18.1.5Organisms and Their Environment
Nutrient Cycles — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 6 of 6
Section 1 of 9Why nutrients are recycled
1Finite amount
- Finite amount there is a limited amount of each element on the planet.
2Recycling
- Recycling nutrients such as carbon and nitrogen must be recycled so new organisms can be made and grow.
Section 2 of 9The carbon cycle
1Photosynthesis
- Photosynthesis plants remove carbon dioxide from the atmosphere.
2Feeding
- Feeding carbon is passed to animals and microorganisms.
3Respiration
- Respiration plants, animals and microorganisms return carbon dioxide to the atmosphere.
4Decomposition
- Decomposition decomposers release carbon dioxide from dead material.
5Fossil fuels
- Fossil fuels if organisms die where decomposers are absent, their carbon can become fossil fuels over millions of years.
6Combustion
- Combustion burning fossil fuels combines carbon with oxygen and releases carbon dioxide.
Section 3 of 9Human effects on the carbon cycle
1Fossil fuels
- Fossil fuels increased use raises atmospheric carbon dioxide.
2Deforestation
- Deforestation reduces the number of producers that remove carbon dioxide.
3Burning forests
- Burning forests clearing land by burning releases yet more carbon dioxide.
Section 4 of 9The nitrogen cycle: needs and sources
1Need
- Need nitrogen is required to make proteins.
2Nitrogen gas
- Nitrogen gas neither plants nor animals can absorb it, as it is very stable.
3Nitrogen-fixing bacteria
- Nitrogen-fixing bacteria convert nitrogen gas to ammonium compounds, then to nitrates.
- They can be free-living in soil or live in root nodules of some plants.
4Lightning
- Lightning splits the N–N bond, forming oxides that dissolve in rainwater and enter the soil.
Section 5 of 9Steps in the nitrogen cycle
1
Plants absorb nitrates from the soil and use them to build proteins.
2
Animals eat plants and get nitrogen from plant proteins, passing it up the food chain.
3
Animal waste, such as the urea in urine, returns nitrogen to the soil as ammonium compounds.
4
Dead plants and animals decay and decomposers break proteins into ammonium compounds.
5
Nitrifying bacteria convert ammonium to nitrites and then nitrates, which plants absorb.
6
Denitrifying bacteria convert nitrates back into nitrogen gas.
Section 6 of 9Denitrification
Effect
- Effect it reduces soil fertility and is bad for plant growth.
Conditions
- Conditions denitrifying bacteria are anaerobic.
Control
- Control aerating the soil, by reducing waterlogging and ploughing, reduces denitrification.
Section 7 of 9More from the specification
1
Biogeochemical Carbon Cycling Dynamics
- Carbon atoms cycle continuously between the atmosphere and biosphere via photosynthetic fixation into organic glucose by producers,
- and return via respiration, decomposition,
- and fossil fuel combustion.
2
Nitrogen Cycle Microbial Conversions
- Specialized nitrifying bacteria convert ammonium ions into nitrites and subsequently into nitrates for plant root uptake,
- while nitrogen-fixing bacteria fix atmospheric N2 and denitrifying bacteria return N2 to the atmosphere under anaerobic conditions.
3
Role of Decomposers in Nutrient Recycling
- Saprotrophic fungi and bacteria secrete extracellular enzymes that digest dead organic matter, liberating locked mineral ions back into the soil solution to be reabsorbed by autotrophic producers.
4
Source
- A part of a plant that releases sucrose or amino acids.
5
Energy transfer
- The movement of energy between trophic levels through feeding.
6
Primary consumer
- Organism that eats producers.
7
Secondary consumer
- An organism that eats primary consumers.
8
Population
- A group of organisms of one species in the same area at the same time.
9
Biomass
- The dry mass of living matter in an organism or trophic level.
10
Consumer
- An organism that gets energy by feeding on other organisms.
11
Ecosystem
- Community plus environment.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Nitrogen fixation | Conversion of nitrogen gas to ammonium compounds by bacteria or lightning. |
| Nitrification | Conversion of ammonium to nitrites and then nitrates. |
| Denitrification | Conversion of nitrates back to nitrogen gas. |
| Decomposition | Breakdown of dead material by decomposers. |
| Combustion | Burning, which releases carbon dioxide. |
| Fossil fuel | Fuel formed from long-dead organisms. |
Section 9 of 9Exam technique
1
Saying plants absorb ammonium directly — nitrifying bacteria convert it to nitrates first.
2
Saying plants can absorb nitrogen gas — only bacteria and lightning make it usable.
3
Saying respiration removes carbon dioxide — photosynthesis removes it and respiration adds it.
4
Saying denitrification improves fertility — it reduces it.
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