spec points:

• To include the role of decomposers and the roles of microorganisms in recycling nitrogen within ecosystems (including Nitrosomonas, Nitrobacter, Azotobacter and Rhizobium)

• the importance of the carbon cycle to include the role of organisms (decomposition, respiration and photosynthesis) and physical and chemical effects in the cycling of carbon within ecosystems.

nitrogen cycle

nitrogen: essential component of metabolism, reacquired for synthesis of proteins and nucleic acids

cannot be used in gas form, to be of any use to plants, needs to be fixed in either form of ammonium ions/nitrates

key processes

• Saprobiotic nutrition, 

• Ammonification, 

• Nitrification, 

• Nitrogen fixation 

• Denitrification.

1. nitrogen fixation

• Nitrogen gas ➔ nitrogen-containing compounds

nitrogen first fixed by bacteria (Rhizobium) which live in root nodules of leguminous plants e.g. pea plants. Azotobacter + Rhizobium bacteria contain nitrogenase enzymes to convert nitrogen and oxygen into ammonia. bacteria have mutualistic relationship with plant where they exchange fixed nitrogen for glucose

• Azotobacter (Free-living bacteria ) – produce ammonia from nitrogen gas.  Make amino acids.  Release them when they die.

• Mutualistic bacteria – live in root nodules in peas and beans.  Obtain carbohydrates from plants and plants get amino acids from bacteria.

2. ammonification

anaerobic conditions (maintained with use of special oxygen-absorbing proteins) enable nitrogen reductase to reduce nitrogen gas to ammonium ions. Ammonium ions subsequently released by bacteria in putrefaction of proteins from dead organic matter.

• Production of ammonia from organic compounds e.g. urea, proteins, nucleic acids.  

• Saprobiotic microorganisms (bacteria & fungi) feed on these to release ammonia into the soil

3. nitrification

• Some microorganisms get energy from reactions involving inorganic ions (chemotrophic bacteria (Nitrosomonas) oxidise Ammonium ions ➔ nitrite ions (NO₂^-))

• This is an oxidation & releases energy

Nitrobacter subsequently oxidise nitrites to nitrates in the presence of oxygen

Nitrite ions ➔ Nitrate Ions (NO₃^-)

• This requires oxygen – occurs in soil with air pockets 

  • e.g. aerated, well-drained soil

4. denitrification

Plants absorb nitrates from soil for nucleotide synthesis

In anaerobic conditions, denitrifying bacteria convert nitrates back into oxygen gas for respiration. Nitrogen gas and nitrous oxide are produced in the process

• Occurs when soils become waterlogged – short of O₂

• Anaerobic bacteria carry out denitrification

• Soil nitrates ➔ nitrogen in the atmosphere

• Reduced Nitrogen compounds available to plants – for land to be productive it must be kept well aerated.

carbon cycle

carbon: component of all organic molecules and as such recycled through environment by processes of photosynthesis, feeding, respiration and decomposition

role of organisms in carbon cycle

• carbon is constantly being recycled around the biospher so that number of carbon atoms in the biosphere is essentially constant; carbon atoms merely swap from one compound to another by various processes in CC

• stored in many forms

  • atmosphere (CO₂)

  • sedimentary rocks

  • fossil fuels like coal, oil and gas; coal is almost pure carbon

  • soil + other organic matter

  • vegetation (cellulose)

  • dissolved in oceans (CO₂)

photosynthesis

• Autotrophs use energy of sunlight to 'fix' carbon dioxide, turning carbon into sugars + other organic molecules - removing carbon from atmosphere

• Calvin cycle is where CO₂ is fixed, by enzyme Rubisco (carboxylates RuBP)

• Terrestrial plants use gaseous CO₂ directly from air

• Aquatic organisms use CO₂ dissolved in water

• As much CO₂ is fixed from ocean microorganisms, as from terrestrial plants

sedimentation

• Saprobionts don’t fully decompose plants that die; their bodies form layers of sediment - can accumulate over millions of years, locking carbon into ground

• sediment = store of energy, can form fossil fuels like peat + coal

• aquatic organisms that die also form sediments on sea bed; can go on to form other fossil fuels e.g. oil, gas

• shells + other calcium-containing body parts can form sedimentary rocks e.g. limestone

• existence of life forms over billions of years has shaped biosphere, in that their remains are still being recycled

respiration

• All life forms respire, including autotrophs

• Heterotrophs rely on respiration for all their energy needs

• Respiration puts CO₂ into the atmosphere, in the opposite direction to photosynthesis

• CO₂ is released in the Link Reaction and the Krebs Cycle of aerobic respiration

• Anaerobic respiration also releases CO₂ into the atmosphere, via fermentation by yeast, moulds and bacteria

feeding

• Carbon passed from autotroph to heterotroph during feeding

• Carbon is also passed from primary consumer to secondary consumer

• Biomass transfer always includes the transfer of carbon, the main element in biomass

decay & decomposition

• dead plants + animals fed upon by detritivores + decayed by saprophytes

  • Releasing carbon into surroundings

  • Supplying carbon to detritivores

  • Supplying carbon to saprophytes

• Waste matter e.g. faeces, urine used by decaying saprobionts

• Such processes can release CO₂ back into the air

burning fossil fuels

• since mid 19^th century, humans extracted + burned increasing amounts of fossil fuels

• CO₂ is being returned to atmosphere faster than it’s absorbed by plants + aquatic producers

• CO₂ level in atmosphere is approx. double that of 800,000 years ago

• warmer temp. = less CO₂ can be dissolved in oceans, so is released into air

• has caused dramatic climate change + affected many other species, mainly through changing habitats