The Nitrogen Cycle

Although nitrogen gas is abundant in the atmosphere, plants obtain usable nitrogen from nitrogen-containing ions in the soil, particularly nitrate ions. The nitrogen cycle connects atmospheric nitrogen → soil nitrogen compounds → plant proteins → animal proteins → soil nitrogen compounds → atmospheric nitrogen, and Cambridge requires Extended candidates to understand seven linked processes: nitrogen fixation, nitrification, uptake by plants, protein production and feeding, decomposition, deamination and denitrification.

Nitrogen fixation

Nitrogen fixation converts atmospheric nitrogen into nitrogen-containing compounds that can enter the biological cycle. Cambridge requires two routes: nitrogen-fixing bacteria, which convert atmospheric nitrogen directly into nitrogen compounds, and lightning, which can also cause atmospheric nitrogen to form nitrogen compounds that eventually enter the soil. Without fixation, atmospheric nitrogen wouldn’t readily enter the pathways plants use to build amino acids and proteins at all — it’s the entry point for the whole cycle.

Decomposition to ammonium ions

Plants and animals contain proteins, and when they die — or when protein-containing material enters waste — decomposers break this material down. Microorganisms involved in decomposition convert nitrogen-containing organic material into ammonium ions: protein in dead organisms and waste → ammonium ions. This is what returns nitrogen from organic material back to the soil.

Nitrification

Nitrification converts ammonium compounds into nitrate ions, carried out by microorganisms. The important direction to remember is ammonium ions → nitrate ions, since it’s the nitrate ions produced here, not the ammonium ions themselves, that plants are able to absorb.

Absorption of nitrate ions and production of proteins

Plants absorb nitrate ions from the soil through their roots and use this nitrogen to build amino acids, then proteins. At this point, nitrogen has moved from an inorganic form in the soil into organic compounds inside a living organism.

Feeding and digestion of proteins

Animals obtain nitrogen-containing compounds by feeding — plant protein → herbivore, or animal protein → carnivore. Proteins in food are digested into amino acids, and those amino acids can then be used to build new proteins in the consumer’s own body. Feeding therefore transfers nitrogen along the same food chains that transfer energy.

Deamination

Animals often have more amino acids than they need for protein synthesis, and excess amino acids can’t simply be stored as amino acids, so they’re deaminated instead. Deamination removes the nitrogen-containing part of the amino acid, and the resulting nitrogen-containing waste is eventually removed from the body, later re-entering environmental nitrogen pathways. The full physiological detail of how that waste is processed and excreted belongs to excretion rather than to this cycle — here, what matters is that excess amino acids don’t simply disappear; their nitrogen is released back into the system.

Denitrification

Denitrification returns nitrogen to the atmosphere. Denitrifying microorganisms convert nitrate compounds into nitrogen gas: nitrate ions in the soil → atmospheric nitrogen. Nitrogen fixation and denitrification act in opposite overall directions — fixation moves nitrogen from the atmosphere into usable compounds, while denitrification moves it from those compounds back into the atmosphere — which is a useful pair to hold in mind together rather than as two unconnected facts.

The role of microorganisms in the nitrogen cycle

Microorganisms are required at four distinct points in this cycle:

Cambridge specifically states that the generic names of individual bacteria aren’t required, so it’s far more valuable to be able to state what each group of microorganisms does at each stage than to try to memorise bacterial names you won’t be asked for.