3.3Unit 2 · Ecology & the Environment
Cycles within Ecosystems — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
5 min read 4 parts Lesson 4 of 5 1 quiz
Lesson summary
Describe the stages of the carbon cycle.Plants cannot use nitrogen gas directly; they absorb nitrate ions.Nitrogen is needed to make amino acids, proteins and DNA.Leguminous plants (peas, beans, clover) have nitrogen-fixing bacteria in root nodules.Ploughing aerates soil, helping…
- Carbon cycle overview
- Removal of CO2
- Return of CO2
- Human impact on carbon cycle
- Nitrogen cycle overview
- Decomposition and nitrification
Section 1 of 4Cycles within Ecosystems — Unit 2
Carbon cycle overview:
carbon is found as CO2 in the air, in carbon compounds in organisms, dissolved in oceans, and in fossil fuels and limestone.Removal of CO2:
plants and algae take in CO2 for photosynthesis, making glucose which becomes starch, cellulose, proteins and fats. Carbon then passes to animals when they feed.Return of CO2:
all organisms respire, releasing CO2.- When organisms die, decomposers (bacteria and fungi) feed on them and respire, releasing CO2.
- If decay cannot happen, remains may form fossil fuels over millions of years; burning (combustion) of these fuels releases the stored carbon.
Human impact on carbon cycle:
burning fossil fuels and deforestation increase CO2 in the atmosphere, contributing to the enhanced greenhouse effect.Nitrogen cycle overview:
air is about 78% nitrogen but it is unreactive, so plants rely on nitrates in soil to make proteins. Animals get nitrogen by eating plants or other animals.Fixation:
nitrogen-fixing bacteria in soil and in the root nodules of legumes convert N2 into ammonium compounds. Lightning and the industrial Haber process also fix nitrogen.Decomposition and nitrification:
decomposers convert proteins in dead organisms, faeces and urea into ammonium ions. Nitrifying bacteria oxidise ammonium to nitrite and then nitrate, which plants absorb. These bacteria need oxygen.Denitrification:
in waterlogged soils lacking oxygen, denitrifying bacteria convert nitrates into nitrogen gas, reducing soil fertility. Farmers drain and plough soil to prevent this.
Section 2 of 4Process & sequence
- Nitrogen gas in the air is fixed by bacteria in root nodules or by lightning.
- Plants absorb nitrates from soil by active transport and make amino acids and proteins.
- Animals eat plants, gaining nitrogen as protein.
- Death and excretion return nitrogen compounds to soil; decomposers make ammonium.
- Nitrifying bacteria convert ammonium to nitrites then nitrates.
- Denitrifying bacteria in waterlogged soil return nitrogen gas to the air.
The nitrogen cycle — write these steps in this order; each step is a marking point.
- N2 fixed by bacteria/lightning
- Nitrates absorbed by plants
- Proteins passed to animals
- Decomposers make ammonium
- Nitrifying bacteria make nitrate
- Denitrifying bacteria release N2
Section 3 of 4Compare
| Process | Effect on atmospheric CO2 | Carried out by |
|---|---|---|
| Photosynthesis | Removes | Plants, algae |
| Respiration | Adds | All living organisms |
| Decomposition | Adds (via respiration) | Bacteria and fungi |
| Combustion | Adds | Burning fossil fuels and wood |
| Feeding | No direct change – carbon passes along food chains | Animals |
| Bacteria | Conversion | Conditions |
|---|---|---|
| Nitrogen-fixing | N2 → ammonium compounds | Root nodules of legumes, or free in soil |
| Decomposers | Proteins/urea → ammonium | Soil |
| Nitrifying | Ammonium → nitrite → nitrate | Aerobic, well-aerated soil |
| Denitrifying | Nitrate → N2 | Anaerobic, waterlogged soil |
Section 4 of 4Worked examples
Explain why farmers often grow clover or beans in a field before growing wheat.
- Legumes have root nodules containing nitrogen-fixing bacteria that convert nitrogen gas into ammonium compounds.
- When the crop is ploughed in, decomposers and nitrifying bacteria convert these into nitrates,
- increasing soil nitrate for the following wheat crop to make proteins.
- When the crop is ploughed in, decomposers and nitrifying bacteria convert these into nitrates,
- Legumes have root nodules containing nitrogen-fixing bacteria that convert nitrogen gas into ammonium compounds.
Describe how carbon in a dead leaf can end up in the atmosphere.
- Decomposers such as fungi and bacteria digest the carbon compounds in the leaf and absorb the products.
- They use them in respiration, which releases carbon dioxide into the atmosphere.
- Decomposers such as fungi and bacteria digest the carbon compounds in the leaf and absorb the products.
Marked practiceQuizzes for this lesson
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