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<blurb target="co2" width="420">
<short><![CDATA[The primary source of carbon for living organisms is the atmospheric pool of carbon dioxide gas (two atoms of oxygen attached to a single atom of carbon, CO2). Carbon dioxide comprises only about 0.04 percent of the atmosphere on average, but levels fluctuate slightly due to seasonal changes in carbon dioxide uptake and release by plants. ]]></short> </blurb>

<blurb target="respiration" width="420"><short><![CDATA[All living organisms provide energy for themselves through the chemical breakdown of food — a process called respiration. While a few microbes have found ways to do this without oxygen, respiration more typically involves the intake of oxygen and the release of carbon dioxide. The oxygen is used in chemical reactions that release the energy stored in carbon-based food molecules, and carbon dioxide is a byproduct.]]></short></blurb>
<blurb target="methane" width="420"><short><![CDATA[Methane gas (one carbon atom surrounded by four hydrogen atoms, CH4) is a natural byproduct of digestion. Bacteria that live in the digestive tracts of animals produce it as they help break down food. But the red arrow in the diagram indicates that the amount of methane released into the atmosphere has been greatly influenced by human agricultural practices, especially intensive livestock production.]]></short></blurb>
<blurb target="microbe" width="420"><short><![CDATA[Dead plants and animals are an abundant source of food for microbes, primarily bacteria and fungi. The microbes break down the large organic carbon molecules in the dead organism’s body into smaller and smaller pieces. They use the energy they obtain from these organic molecules and release carbon dioxide and methane.]]></short></blurb>
<blurb target="fuel" width="420"><short><![CDATA[Humans have developed technologies to remove coal, petroleum, and natural gas from sedimentary layers in the Earth’s crust and release the energy stored in their molecules through controlled combustion. In burning, carbon atoms from the fuel combine with oxygen atoms from the air, creating carbon dioxide and carbon monoxide (CO) gases, which are released back to the atmosphere.]]></short></blurb>
<blurb target="pet" width="420"><short><![CDATA[Petroleum, coal, and natural gas are called "fossil fuels" because they were formed from the remains of plants and animals that lived eons ago. Much of this ancient life was microbial. Trapped between layers of sediment and subjected to conditions of high temperature and pressure, this organic carbon was transformed into hydrocarbons — chains of carbon atoms with hydrogen atoms attached.]]></short></blurb>
<blurb target="dead" width="420"><short><![CDATA[The pathway of carbon atoms that make up the body of a plant or animal depends on how the organism dies. If the organism is eaten, the carbon in its body is used directly to provide energy and build new tissues in the body of the consumer.  Otherwise, natural causes of death, including diseases brought on by pathogenic microbes, lead to decomposition.]]></short></blurb>
<blurb target="photo" width="420"><short><![CDATA[Photosynthesis is the process in which green plants and algae combine carbon dioxide and water to form sugar molecules using sunlight as a source of energy. The carbon-based sugars produced serve as food for the organisms themselves, or they may be stored in the organisms’ tissues. In plants, much of this stored carbon is in the form of cellulose.]]></short></blurb>
<blurb target="cow" width="420"><short><![CDATA[Herbivores (plant eaters) lack digestive enzymes capable of turning cellulose (a complex type of sugar that is the main building block of plant tissues) into a usable form of carbon. They depend on bacteria and protozoa in their digestive tracts to break down the cellulose for them. These carbon building blocks are then reassembled into their own tissues.]]></short></blurb>

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