Earth as a System: Energy, Matter, and Life · Lesson 10 of 13
Carbon Cycle
“Carbon keeps changing jobs: gas, leaf, lunch, shell, rock, and occasionally fuel.”
• Explain why carbon is essential to living organisms. • Distinguish fast and slow carbon pathways. • Trace carbon through photosynthesis, feeding, respiration and decomposition. • Describe carbon exchange with oceans. • Interpret the long-term rise and seasonal variation in atmospheric carbon dioxide. • Explain how fuel combustion changes the cycle.
A carbon atom in the air can enter a leaf, become part of a grain, move into an animal and return to the atmosphere through respiration. Another carbon atom may remain locked in sediment or fossil fuel for millions of years. The carbon cycle contains both quick biological exchanges and very slow geological storage.
The carbon cycle is the movement of carbon among the atmosphere, oceans, rocks, soil and living organisms.
Green plants and phytoplankton take in carbon dioxide during photosynthesis and build energy-rich organic molecules. Carbon passes through food chains when organisms eat plants or other organisms. Respiration releases carbon dioxide as cells obtain usable energy. When organisms die, decomposers break down their remains, returning carbon dioxide and carbon compounds to air, water and soil.
Problem
Trace some carbon from air to a person and back.
- 1.A rice plant absorbs carbon dioxide during photosynthesis.
- 2.Carbon becomes part of sugars, starch and other molecules in the grain.
- 3.A person eats the rice and digests these molecules.
- 4.Cells use some of the material in respiration.
- 5.Carbon dioxide is released and eventually exhaled into the atmosphere.
Under conditions where dead material is buried faster than it decomposes, carbon may remain stored in sediments. Over very long periods, heat and pressure can contribute to the formation of coal, petroleum and natural gas. Burning these fuels releases carbon dioxide within hours, transferring carbon from a slow geological store into the fast atmosphere-ocean-biosphere system.
Problem
Why does burning fuel disturb the timing of the carbon cycle?
- 1.The carbon was stored underground over an extremely long period.
- 2.Extraction moves the material to the surface.
- 3.Combustion rapidly combines carbon with oxygen.
- 4.Carbon dioxide enters the atmosphere much faster than the store is naturally rebuilt.
- 5.The imbalance raises atmospheric carbon dioxide when removal processes cannot keep pace.
Carbon dioxide dissolves in seawater and can form bicarbonate and carbonate ions. Phytoplankton use dissolved carbon in photosynthesis. Many organisms build calcium-carbonate shells or skeletons; after death, some material sinks and may become sediment. The ocean is therefore both an active exchange region and a major carbon store.
Pause and Ponder
A graph of atmospheric carbon dioxide measured over decades shows an overall rise with a small repeating saw-tooth pattern. The long-term increase reflects the addition of carbon dioxide, especially from fuel combustion and loss of forests. The yearly rise and fall reflects seasonal changes in photosynthesis and respiration, especially across the land-rich Northern Hemisphere.
Threads of Curiosity
The concentration rose from roughly 315 parts per million around 1960 to more than 420 parts per million by 2025 in the long record shown. The exact value changes seasonally, but the long-term direction is clear. Carbon makes up about 49 per cent of the dry mass of living organisms, while the oceans contain about 71 per cent of global carbon and the atmosphere only about 1 per cent. A small atmospheric reservoir can still have a powerful effect because its carbon dioxide directly influences heat balance.
Both pathways belong to one carbon cycle. Fast biological exchange may take days to years, while geological storage and release can span thousands to millions of years.
Quiz
Which description best matches Carbon Cycle?
Which term matches this description: The carbon cycle is the movement of carbon among the atmosphere, oceans, rocks, soil and living organisms.
Which statement is a key takeaway from this lesson?
Which additional statement is also a key takeaway from this lesson?
Which further statement is also a key takeaway from this lesson?
Practice Problems
- Trace carbon from atmospheric carbon dioxide to a decomposer.
- Explain how oceans both absorb and store carbon.
- Distinguish the saw-tooth seasonal pattern from the long-term trend.
- Why can planting trees help but not instantly reverse rapid fossil-fuel emissions?
Key Takeaways
• Carbon forms the framework of many biological molecules. • Photosynthesis removes carbon dioxide; respiration and decomposition return it. • Oceans exchange and store large amounts of carbon. • Combustion transfers carbon rapidly from geological stores to the atmosphere.