Carbon is the common currency of life. The major biological molecules are all constructed from a framework of carbon, and so living organisms need this element in especially large quantities if they are to grow and maintain their tissues. Carbon-containing molecules also serve as a store of energy for cells to work; the bonds within the molecules are broken and energy is released. For these two purposes—building bodies and fueling them—organisms are always grabbing carbon from one another, or in the case of plants, directly out of the atmosphere by photosynthesis. While many other important elements, like calcium and sulfur, are transferred too, carbon is needed in the greatest quantities and generally the most urgently.
Carbon atoms are shuttled from one molecule to another within the cells and tissues of an organism, and from one organism to another (by predation, parasitism, herbivory, decay and all the other ways in which organisms interact), until they are eventually "oxidized"—in effect, burnt—to give carbon dioxide, which goes into the atmosphere. Each C02 molecule may later be taken up by plants in photosynthesis, perhaps in a completely different part of the world because molecules carried by the wind can travel hundreds of miles in a few days. My friend and colleague Tyler Volk of NYU has calculated that from each individual breath a person has taken during the last several years, there are on average 60 carbon atoms present in any particular leaf taken from anywhere in the world! Having been taken up by a plant, the carbon atom can go shooting down a food chain once again. It may end up in soil, and perhaps only released by decay after many decades. 0r the C02 molecule may drift out across an ocean, dissolve in its waters and stay there for thousands of years before it eventually wanders back out again. It might even end up in sediment on the sea floor, be buried, folded down into the earth and only released millions of years later when it gets spat out of a volcano. The global shuttling process of carbon atoms between organisms, atmosphere, oceans, rocks and soil is together known as the "carbon cycle" (Figure 7.1). Carbon has been recycled between these different compartments since the beginning of life on earth, billions of years ago. For
Carbon dioxide 725 Methane 3
Carbon monoxide 0.2
Erosion A 0.4^
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.