GED Science Practice Test: Carbon Cycle

Another important cycle that ecologists study is the carbon cycle. As with the nitrogen and water cycles, carbon is present in living organisms, in the atmosphere and in the soils and other layers of the earth.  All living organisms are earth are considered to be “carbon-based life forms;” carbon is the building block of all life on earth.  In the atmosphere, carbon is present as part of a compound called carbon dioxide.  Humans and other animals that breathe, or conduct respiration, exhale carbon dioxide.  Plants use carbon dioxide in the atmosphere to conduct photosynthesis and produce their own food.  Thus, carbon is very important to plant and animal populations and communities.

The carbon dioxide that is in the atmosphere serves another important purpose, which is to keep earth warm.  Carbon dioxide is considered a greenhouse gas.  Greenhouse gases are those substances which exist in the atmosphere and trap heat close to the earth.  The greenhouse effect refers to this trapping of heat. Some people describe the greenhouse gases as creating a sort of “blanket” over the earth. the greenhouse effect, most life on earth would cease to exist because the earth would be too cold!

Many people confuse the greenhouse effect with global warming.  Global warming is a bad result of an imbalance in the carbon cycle.  Many human industrial processes release carbon dioxide into the air.  When the amount of carbon dioxide in the atmosphere becomes too great, the earth’s temperature can increase beyond normal ranges.  If we continue the blanket analogy from above, human processes create too many greenhouse gases, and we have more blankets than we need…we get too hot.

In addition to being the building block of life, and existing in the atmosphere, carbon can also be found on the earth as the main component in fossil fuels.  Over very long periods of time, decaying plant and animal material compresses to form fossil fuels.  The burning of fossil fuels also releases carbon dioxide into the atmosphere.  Carbon is also found in large amounts in the oceans.  However, too much carbon dioxide in the earth’s oceans can lead to ocean acidification, which has drastic effects on shellfish and other populations.

Like the water and nitrogen cycles, when ecologists talk about the carbon cycle, they are concerned by too much or too little carbon in certain places.  Within the carbon cycle, then, ecologists talk about carbon sources, places or things that produce carbon, and carbon sinks, which are places or things that retain carbon.  Using our diagram from above, human industrial processes are a carbon source, while oceans are a carbon sink. Ecologists are concerned because the human processes that release carbon dioxide into the atmosphere are happening at a faster rate than carbon sinks can react to properly.  Ecologists refer to this balance (or imbalance) of carbon as the carbon budget.


Use the diagram below to answer questions 1 through 3:



You have seen 1 out of 15 free pages this month.
Get unlimited access, over 1000 practice questions for just $29.99. Enroll Now