
carbon
+ water
glucose +oxygen
dioxide
chlorophyll
C6H12O8(aq) + 602(g) → 6CO2(g) + 6H2O(I)
6CO2(g) + 6H20(I) → C6H12O6(aq) + 6O2(g)
c
d
e
g
f
a
b
c
d
e
f
g
carbon dioxide in the air
sunlight
plants take in carbon dioxide and give out oxygen
animals take in oxygen, eat plants and vegetables, and breath out CO2
death and decay
carbon compounds (e.g., in oil and coal)
burning fuel produces CO2