The nitrogen cycle for A-Level Biology

The nitrogen cycle is the set of biological and chemical processes that move nitrogen between the atmosphere, the soil, and living organisms. Nitrogen makes up about 78 percent of the atmosphere as N2 gas, but plants and animals cannot use it in that form. The cycle takes inert N2 and converts it into forms organisms can use, and eventually returns nitrogen back to the atmosphere.

Four key stages drive the cycle: Nitrogen fixation, ammonification, nitrification, and denitrification. This guide covers what each stage does, the bacteria involved, and the wording AQA examiners reward in 6-mark questions on the cycle.

Nitrogen is a key component of amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA), chlorophyll, and ATP. Without a steady supply of nitrogen in usable form, plants cannot grow, and the animals that eat them cannot build new tissue.

The problem is that atmospheric nitrogen (N2) is held together by a strong triple bond that makes it almost inert. Plants and animals cannot break this bond. The nitrogen cycle exists because a few specialised bacteria can break it, and once broken, the nitrogen can be reused over and over by other organisms.

Nitrogen fixation is the conversion of atmospheric N2 into ammonia (NH3) or ammonium ions (NH4+). The triple bond in N2 has to be broken, which takes a lot of energy. Only a few organisms can do it, plus a small amount happens through lightning.

The main biological route is through nitrogen-fixing bacteria. Rhizobium lives in root nodules on legumes (peas, beans, clover) in a mutualistic relationship: The plant gives the bacteria sugars, and the bacteria give the plant fixed nitrogen. Azotobacter lives freely in the soil and fixes nitrogen on its own.

Ammonification is the conversion of organic nitrogen (in proteins, urea, and DNA from dead organisms or waste) into ammonia or ammonium ions. It is the recycling step. Without it, all the nitrogen locked inside living tissue would stay there forever.

Decomposers do this job: Saprobiotic bacteria and fungi feed on dead plants, dead animals, and animal waste. They secrete enzymes that break proteins down into amino acids, then into ammonia. The ammonia released into the soil becomes available for the next stage.

Nitrification is the oxidation of ammonium ions into nitrites (NO2-) and then nitrates (NO3-). It happens in two steps, each driven by a different group of bacteria.

First, Nitrosomonas oxidises ammonium ions (NH4+) to nitrites (NO2-). Then Nitrobacter oxidises nitrites to nitrates (NO3-). Both steps are oxidation reactions that release energy, which the bacteria use to fix carbon. Nitrates are the form of nitrogen that most plants prefer to absorb through their roots.

Denitrification is the conversion of nitrates back into atmospheric N2 gas. It is the only step that takes nitrogen out of soils and returns it to the air. From a plant's point of view it is a loss, but it is what closes the cycle.

Denitrifying bacteria such as Pseudomonas live in anaerobic conditions, typically in waterlogged or compacted soils. They use nitrate as an electron acceptor in respiration when no oxygen is available, releasing N2 gas as a by-product.

Plants take up nitrate ions from the soil through their roots by active transport. Inside the plant, nitrates are reduced and combined with carbohydrates to make amino acids, which are then assembled into proteins, nucleotides, and chlorophyll.

Animals get their nitrogen by eating plants or other animals. They break dietary proteins down into amino acids and reassemble them into their own proteins. When animals excrete waste (mostly urea) or die, decomposers return the nitrogen to the soil via ammonification.

Question: Describe the role of bacteria in the nitrogen cycle. (6 marks)

Model answer: Nitrogen-fixing bacteria such as Rhizobium and Azotobacter convert atmospheric nitrogen (N2) into ammonia, which forms ammonium ions in the soil. Saprobiotic bacteria carry out ammonification by decomposing dead organisms and waste, releasing ammonium ions. Nitrifying bacteria then oxidise these ammonium ions: Nitrosomonas converts ammonium to nitrite, and Nitrobacter converts nitrite to nitrate. Plants absorb nitrate ions through their roots. Finally, denitrifying bacteria such as Pseudomonas convert nitrate back into nitrogen gas under anaerobic conditions, returning it to the atmosphere.

This answer covers all four stages, names specific bacteria for each, and links each step to the next. It is a comfortable Level 3 / full-mark response.