recycling within ecosystems

• why do nutrients have to be recycled?

  • they are used up by living organisms
  • no external energy source constantly replenishing nutrients in the way that eg the sun provides energy
• define decomposition

  chemical process in which a compound is broken down into its constituent elements
• why are organic compounds containing carbon and nitrogen broken down?

  • can't be used directly by an organism in the organic form its in
  • must be processed and broken down into simpler inorganic elements and compounds, which are a much more usable form
  • these are then returned to the environment
• what is a decomposer?

  an organism that feeds on and breaks down dead plant or animal matter, and converting organic compounds into inorganic ones, available to photosynthetic producers in the ecosystem
• what species of microorganism are decomposers mainly?

  fungi and bacteria
• why are decomposers referred to as saprotrophs?

  because they obtain their energy from dead or waste organic material
• how do decomposers digest waste?

  • secrete enzymes onto dead organisms or waste matter
  • breaks it down into simpler soluble molecules
  • decomposers absorb these molecules
  • through this process, decomposers release stored inorganic compounds and elements back into the environment
• what are detritivores?

  • speed up the process of decay by feeding on detritus
  • break it down into smaller pieces of organic material which increases the surface area for decomposers to work on
  • detritivores perform internal digestion
• do detritivores perform internal or external digestion?

  internal
• why is nitrogen an essential element for plants and animals?

  • for making amino acids and therefore proteins for growth
  • for making nucleic acids
• what microorganism converts nitrogen into a form usable by plants?

  bacteria
• what is the role of nitrogen fixing bacteria?

  • they all contain the enzyme nitrogenase
  • this enzyme combines atmospheric nitrogen with hydrogen to produce ammonia
  • ammonia is a form of nitrogen that can be used by plants
  • this process is called nitrogen fixation
• why is nitrogen-fixing bacteria said to have symbiotic mutualistic relationships with plants?

  both organisms benefit from each other:

  • plants gain amino acids from bacteria
  • bacteria gain carbohydrates from the plants produced during photosynthesis, which bacteria use as an energy store
• what four steps are involved in the nitrogen cycle?

  • nitrification
  • denitrification
  • ammonification
  • nitrogen fixation
• describe the stages of the nitrogen cycle

  • nitrogen fixation- nitrogen fixing bacteria (eg rhizobium) contain enzyme nitrogenase, which helps to combine $N_2$ and H2 to form ammonia- this can be absorbed by plants
  • ammonification- decomposers convert nitrogen containing molecules in dead organisms and faeces into ammonium compounds
  • nitrification- ammonium compounds in the soil converted to nitrogen-containing molecules used by plants. Nitrifying bacteria are involved in this. Oxidation reactions take place in 2 steps- nitrosamonas oxidises ammonium compounds to nitrites (NO2-). Then nitrobacter oxidisers nitrites to nitrates ($NO_3^-$)
  • denitrification- under anaerobic conditions, eg when the plant is waterlogged, denitrifying bacteria convert nitrates back to nitrogen gas. The bacteria use nitrates as an energy source to respire