The Carbon Cycle
Describe the carbon cycle, tracing carbon through photosynthesis, respiration, feeding, decomposition, and combustion, and explain the role of each process
What a learner can do afterwards
- Draws or labels a diagram of the carbon cycle showing the main processes
- Explains how photosynthesis removes carbon dioxide from the atmosphere
- Explains how respiration, combustion, and decomposition return carbon dioxide to the atmosphere
- Discusses how human activities (burning fossil fuels, deforestation) disrupt the carbon cycle
1 · Read
Carbon is the second most common element in your body, and it sits inside sugars, wood, skin and many other parts of living things. Carbon never disappears. It just moves. It travels in a loop between the air, living things, the soil, and the oceans called the carbon cycle. You already know plants pull carbon dioxide out of the air during photosynthesis. That is where most carbon enters living things.
Think of the cycle as two loops running at different speeds. The fast loop runs between living things. Plants build carbon into sugar, animals eat the plants, and both breathe carbon dioxide back out through respiration. When things die, decomposers break them down and release the rest. The slow loop runs through rocks and oceans. Carbon dioxide dissolves into seawater, sea creatures build it into shells, and over millions of years those shells can harden into limestone rock. Buried plants can slowly turn into coal, oil, and gas. Burning also returns carbon to the air.
Take a wooden table. That carbon was once carbon dioxide floating in the air. A tree absorbed it through photosynthesis and used the carbon to build sugar, then wood. When the tree was cut down, the carbon stayed locked in your table's wood.
Burning coal, oil, and gas releases carbon that has been locked underground for millions of years. Cutting down forests removes trees that would pull CO2 back out of the air. Both add extra carbon to the air faster than the cycle can take it back, which scientists link to our changing climate.
Carbon keeps moving between air, living things, oceans, and rocks, in a fast loop through life and a slow loop through the ground. Human activity is adding it to the air faster than the cycle can absorb it.
Adapted from OpenStax Biology (CC-BY 4.0), openstax.org · license
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.