4.3 Cycles within Ecosystems

REVISION NOTES

IGCSE Edexcel Biology

4.3 Cycles within Ecosystems

4.3.1 Describe the stages in the carbon cycle, including respiration, photosynthesis, decomposition and combustion

Finite amounts of carbon exists on this planet thus it is recycled for the growth of new organisms

Respiration:

CO2 is returned to the atmosphere through respiration of organisms:
- Plants
- Animals
- Decomposers

Photosynthesis:

Carbon is removed from the atmosphere in the form of CO2 by plants for photosynthesis

Decomposition:

Decomposers break down dead animals and plants
CO2 produced is returned to the atmosphere
In the absence of decomposers, carbon from their bodies cannot be converted
- Under high pressure and over time, these organisms turn into fossil fuels

Combustion:

Combustion of plants and fossil fuels leads to carbon binding to oxygen
CO2 is released into the atmosphere
Increased use of fossil fuels contributes to the increase in CO2 levels

4.3.2B Describe the stages in the nitrogen cycle, including the roles of nitrogen fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria (specific names of bacteria are not required)

Nitrogen is required for protein production in plants or animals. It cannot be absorbed from the atmosphere like carbon.

Nitrogen cycle:

Plants absorb nitrates from the soil
- Used for protein production
Animals get their nitrates from the consumption of plants or other animals

Nitrogen fixing bacteria:

Removes nitrogen from the environment and converts it for absorption
- N2 gas converted it into nitrates
Nitrogen fixing bacteria are found in soil and root nodules of some plants

Lightning:

Splits the bond between N2 atoms
Converts them into nitrous oxides
Nitrous oxides dissolve into rainwater and enter the soil

Decomposers:

Decomposers break down proteins in plants and animals into ammonium
Ammonium is returned to the soil

Nitrifying bacteria:

Waste such as urine or faeces from animals returns the nitrogen into the soil as ammonium
Plants are unable to absorb ammonium based compounds
Nitrifying bacteria converts ammonium to nitrites
Nitrites are converted back into nitrates that are absorbed by plants

Denitrifying bacteria: