The Carbon Cycle and Long-Term Carbon Storage

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  • 0:05 The Carbon Cycle
  • 3:22 Long-Term Carbon Storage
  • 5:04 Lesson Summary
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Lesson Transcript
Instructor: Joshua Anderson
All living organisms have a role in the carbon cycle. Do you know you understand how humans, animals and plants use carbon? This lesson will introduce you to the carbon cycle and explain how it functions on a global scale.

The Carbon Cycle

In the carbon cycle, the main source of carbon is in the atmosphere
Main Source of Carbon Atmosphere

Perhaps the most studied biogeochemical cycle is the carbon cycle, which, as you may have guessed, describes the pathways that carbon takes through living organisms and the environment. The carbon cycle is very different from either the nitrogen or phosphorus cycle. One reason is that the primary source of usable carbon for plants is carbon dioxide in the atmosphere, not the soil, which is where the main sources of phosphorus and nitrogen are for terrestrial plants. Because carbon dioxide is a gas that can move freely through the air and water, the carbon cycle is truly global.

In contrast, the phosphorus and nitrogen cycles depend on local concentrations of usable nitrogen and phosphorus in the soil or water. If these local levels are changed, there can be major consequences for the local ecosystem, but since the carbon cycle is global, changes in the carbon cycle have more global consequences.

As with the other biogeochemical cycles, consumers still get their carbon from eating other organisms, but again, there is a major difference between carbon and other essential elements. The difference is that carbon is used by living organisms as the primary means of energy storage, transfer and usage. Let's take a look at how using carbon as a means to store energy affects the carbon cycle. Plants take atmospheric carbon dioxide, which has very little potential energy, and convert it to high energy-containing carbohydrates. They do this through photosynthesis, which is the process by which autotrophs convert light energy into chemical energy. Energy can also be stored in the carbon bonds of lipids and proteins.

Organisms using carbon as a means to store energy affect the carbon cycle
Carbon Used to Store, Transfer and Use Energy

Of course, if carbohydrates and other organic molecules are used to store energy, there has to be a way for organisms to release that energy for cellular functions. This process in which oxygen is used to convert organic molecules to carbon dioxide and water and provide energy for the cell is called cellular respiration. Plants use cellular respiration to tap into their energy stores, and in this way, they do return a lot of the carbon back into the atmosphere as carbon dioxide. Plants are constantly using carbon to store energy by forming carbohydrates and then breaking them up and returning carbon dioxide to the atmosphere when they use the energy. This results in a much more rapid turnover rate for carbon compared to the turnover rate for nitrogen and phosphorus in plants.

Animals are completely reliant on cellular respiration to provide for all of their energy needs. They consume plants and other animals and then break down most of the organic molecules to produce energy through cellular respiration. So, most of the carbon consumed by animals is quickly returned to the atmosphere in the form of carbon dioxide, and a small percentage is incorporated into the biomass of the animal itself. Dead animal and plant matter is consumed by detritivores, which can also use cellular respiration as their main energy source and release carbon dioxide back into the air.

Long-Term Carbon Storage

Photosynthesis and cellular respiration are both very fast processes that are almost evenly balanced on a global scale. I say 'almost' because the rate of photosynthesis exceeds the rate of cellular respiration. However, some of that difference is made up for by wildfires that burn plant matter and release carbon dioxide back into the atmosphere, much like cellular respiration does. Wildfires make up for most of that difference, but not quite. A very small amount of organic material is buried under sediment and taken out of the short-term carbon cycle that cycles carbon between living organisms, the atmosphere and the surface of the oceans. This buried organic material may eventually be turned into fossil fuels, but that's not the only way that carbon can be taken out of the short-term carbon cycle.

Coral reefs take carbon out of the short-term carbon cycle to form calcium carbonate exoskeletons
Coral Reefs Take Most Carbon Out of Short-Term Cycle

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