6.3.1(c) - Carbon cycling in ecosystems

Why Carbon Cycles

Every carbohydrate, lipid, protein and nucleic acid in an organism contains carbon. That means carbon is not a small extra detail in ecosystems: it is part of the structure of living matter. When a plant grows new leaves, when a herbivore builds muscle from food, or when a fungus breaks down dead wood, carbon atoms are being moved from one store to another.

A useful way to read the cycle is to separate stores from processes. A store is where carbon is held for a time, such as carbon dioxide in the atmosphere or carbon compounds in plant biomass. A process is the arrow that moves carbon between stores, such as photosynthesis, respiration, feeding, decomposition or combustion.

This matters because ecosystems recycle matter but do not recycle energy in the same way. Carbon atoms can be reused many times. Energy enters most ecosystems as light, is transferred through food chains, and is eventually dissipated as heat during respiration and other transfers.

Because matter can return to earlier stores while transferred energy is dissipated as heat, cycling carbon is different from tracing energy flow.

Photosynthesis And Feeding

The main biological route into a food web is photosynthesis. Producers such as plants, algae and some bacteria take in carbon dioxide and use light energy to make organic molecules. In land ecosystems, the carbon dioxide is usually taken from the air. In aquatic ecosystems, producers can use dissolved carbon dioxide and related dissolved inorganic carbon.

For this lesson, you do not need to revisit the detailed light-dependent and light-independent stages of photosynthesis. What matters is the direction of carbon movement: carbon dioxide is converted into organic carbon in producer biomass. Some of that organic carbon may become glucose, starch, cellulose, lipids, proteins or nucleic acids.

The producer is now a carbon store. When a herbivore eats a plant, some organic carbon passes into the herbivore. When a predator eats the herbivore, carbon can move again into a higher consumer. This is not perfectly efficient: some carbon is lost in faeces, some remains in uneaten plant material, and all living organisms respire.

If a carbon-cycle question asks for the process that removes carbon dioxide from the atmosphere, the safest answer is photosynthesis. If it asks what happens to that carbon next, say it is converted into organic compounds/biomass in producers.

In this photosynthetic food web, feeding moves fixed organic carbon onward after photosynthesis has performed the initial fixation step.

Respiration And Decomposition

Respiration returns carbon to the environment as carbon dioxide. A common mistake is to think only animals respire. Producers, consumers and decomposers all respire because they all need ATP for cell activities.

This equation is useful as a carbon-cycle summary, not as a claim that photosynthesis and respiration are identical reverse pathways in every detail. The important OCR idea here is that respiration oxidises organic carbon compounds and releases carbon dioxide.

Decomposition begins when decomposers, mainly bacteria and fungi, break down dead organisms and waste materials. They secrete enzymes onto complex organic matter, digest it into smaller soluble molecules, absorb those products, and use them in growth and respiration. Some carbon therefore enters decomposer biomass, while much is returned as carbon dioxide during decomposer respiration.

Dead organic matter includes fallen leaves, dead animals, dead microorganisms, faeces and other waste. Without decomposers, carbon would remain locked in dead material for much longer, reducing the return of carbon dioxide that producers need for photosynthesis.

Following Carbon Through A Fallen Leaf

A leaf falls from a tree. Its cellulose, starch, proteins and lipids contain organic carbon. Decomposer microorganisms digest the leaf material, absorb small molecules and respire. Carbon atoms from the leaf can become part of decomposer biomass, remain for a time in soil organic matter, or be released as carbon dioxide during respiration.

The decomposer step therefore links carbon in dead organic matter to both decomposer biomass and carbon dioxide released in respiration.

Physical And Chemical Routes

The carbon cycle is not only a set of feeding and respiration arrows. OCR also expects physical and chemical effects in carbon cycling. The diagram pulls these routes together.