3.3Unit 2 · Ecology & the Environment
Cycles within Ecosystems — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
5 min read 7 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 7Cycles within Ecosystems
1
Carbon cycle overview
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.
2
Removal of CO2
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.
3
Return of CO2
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.
4
Human impact on carbon cycle
Human impact on carbon cycle burning fossil fuels and deforestation increase CO2 in the atmosphere, contributing to the enhanced greenhouse effect.
5
Nitrogen cycle overview
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.
6
Fixation
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.
7
Decomposition and nitrification
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.
8
Denitrification
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 7Process & sequence
1
Nitrogen gas in the air is fixed by bacteria in root nodules or by lightning.
2
Plants absorb nitrates from soil by active transport and make amino acids and proteins.
3
Animals eat plants, gaining nitrogen as protein.
4
Death and excretion return nitrogen compounds to soil; decomposers make ammonium.
5
Nitrifying bacteria convert ammonium to nitrites then nitrates.
6
Denitrifying bacteria in waterlogged soil return nitrogen gas to the air.
The nitrogen cyclewrite these steps in this order; each step is a marking point.
1
N2 fixed by bacteria/lightning
2
Nitrates absorbed by plants
3
Proteins passed to animals
4
Decomposers make ammonium
5
Nitrifying bacteria make nitrate
6
Denitrifying bacteria release N2
Section 3 of 7Compare
| 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 7Worked examples
1Explain 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,
2Describe 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.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Carbon cycle · Nitrogen cycle · Decomposer · Nitrogen fixation · Nitrification · Denitrification · Ammonification · Root nodules · Combustion
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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