How are carbon and nitrogen recycled through ecosystems?
The carbon cycle including photosynthesis, respiration, combustion and decomposition, and the nitrogen cycle including the roles of decomposers and nitrogen-fixing, nitrifying and denitrifying bacteria.
A focused CCEA GCSE Biology answer on nutrient cycles, covering the carbon cycle through photosynthesis, respiration, combustion and decomposition, and the nitrogen cycle including the roles of the different bacteria.
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What this dot point is asking
CCEA wants you to describe the carbon cycle (photosynthesis, respiration, combustion and decomposition) and the nitrogen cycle, including the roles of nitrogen-fixing, nitrifying and denitrifying bacteria and decomposers.
The carbon cycle
The nitrogen cycle
Plants need nitrogen to make proteins, but they cannot use nitrogen gas directly. Bacteria make it available:
- Nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds (nitrates); some live in the root nodules of legumes such as peas and beans.
- Decomposers break down dead organisms and waste, releasing ammonia.
- Nitrifying bacteria convert ammonia into nitrites and then nitrates, which plants absorb through their roots.
- Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air.
Examples in context
- Example 1. Why farmers grow legumes or add fertiliser
- Crops remove nitrates from the soil, which can leave it short of nitrogen. Farmers restore it either by adding nitrate fertiliser or by growing legumes such as clover, whose root nodules contain nitrogen-fixing bacteria that enrich the soil. Ploughing the legumes back in returns the nitrogen compounds to the soil. This applied use of the nitrogen cycle is a common CCEA context question.
- Example 2. Why dead leaves rot away
- A fallen leaf is broken down by decomposers, bacteria and fungi, which respire as they feed on it. This releases carbon dioxide back into the air (part of the carbon cycle) and releases nitrogen compounds such as ammonia into the soil (part of the nitrogen cycle). Without decomposers, dead material and the nutrients locked inside it would pile up and the cycles would stop. This shows why decomposers are central to both nutrient cycles.
- Example 3. Why fossil fuels disturb the carbon cycle
- For millions of years carbon was locked away in coal, oil and gas, removed from the active carbon cycle. Burning these fossil fuels now releases that stored carbon as carbon dioxide much faster than photosynthesis can remove it. As a result the amount of carbon dioxide in the air rises, adding to the greenhouse effect and global warming. This shows that the natural carbon cycle is roughly balanced (photosynthesis removing carbon dioxide, respiration and decomposition returning it), but human activities such as combustion and deforestation tip the balance. CCEA links this idea to both the carbon cycle and human impact on the environment.
Try this
Q1. Name the process that removes carbon dioxide from the air. [1 mark]
- Cue. Photosynthesis.
Q2. What do nitrifying bacteria do? [1 mark]
- Cue. Convert ammonia into nitrites and then nitrates that plants can absorb.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20205 marksDescribe how carbon is cycled between the air and living organisms.Show worked answer →
Five marks for the main processes that add and remove carbon dioxide.
Carbon dioxide is removed from the air by plants during photosynthesis, and the carbon becomes part of glucose and other plant compounds.
The carbon passes along food chains when animals eat plants.
Carbon dioxide is returned to the air when plants, animals and microorganisms respire.
It is also returned when decomposers (bacteria and fungi) break down dead organisms and waste, respiring as they do.
Burning (combustion) of wood and fossil fuels also releases carbon dioxide into the air.
Markers reward photosynthesis removing carbon dioxide, respiration, decomposition and combustion returning it, with feeding moving carbon along the chain.
CCEA 20184 marksExplain the role of bacteria in the nitrogen cycle.Show worked answer →
Four marks for the different bacteria and what they do.
Nitrogen-fixing bacteria convert nitrogen gas from the air into nitrogen compounds (such as nitrates) that plants can use; some live in root nodules of legumes.
Decomposers (and ammonifying bacteria) break down dead organisms and waste, releasing ammonia.
Nitrifying bacteria convert ammonia into nitrites and then nitrates, which plants absorb through their roots.
Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the air.
Markers reward at least three of: nitrogen-fixing, nitrifying, denitrifying and decomposer bacteria, each with the correct conversion.
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)