Figure 24.8: The water cycle. Seehttp://www.youtube.com/watch?v=4Cb3SIMRCIE&
NR=1for an animation of the water cycle. ( 15 )
The Carbon Cycle
Carbon is one of the most abundant elements found in living organisms. Carbon chains form
the backbones of carbohydrates, proteins, and fats. Carbon is constantly cycling between
living things and the atmosphere (Figure24.9).
In the atmosphere, water is in the form of carbon dioxide. Producers capture this carbon
dioxideand convertit to food through the process of photosynthesis (discussed in the chapter
titledCells and Their Structures). Asconsumerseatproducersorotherconsumers, theygain
the carbon from that organism. Some of this carbon is lost, however, through the process
of cellular respiration. When our cells burn food for energy, carbon dioxide is released. We
exhale this carbon dioxide and it returns to the atmosphere. Also, carbon dioxide is released
to the atmosphere as an organism dies and decomposes.
Millions of years ago there was so much organic matter that it could not be completely
decomposed before it was buried. As this buried organic matter was under pressure for
millions of years, it formed intofossil fuels such as coal, oil, and natural gas. When
humans excavate and use fossil fuels, we have an impact on the carbon cycle (Figure24.10).
The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used
by photosynthesis. Therefore the net amount of carbon dioxide in the atmosphere is rising.
Carbon dioxide is known as a greenhouse gas since it lets in light energy but does not let
heat escape, much like the panes of a greenhouse. The increase of greenhouse gasses in
the atmosphere is contributing to a global rise in Earth’s temperature, known asglobal
warming(see theEnvironmental Problemschapter for additional information).