The carbon cycle describes how carbon atoms move among the atmosphere, living things, oceans, rocks, soils, and fossil fuels. Carbon matters because it is the backbone of biological molecules such as carbohydrates, fats, proteins, and DNA. It also affects climate because carbon dioxide and methane trap heat in Earth’s atmosphere.
Understanding the cycle helps explain how ecosystems function and how human activities change the planet.
Key Facts
- Photosynthesis stores carbon: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2.
- Cellular respiration releases carbon: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy.
- Combustion releases stored carbon: fuel + O2 -> CO2 + H2O + energy.
- Carbon moves from the atmosphere into food webs when producers fix CO2 into organic molecules.
- Oceans absorb and release CO2, and some dissolved carbon becomes carbonate ions used by shells and coral.
- Human burning of fossil fuels transfers carbon from long-term geologic storage to the atmosphere much faster than natural processes remove it.
Vocabulary
- Carbon cycle
- The carbon cycle is the movement of carbon through the atmosphere, organisms, oceans, soil, rocks, and human-made systems.
- Photosynthesis
- Photosynthesis is the process by which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into sugars and oxygen.
- Cellular respiration
- Cellular respiration is the process by which cells break down sugars with oxygen to release energy, carbon dioxide, and water.
- Fossil fuel
- A fossil fuel is coal, oil, or natural gas formed from ancient organic matter and used as an energy source.
- Carbon sink
- A carbon sink is a reservoir, such as a forest, soil, or ocean, that absorbs more carbon than it releases over a period of time.
Common Mistakes to Avoid
- Thinking carbon is only found in carbon dioxide is wrong because carbon is also in sugars, fats, proteins, DNA, shells, rocks, fossil fuels, and many other materials.
- Forgetting that respiration occurs in plants is wrong because plants both photosynthesize and respire, using sugars for energy and releasing CO2.
- Treating the carbon cycle as a perfect circle with equal flows is wrong because carbon can stay in reservoirs for very different lengths of time, from days in leaves to millions of years in fossil fuels.
- Assuming oceans only absorb CO2 is wrong because oceans both absorb and release carbon dioxide depending on temperature, chemistry, currents, and biological activity.
Practice Questions
- 1 A plant takes in 24 molecules of CO2 during photosynthesis. Using 6CO2 + 6H2O -> C6H12O6 + 6O2, how many molecules of glucose can it make, and how many molecules of O2 are released?
- 2 A power plant burns fuel and releases 5000 kg of CO2 in one hour. If a nearby forest absorbs 1200 kg of CO2 in the same hour, what is the net CO2 added to the atmosphere?
- 3 Explain why cutting down a forest and burning the wood can increase atmospheric CO2 in two different ways.