Photosynthesis is the primary mechanism for removing carbon dioxide () from the atmosphere and converting it into organic matter.
Green plants and algae capture light energy to fix carbon into glucose (), which serves as the energy source and structural material for the organism.
This process effectively transforms inorganic carbon into biomass, making it available to the rest of the food web.
Respiration is the biological process that returns carbon to the atmosphere as . It occurs in the cells of plants, animals, and microorganisms.
Decomposition involves microorganisms like bacteria and fungi breaking down dead organisms and waste products (detritus).
During decomposition, these microbes perform respiration to obtain energy from the waste, releasing back into the environment as a byproduct.
If organic matter is buried in environments lacking oxygen (anaerobic conditions), decomposers cannot function, and the carbon may eventually be converted into fossil fuels over millions of years.
Combustion is the chemical process of burning these fossil fuels or contemporary biomass (like wood) in the presence of oxygen.
This reaction rapidly releases stored carbon back into the atmosphere as , often at a much faster rate than it was originally sequestered.
| Process | Direction of Carbon Flow | Primary Organisms Involved |
|---|---|---|
| Photosynthesis | Atmosphere Organism | Plants, Algae, Cyanobacteria |
| Respiration | Organism Atmosphere | All living cells (Plants & Animals) |
| Decomposition | Dead Matter Atmosphere | Bacteria and Fungi |
| Combustion | Fossil/Biomass Atmosphere | Human activity / Natural fires |
Follow the Arrows: In exam diagrams, always check the direction of the arrows. Arrows pointing away from the atmosphere represent carbon fixation (photosynthesis), while arrows pointing toward the atmosphere represent release (respiration, decomposition, or combustion).
Don't Forget Plants: A common mistake is assuming plants only perform photosynthesis. Remember that plants respire 24/7, meaning they are constantly releasing some even while they are absorbing it during the day.
Decomposers are Respirers: When asked how carbon returns to the air from dead leaves, the answer is 'microbial respiration' during decomposition, not just 'rotting'.
Check for Fossilization: If a diagram shows an arrow from dead matter that does not go to the atmosphere, it likely represents the formation of fossil fuels due to specific environmental conditions.
