Nutrient Cycles
Senior 3 · Biology
Nutrient Cycles
Lesson 2 of 4
25 min
Trucks loaded with sacks of charcoal are a common sight on Rwanda's rural roads. Every piece of that charcoal was made from wood, and the wood was made by a tree from carbon dioxide taken out of the air by photosynthesis. When the charcoal burns in a kitchen stove, the carbon goes back into the air as carbon dioxide. Carbon moves round and round between the air, living things, the soil, rocks and water: the carbon cycle. All the carbon in your body was once carbon dioxide in the air.
Definition
Carbon cycle: the continuous movement of carbon between the air (as ), living organisms (as carbon compounds such as carbohydrates, fats and proteins), dead matter, fossil fuels, rocks and water.
Photosynthesis removes from the air: (using light energy).
Respiration returns to the air: (releasing energy).
Combustion: burning a fuel in oxygen, which releases the carbon it contains as .
Fossil fuels: coal, oil and natural gas, formed over millions of years from the remains of organisms that did not fully decay. Peat is partly decayed plant matter that builds up in waterlogged wetlands.
Carbon store (sink): a place where carbon is held for a long time, such as forests, soils, peat bogs, fossil fuels, oceans and lakes.
Figure 1: The carbon cycle. Follow the numbered arrows with the key below.
| Arrow | Process | Effect on CO₂ in the air |
|---|---|---|
| 1 | Photosynthesis by green plants and algae | removes |
| 2, 4, 8 | Respiration by plants, animals and decomposers | adds |
| 3 | Feeding: carbon compounds pass along food chains | none directly |
| 5, 6 | Death, faeces, fallen leaves and other wastes | none directly |
| 7 | Decay: decomposers feed on dead matter | none directly (see 8) |
| 9 | Fossilisation: remains that do not decay become peat, coal, oil and gas (millions of years) | locks carbon away |
| 10 | Combustion of fossil fuels and peat | adds |
| 11 | Burning wood, charcoal and forests | adds |
| 12 | dissolving in, and escaping from, lakes and oceans | both |
Worked example 1: carbon dioxide from firewood
Dry wood is about 50% carbon by mass. A family burns 10 kg of dry firewood. What mass of carbon dioxide is released? (Relative atomic masses: C = 12, O = 16.)
Answer: about 18 kg of carbon dioxide, almost twice the mass of the wood, because each carbon atom joins two oxygen atoms from the air. If new trees grow to replace the wood, their photosynthesis takes this carbon back out of the air; if they do not, the carbon stays in the air.
Lake Kivu is unusual. Its deep water, below about 60 m, never mixes with the surface water, and it holds huge amounts of dissolved gases: scientists estimate about 300 km³ of carbon dioxide and 60 km³ of methane (measured as gas at 0 °C and normal air pressure). The carbon dioxide comes mainly from volcanic sources under the lake; most of the methane is made by bacteria from carbon dioxide and from the decay of dead matter sinking from above. Since 2015 the KivuWatt plant has pumped deep water up, separated the methane and burned it to make electricity; Rwanda Energy Group reports a capacity of 26 MW.
For thousands of years, photosynthesis and dissolving (arrows 1 and 12) roughly balanced respiration and the release of from water. Human activities now add carbon to the air faster than it is removed:
Carbon dioxide is a greenhouse gas: it lets sunlight through but absorbs some of the heat that Earth gives off. More traps more heat, which is warming Earth's climate.
Figure 2: Yearly mean carbon dioxide in the air at Mauna Loa, Hawaii, 1960–2025, in parts per million (ppm). Data: NOAA Global Monitoring Laboratory and Scripps Institution of Oceanography.
Worked example 2: is the rise speeding up?
Use Figure 2: 316.91 ppm in 1960, 414.21 ppm in 2020, 401.01 ppm in 2015 and 427.35 ppm in 2025. Compare the mean rate of increase from 1960 to 2020 with the rate from 2015 to 2025.
Conclusion: carbon dioxide is not only rising; it is rising faster, because more fossil fuel is being burned each year than photosynthesis and the oceans can absorb.
Warning
Common mistakes. (1) "Plants only photosynthesise and animals only respire": plants respire all the time, day and night. (2) Forgetting the decomposers: their respiration returns most of the carbon in dead matter to the air. (3) Reading Figure 2 as if the vertical axis started at zero: it starts at 300 ppm, so the rise is about 35% (from 317 to 427), not ten-fold.
GCSE Biology: Carbon Cycle and Water Cycle (Cognito). Watch the carbon cycle section and match each process in the video to a numbered arrow in Figure 1.
Tip
Memory hook: "one way in, four ways out". Carbon enters living things by one main route, photosynthesis. It returns to the air by respiration (plants, animals, decomposers) and by combustion. In an exam diagram, check that every arrow points the right way.
Practice: the carbon cycle
Key points
Sources and credits
Written by Ganzaa: Figures 1 and 2 (Figure 2 drawn from the NOAA data), the worked examples and the practice questions.
