an ecosystem for two reasons. First, all living things are made of carbon. Carbon is a building block for carbohydrates, proteins, DNA and fats.

The second reason why we need to understand the carbon cycle is because of its effect on the physical environment. Carbon dioxide is released during the combustion of fossil fuels, as well as the respiration of organisms. As we will see later, this can have a tremendous effect on our climate, since carbon dioxide is a greenhouse gas.

Carbon can be found in two forms in the carbon cycle: as a gas or as a solid. As a gas, it is mostly in the form of carbon dioxide, but it can also be found in compounds like methane and carbon monoxide. Methane and carbon dioxide are both greenhouse gases. Carbon dioxide can be taken out of the atmosphere by photosynthesis in plants. When this happens, the carbon is changed into sugars/carbohydrates. Carbon can be put back into the air during respiration, as carbon dioxide. Another way carbon can be removed from the atmosphere is by being absorbed by water. If that happens, that carbon becomes available to water plants for photosynthesis, as well as being available to form compounds such as calcium carbonate (chalk and shells of organisms).

The reason why many people are concerned about the carbon cycle is because human uses have caused the carbon cycle to change. By burning fossil fuels, humans have upset the balance of the carbon cycle and greatly increased the rate at which carbon is released in to the atmosphere.