Question
Explain the carbon cycle, showing how carbon moves through the atmosphere, biosphere, hydrosphere, and lithosphere.
Solution — Step by Step
flowchart TD
A[Atmospheric CO2] -->|Photosynthesis| B[Plants - Organic Carbon]
B -->|Respiration| A
B -->|Consumed by| C[Animals]
C -->|Respiration| A
B -->|Death/Decomposition| D[Soil Organic Matter]
C -->|Death/Decomposition| D
D -->|Decomposer Respiration| A
D -->|Fossilization over millions of years| E[Fossil Fuels]
E -->|Combustion| A
A -->|Dissolves in| F[Ocean CO2]
F -->|Marine Photosynthesis| G[Marine Organisms]
G -->|Shell formation| H[Limestone/Sediments]Plants and algae absorb atmospheric CO and convert it to organic carbon (glucose) through photosynthesis: . This is the primary route for carbon to enter living systems.
Herbivores eat plants, carnivores eat herbivores — carbon moves up the food chain as organic molecules (carbohydrates, fats, proteins).
All living organisms release CO through cellular respiration. When organisms die, decomposers (bacteria, fungi) break down organic matter, releasing CO back to the atmosphere.
Some organic matter gets buried before decomposing and, over millions of years, forms fossil fuels (coal, petroleum, natural gas). Marine organisms form calcium carbonate shells that accumulate as limestone. These are long-term carbon sinks.
Burning fossil fuels releases stored carbon as CO. Volcanic eruptions also release CO from deep within the Earth. Human activity has dramatically increased the combustion pathway, causing rising atmospheric CO levels.
Why This Works
The carbon cycle maintains atmospheric CO at levels suitable for life. Carbon is the backbone of all organic molecules, so its cycling between the atmosphere, biosphere, and lithosphere is fundamental. The current increase in atmospheric CO (from ~280 ppm pre-industrial to ~420 ppm now) is because fossil fuel combustion releases carbon faster than photosynthesis and ocean absorption can remove it.
Common Mistake
Students write that “only plants remove CO from the atmosphere.” The oceans are a major carbon sink — they absorb about 25% of human-emitted CO. This dissolved CO forms carbonic acid, contributing to ocean acidification. Both terrestrial photosynthesis and ocean absorption must be mentioned for a complete answer.