Question
Explain how carbon moves through the atmosphere, biosphere, and lithosphere. Describe the major processes involved in the carbon cycle.
(NEET, CBSE Class 12 — Ecosystem)
Solution — Step by Step
Green plants fix atmospheric CO into organic molecules (glucose) through photosynthesis: . This is the primary pathway by which carbon enters living systems. Approximately 4 x 10 kg of carbon is fixed annually.
Herbivores eat plants and incorporate carbon into their bodies. Carnivores eat herbivores. At each trophic level, carbon is passed along in the form of organic compounds (carbohydrates, fats, proteins).
All living organisms (plants, animals, decomposers) release CO back to the atmosphere through cellular respiration: . This is the reverse of photosynthesis in terms of carbon flow.
When organisms die, decomposers (bacteria, fungi) break down the organic matter, releasing CO into the atmosphere. Some carbon enters the soil as humus.
Some organic matter gets buried under sediments over millions of years, forming fossil fuels (coal, petroleum, natural gas). Marine organisms form calcium carbonate shells that become limestone. Burning fossil fuels and volcanic activity release this stored carbon back as CO.
graph TD
A["Atmospheric CO₂"] -->|"Photosynthesis"| B["Plants"]
B -->|"Food chains"| C["Animals"]
B -->|"Respiration"| A
C -->|"Respiration"| A
B --> D["Dead organic matter"]
C --> D
D -->|"Decomposition"| A
D -->|"Burial over millions of years"| E["Fossil Fuels / Limestone"]
E -->|"Combustion / Volcanic activity"| A
A -->|"Dissolves in ocean"| F["Ocean CO₂"]
F -->|"Marine photosynthesis"| AWhy This Works
Carbon is the backbone of all organic molecules. The carbon cycle ensures that carbon is continuously recycled between the atmosphere, living organisms, oceans, and the Earth’s crust.
The atmospheric CO concentration (about 0.04%) is maintained by a balance between photosynthesis (which removes CO) and respiration + combustion (which add CO). Human activities (burning fossil fuels, deforestation) are adding extra CO, disrupting this balance and causing global warming.
Alternative Method — Ocean Carbon Sink
Oceans absorb about 25% of human-generated CO. CO dissolves in seawater, forming carbonic acid. Marine phytoplankton also fix carbon via photosynthesis. When these organisms die, their shells sink to the ocean floor — this is called the biological pump.
For NEET, the key fact about the carbon cycle: only 0.04% of atmospheric gases are CO, yet this tiny fraction sustains all life through photosynthesis. The cycle has both a rapid component (photosynthesis-respiration, years) and a slow component (fossil fuel formation, millions of years).
Common Mistake
Students often forget that plants also respire. Plants are not just CO absorbers — they release CO through respiration 24 hours a day. Net CO absorption by plants happens only because photosynthesis rate exceeds respiration rate during daylight hours.