ecosystems

• Azotobacter is an example of a free-living soil bacterium.
• Rhizobium live inside root nodules.
• Azotobacter and Rhizobium have a symbiotic mutualistic relationship with the plant as both organisms benefit. The plant gains amino acids from Rhizobium and the bacteria gain carbohydrates produced by the plant which are used as an energy store.
• Nitrification is the process by which ammonium compounds in the soil are converted into nitrogen-containing molecules that can be used by plants.
• Nitrification is an oxidation reaction and so only occurs in well-aerated soil.
• Nitrification takes place in two steps:

  • Nitrifying bacteria oxidise ammonium compounds into nitrites
  • Nitrobacter oxidise nitrites into nitrates
• In the absence of oxygen (e.g. in waterlogged soil), denitrifying bacteria convert nitrates in the soil back to nitrogen gas.
• Denitrification only happens under anaerobic conditions. The bacteria use the nitrates as a source of energy for respiration and nitrogen gas is released.
• Ammonification is the process by which decomposers convert nitrogen-containing molecules in dead organisms, faeces and urine into ammonium compounds.
• Decomposer= an organism that feeds on and breaks down dead plant or animal matter turning organic compounds into inorganic ones for photosynthetic producers in the ecosystem. Decomposers are primarily fungi and bacteria.
• Decomposers are saprotrophs because they obtain their energy from dead or waste organic material.
• Decomposers digest their food externally by secreting enzymes onto dead organisms or organic waste matter. The enzyme breaks down complex organic molecules into simpler soluble molecules-the decomposers then absorb these molecules.
• Detritivores are another class of organism involved in decomposition. They help speed up decay by feeding on detritus (dead and decaying material). They break it down into smaller pieces of organic material, which increases the surface area for the decomposers to work on.
• There are two types of succession:

  • primary succession-this occurs on an area of land that has been newly exposed, there is no soil or organic material present to begin with.
  • secondary succession-occurs on an area of land where soil is present but it contains no plant or animal species
• Population= group of organisms of the same species living in the same habitat
• Community= all the populations of different species in the same area at the same time
• Ecosystem= A community and the non-living components of an environment (the biotic and abiotic factors).
• Niche= an organism's role within an ecosystem
• Plants and animals cannot obtain nitrogen through gas exchange due to the triple bond in nitrogen. Microorganisms are needed to convert nitrogen gas into nitrogen containing substances that plants/animals can absorb (nitrates and ammonium ions).
• Nitrogen-fixing bacteria convert nitrogen gas into nitrogen containing substances such as ammonium (this is called ammonification).
• Nitrifying bacteria oxidise ammonium into nitrites (this is called nitrification).
• Nitrifying bacteria can also oxidise nitrites into nitrates. These nitrates can be absorbed into plants by active transport to be used to make proteins, DNA and ATP.
• Animals will eat plants containing nitrates and digest them to release nitrogen containing compounds which can be assimilated into the proteins, DNA, RNA and ATP.
• Denitrifying bacteria can convert nitrates back into nitrogen gas only in anaerobic conditions (e.g. waterlogged soil- so not enough oxygen).