To trace carbon through a system, follow one carbon atom from store to store and name the process on every arrow. The skill appears whenever a question supplies a cycle diagram or a short description of an ecosystem and asks you to explain how carbon moves.
It builds on photosynthesis, respiration and feeding, and sets up decomposition and nutrient recycling. The wider topic is in nutrient cycles and populations.
How does the idea build up?
Carbon sits in a few main stores: carbon dioxide in air or water, organic compounds in living things, dead organic matter, and fossil fuels. Arrows between the stores are processes.
- Photosynthesis moves carbon from carbon dioxide into carbohydrates in producers.
- Feeding passes organic carbon from one organism to the next along the food chain.
- Respiration releases carbon dioxide from organisms back to air or water.
- Death and decomposition move carbon from organisms into dead matter, and decomposers respire it back to carbon dioxide.
- Combustion of wood or fossil fuels releases carbon dioxide quickly.
A good route always alternates a store, then the process that moves it.
Worked example
A supplied diagram shows a small forest. Invented yearly flows, in units of carbon: photosynthesis takes in 120, plant respiration releases 60, animal respiration releases 6, decomposer respiration releases 50.
Question: Trace one carbon atom from the air to a bird, and back. Then decide whether the forest is gaining or losing carbon.
Step 1, air to plant: the atom is in carbon dioxide. Photosynthesis in a leaf builds it into glucose.
Step 2, plant to caterpillar: the caterpillar eats the leaf, so the atom passes on in organic compounds by feeding.
Step 3, caterpillar to bird: the bird eats the caterpillar, and the atom is in the bird’s tissues.
Step 4, back to air, route A: the bird respires, and the atom leaves as carbon dioxide.
Step 4, back to air, route B: the bird dies, decomposers feed on the body, and their respiration releases the atom as carbon dioxide.
Step 5, balance: carbon released = 60 + 6 + 50 = 116. Carbon taken in = 120. Net change = 120 − 116 = +4, so the forest stores 4 more units of carbon than it releases this year, for example as growing wood.
This is invented data for practice, not measurements from a real forest.
What mistake should I watch for?
Mistaken answer: “The plant takes carbon from the air, the animal eats it, and then the carbon goes into the soil.”
There are two problems. The first step names no process, so photosynthesis is missing. The second is that carbon does not simply “go into the soil”: it moves to dead matter, and decomposers respire it back to carbon dioxide.
Corrected: “Carbon dioxide is absorbed by the plant and built into glucose by photosynthesis. It passes to the animal by feeding. When the animal dies, decomposers break down the body and respire, releasing carbon dioxide into the air.”
Each arrow now has a process and each store has a form of carbon.
Check yourself
1. Name the process that moves carbon from a grass leaf into a rabbit.
Show answer
Feeding. The rabbit eats the grass, so organic carbon compounds pass from the plant to the animal.
2. In a marsh, photosynthesis takes in 80 units of carbon and all respiration plus decomposition releases 85 units. Is the marsh gaining or losing carbon overall?
Show answer
Net change = 80 − 85 = −5. The marsh is losing 5 units, so it releases more carbon than it stores that year.
3. Give two processes that release carbon dioxide into the air.
Show answer
Any two from respiration (by plants, animals or decomposers) and combustion (of wood or fossil fuels).
Where does this lead next?
Next, see how decomposers recycle nutrients, then test yourself on the mixed practice set.
Students often understand the cycle but lose marks by leaving out a process word. A teacher in online one-to-one Biology tuition can read your written route with you and mark exactly where it skips.