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The Carbon Cycle

The essential components of hydrocarbons are hydrogen and carbon (big surprise right?). These basic components come from the chemical breakdown of carbon dioxide and water, which takes place in plants. As it turns out, understanding hydrocarbons (at least those that make up fossil fuels) begins with understanding biology and geology. In this article, we look at the biology of carbon.

The carbon cycle refers to the natural, biological pathways through which carbon is transferred from non-living substances to living organisms and from one living organism to another. The entire cycle, however, begins with plants.

Plants and Photoautotrophs

Plants are part of a larger group of group of organisms called photoautotrophs. This gigantic term basically means “organisms that make their own food from inorganic (non-living) sources with aide of sunlight.” In other words, photoautotrophs take basic chemicals and, in the presence of light, convert them into more complex molecules that are useful to living things. The basic building blocks of all hydrocarbon molecules come from carbon dioxide (CO2) and water (H2O), which are combined in the process of photosynthesis to create sugars.


Photosynthesis is a chemical reaction that converts “reactants” into “products.” In particular, it is a chemical reaction that builds complex molecules from smaller, simpler molecules. Any process that builds a complicated structure requires energy and the energy for photosynthesis comes from the sun. The basic reaction looks like this.


The sugar from this reaction is then used by plant and the animals that eat those plants for both energy and to produce other molecules. At some point, plants and animals die and the sugar contained in their bodies, if subjected to the right conditions, is turned into coal, oil, and natural gas.

Fossil Fuel Formation

The key to turning a dead plant or animal into a fossil and eventually a fossil fuel is anaerobic conditions. That is to say, if there is no oxygen around, then you are well on your way to forming a fossil fuel. Add to this mixture some heat and pressure and you end up with carbon in various forms that include coal, oil, natural gas, graphite, and even diamond.

The basic story is that pressure from multiple layers of sediment leads to an anoxic (oxygen free) environment that allows for decomposition to take place without oxygen (molecular oxygen in the form of O2, that is). When this is combined with heat from the Earth, the carbon in sugar molecules is rearranged to form other compounds. The material that is buried helps to determine what the final product will be. Animal remains tend to form petroleum (crude oil) while plant matter is more likely to form coal and natural gas.

Completing the Cycle – The Slow Carbon Cycle

You may wonder, if carbon is trapped in fossil fuels, how does the carbon cycle make a full circle? Well, first, not all carbon is trapped in fossil fuels. Many plants and animals are decomposed in oxygen-rich environments where microorganisms are able to reclaim the carbon and put it back into the cycle via respiration (the process of breaking down sugar and releasing water and carbon dioxide). Carbon dioxide that is returned this way is often referred to as participating in the “fast carbon cycle.” There is another branch of the cycle, however, that is sometimes called the “slow carbon cycle” because it make take millions of years for carbon to renter the atmosphere.

The second way that carbon reenters the cycle is through volcanoes. Volcanoes emit somewhere between 130 and 380 million metric tons (U.S. tons) of carbon dioxide per year, which is about 100-300 times LESS than the 30 billion tons that humans emit.

The third way carbon reenters the cycle is through us burning fossil fuels. When we do this, carbon dioxide and water are release into the atmosphere and taken up by plants for use in photosynthesis. Of course, not all of the carbon is immediately taken back up. Some of it remains in the atmosphere, increasing levels of atmosphere carbon dioxide and contributing to the greenhouse effect.