The Carbon Cycle

The atmosphere contains 0.03% by volume of carbon dioxide.
This represents a huge reservoir of carbon, which is continually being removed from and returned to the atmosphere by a variety of processes.

Defination:

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth.

adapted from http://en.wikipedia.org/wiki/Carbon_cycle

How is Carbon Dioxide produced?

There are two main processes which produce Carbon Dioxide.

1.Respiration
All Living things respire. During respiration, carbon and hydrogen compounds in food are converted into carbon dioxide and water. Energy is released during respiraton.

glucose + oxygen ---(in living cells)--> carbon dioxide + water + energy
C6H12O6(aq) + 6 O2(g) ---> 6CO2(g) + 6H2O(l) + energy

2.Combustion of fuels
Coal, petroleum and natural gas are examples of fuels. Most fuels contain both carbon and hydrogen. When these fuels are burnt, carbon dioxide and water are produced. In the combustion of natural gas, which contains mainly methane, the equation for the reaction can be written as

methane + oxygen --> carbon dioxide + water vapour + heat energy
CH4(g) + 2O2(g) --> CO2(g) +2H2O(g) + heat energy

However, if a limited supply of air is used, carbon particles in the form of soot and carbon monoxide, a odourless but colorless gas is given out. This is known as incomplete combustion. The chemical equation can be written as:

Methane + oxygen --> carbon monoxide + water vapour + carbon + heat energy

4CH4(g) + 5O2(g) ---> 2CO(g) + 8 H2O(g) + 2C(s) + heat energy


How is Carbon Dioxide removed?

Planes are important as they help to remove carbon dioxide from the atmosphere. This process is known as photosynthesis, whereby plants absorb raw materials such a water, and carbon dioxide in the presence of sunlight to form glucose and oxygen.

The simplified version of the chemical equation for photosynthesis is shown below.

carbon dioxide + water ---> glucose + oxygen

6CO2(g) + 6H2O(l) --(sunlight + chlorophyll)--> C6H12O6(aq) + 6O2(g)

Therefore, the carbon cycle exists to help regulate the amount of carbon (which exists as Carbon dioxide) in the atmosphere.
The supply of oxygen will always be renewed by plants through photosynthesis and oxygen will be used up in the process of respiration and combustion of fuels.