Topic 19.3 · Core
The Carbon Cycle
Unlike energy, carbon doesn’t leave biological systems — it moves in a loop between the atmosphere, organisms and, over very long timescales, fossil fuels. Cambridge limits the carbon cycle to six named processes, and the mark-losing habit to avoid is naming them without stating which direction carbon actually moves in each one.
Carbon exists both as atmospheric carbon dioxide and as organic compounds inside organisms, and the carbon cycle describes how it moves between those two forms. For this syllabus, the cycle is limited to six processes: photosynthesis, feeding, respiration, decomposition, formation of fossil fuels, and combustion.
Photosynthesis removes carbon dioxide from the atmosphere
Green plants absorb carbon dioxide, and during photosynthesis, carbon from that carbon dioxide becomes incorporated into organic molecules inside plant cells. This moves carbon from atmospheric carbon dioxide into organic compounds in producers. The full mechanism of photosynthesis itself belongs to plant nutrition rather than this chapter — here, the required idea is simply that this is the entry point through which carbon joins the biological part of the cycle.
Feeding transfers carbon between organisms
When an animal eats a plant, carbon-containing organic compounds from the plant pass into the animal, and when one animal eats another, carbon is transferred again. Feeding therefore moves carbon along a food chain: producer → consumer → higher-level consumer. It’s worth being precise here — carbon is being transferred between organisms, not created or destroyed at any point in this step.
Respiration returns carbon dioxide
Plants, animals and decomposers all respire. During respiration, carbon-containing compounds are broken down and carbon dioxide is produced, so carbon can move from organic compounds in organisms back into carbon dioxide in the atmosphere. This returned carbon dioxide is exactly what can later be used again in photosynthesis — respiration is what closes the loop that photosynthesis opens.
Decomposition
When organisms die or produce organic waste, decomposers obtain nutrients and energy from that material, and as they do, they respire and release carbon dioxide. Decomposition therefore provides the pathway through which carbon locked in dead and waste organic material can eventually return to the atmosphere, rather than staying trapped in tissue that’s no longer part of a living organism.
Formation of fossil fuels
Some dead organic material doesn’t decompose completely. Over very long periods of geological time, some of this carbon-containing material can form fossil fuels, so carbon can remain stored — effectively removed from the short-term cycle — until those fuels are eventually used.
Combustion
When fossil fuels are burned, combustion releases the carbon dioxide that had been stored in them back into the atmosphere: fossil fuels → atmospheric carbon dioxide. That completes the cycle Cambridge requires for this syllabus point.
Putting the whole pathway together
The complete syllabus pathway connects as: atmospheric carbon dioxide → photosynthesis → organic compounds in plants → feeding → animals, with carbon returning to the atmosphere through respiration and decomposition, and a smaller share of it passing through dead organic material → fossil fuels → combustion → atmospheric carbon dioxide. For this chapter, these are the processes you need to be able to follow and name in order — additional geological and oceanic carbon pathways sit outside the stated syllabus boundary, so you don’t need them here, even if you’ve encountered them elsewhere.
The nitrogen cycle works on the same basic principle of recycling rather than one-way flow, but through a different set of processes — covered on the nitrogen cycle page.