A2/A-level Biology - Nitrogen cycle

Do you remember the nitrogen cycle in A2/A-level Biology?

• Materials and energy are lost when an organism dies or excretes, and this dead and waste organic material can be broken down by decomposers, including fungi and bacteria (and a small amount of animals).
• These microorganisms involved in decomposition feed in a way which is different from animals, which is described as saprotrophically, and the organisms themselves are described as saprophytic. 
• These organisms secrete enzymes onto dead and waste matter, which digest the material into small molecules which can be absorbed into the decomposers’ bodies.
• Once absorbed, as with other organisms, the molecules are either used for respiration (and other life processes) or stored. 
• If bacteria and fungi did not break down dead organisms, energy and valuable nutrients would remain trapped within them.
• By digesting such materials, microbes get a big enough supply of energy to sustain life and grow, and the trapped nutrients are not wasted, but recycled.
• Microbes have a particularly important role to play in the global carbon cycle, and also the global nitrogen cycle.

• Living things need nitrogen to make proteins and nucleic acids.
• Nitrogen is cycled between the biotic and abiotic components of an ecosystem in a cycle known as the nitrogen cycle.
• The four major processes which occur as part of the cycle are nitrogen fixation, ammonification, nitrification and denitrification.

• Nitrogen, whilst making up almost 80% of the atmosphere, is very unreactive and plants are unable to use it directly, instead, they must have a supply of materials where nitrogen is fixed, such as in ammonium ions (NH4+) or nitrate ions (NO3-).
• Nitrogen fixation occurs when lightning strikes, or through the Haber process, but these only account for 10% of total nitrogen fixation – the majority comes from nitrogen-fixing bacteria. 
• Most of the nitrogen-fixing bacteria exist freely in the soil, fixing nitrogen using it to make amino acids (this nitrogen comes from the air in the soil).
• Nitrogen-fixing bacteria, such as Rhizobium, also live inside the root nodules of many legumes (bean plants).
• They have a symbiotic relationship - the bacteria supply the plants with nitrogen-fixed molecules in return for organic compounds such as glucose from the plant.

• Unlike with nitrogen fixation, the process of ammonification produces nitrogenous compounds (mainly ammonium) from dead matter.
• Dead materials and waste materials are decomposed by the types of microbes and they convert the organic nitrogen (the initial form of nitrogen found in dead/waste matter) back into ammonium.
• This process may also be called mineralisation. 

• When chemoautotrophic bacteria in the soil absorb ammonium, nitrification happens.
• Ammonium is released by bacteria involved in putrefaction of proteins found in waste or dead organic matter.
• Rather than getting their energy from sunlight (like photoautotrophic bacteria), these bacteria gain energy by oxidising ammonium to produce nitrites (NO2-), or by oxidising nitrites to produce nitrates (NO3-). 
• These reactions only happen in well-aerated soils, as they require plenty of oxygen.
• Nitrates can be absorbed by plants straight from the soil to help produce nucleotide bases and amino acids.

• Denitrification uses different bacteria, which convert nitrates back into nitrogen gas.
• This occurs when the bacteria are under anaerobic conditions, because they can break down nitrates to release nitrogen gas and oxygen gas.
• The oxygen is used to fuel aerobic respiration, and the nitrogen gas is released back into the atmosphere.
• Plants cannot take up this nitrogen gas, as it is not fixated – so this step simply returns nitrogen back into the air, which can then be fixated again under aerobic conditions by nitrogen-fixing bacteria. 

That's the end of the topic!

Drafted by Bonnie (Biology)