Cycling of Matter: Carbon Cycle


The carbon cycle diagrams or models the movement of carbon through an ecosystem. Carbon is the element on which life is constructed. Like the wood framing of a house or the steel beams of a skyscraper, carbon atoms make up the basic structures of life. You are most likely familiar with its presence in carbon dioxide, a gas. It is also found in other gases, like methane, and larger organic compounds, like DNA, protein, fats, and carbohydrates (carbohydrates = sugars). Before we start our exploration, let us again define organic compounds. Recall that organic compounds are any chemicals that include carbon and at least one hydrogen atom. Organic compounds make up organic matter and can include other atoms like sulfur, phosphorous, nitrogen, and oxygen. All organisms are built from organic matter. As you explore the carbon cycle, refer to the diagram below; remember, with a cycle, there is no start or end point.


Bromothymol_blue.jpg1. As you already know, carbon dioxide is the gas we and all other animals breath out. Thus, it is present in the air, but it can also be dissolved in water. It is odd to think of a gas existing inside a liquid, but you have already observed this. In a past investigation, we breathed carbon dioxide into a solution of Bromothymol Blue, causing it to turn yellow; the color change was caused by the dissolved carbon dioxide gas in the solution. I highlight this investigation because more carbon dioxide is dissolved in the Earth's oceans than found in the atmosphere. In fact, a recent study shows that nearly half of the carbon dioxide created by humans since the industrial revolution has dissolved into the oceans, which has caused harmful effects in ocean ecosystems. The main idea is that gases are exchanged or transferred between the large bodies of water and the air.

Exhale_CO2.png2. You also know that while animals exhale carbon dioxide, plants mostly absorb it from the air through their leaves. During photosynthesis, plants use the carbon atoms inside carbon dioxide to construct glucose, which is one variety of sugar. Sugars are organic compounds consumed by herbivores, omnivores, and decomposers for energy.

Photosynthesis.png3. Organisms that consume plants and the organisms that eat them use the sugars generated during photosynthesis for energy. The carbon inside the sugars mostly exits the body as carbon dioxide during respiration or breathing. Of course, not all gas exits our mouth. It is estimated that the average human produces 2.3 lbs of carbon dioxide gas each day. In the end, carbon is returned back to the atmosphere, where it can be used again by plants.

Decomposition.JPG4. Not all organism are eaten, rather they are decomposed. Fallen leaves, dead animals, and waste all provide nutrition for decomposers, which are organisms that break down organic matter. They too release much of the carbon as carbon dioxide or methane, but sometimes produce other organic compounds like ethanol as a waste product. Interestingly, the left overs of decomposition can produce other well known substances, including limestone, oil, coal and soil. These substances all include large amounts of carbon. Limestone is mostly made up of the shells and bones of dead organisms, while coal and oil (also known as fossil fuels) are formed from remaining organic matter that becomes trapped underground.

Air_Pollution.jpg5. Humans use both coal and oil to generate energy, and when burned they release the carbon inside these substances back into the air as carbon dioxide. Furthermore, the production of fuel from oil and coal often releases methane, another carbon containing gas. Carbon dioxide and methane are classified as greenhouse gases and contribute to air pollution and global warming. Measurements taken in 2005 show that carbon dioxide levels have increased by 35% since the industrial revolution, a time when coal mining sharply increased.