Just as important as the flow of energy in an ecosystem is the flow of matter. All organisms need to take in nutrients and expel waste products. These nutrients contain energy, but they also contain atoms and molecules that are important for life. For instance, all humans need iron, a primary constituent of hemoglobin which carries oxygen to the cells in the body. As iron leaches out of the body in the waste products that we expel or in the blood that we loose, we must replace it. Therefore, having access to stores of iron is important to humans.

There are six major elements that are important for almost all lifeforms: hydrogen/oxygen (water), nitrogen (amino acids and nucleic acids), phosphorous (nucleic acids), sulfur (amino acids), and carbon. Of these, carbon is vitally important, as it is needed to form all organic molecules. The largest stores of carbon exist in the ocean and ground, mostly in the form of carbonates such as in calcium carbonate (limestone). The largest reservoir that is available to surface organisms, though, is in the air as carbon dioxide. Plants are able to take this carbon out of the air during photosynthesis and to use it to form organic molecules such as carbohydrates. Animals that eat the plants are then able to use the carbon stored in the fibers. What is not used to build other organic molecules in the body is excreted as waste, some of which is in the form of carbon dioxide.

This module takes a look at the carbon cycle. In particular, it will look at how much carbon is stored in a forest, and how much would be released if the forest is burned or cut down and allowed to decay. It also looks at the process of photosynthesis. Finally, it looks at how much carbon dioxide a person emits during the course of a normal year.