19 April 2015

# 66 The nitrogen cycle

Living organisms need nitrogen because nitrogen atoms are an essential part of proteins, nucleic acids and ATP. The air contains about 78% nitrogen gas. However, this is in the form of nitrogen molecules, in which two nitrogen atoms are held together by a very strong triple covalent bond. This is very unreactive.

Nitrogen molecules freely diffuse in and out of the bodies of living organisms, but take no part in the metabolic reactions inside their cells.

Nitrogen fixation

For nitrogen to become involved in metabolic reactions, it must first be converted to a different form by combining with oxygen or hydrogen. This process is called nitrogen fixation. It can be done by:

Lightning, which provides very high temperatures that can cause nitrogen and oxygen molecules in the air to combine to form nitrogen oxides; these can then be washed to the ground in rain.

Industrial processes in which nitrogen is combined with hydrogen to produce ammonia, NH3; this is then used to manufacture fertilisers such as ammonium nitrate.

Nitrogen-fixing bacteria, which use the enzyme nitrogenase to combine nitrogen and hydrogen to produce ammonium ions. Some of these bacteria live free in the soil, lakes or oceans. Others, for example Rhizobium, live symbiotically in root nodules in several different species of plants, particularly legumes such as peas and beans.

Formation of amino acids

Plants are able to take nitrate ions, N03-, or ammonium ions, NH4+, from the soil into their root hairs. This may be done by diffusion or active transport. These ions can be combined with carbohydrates to produce amino acids.

Consumers obtain their nitrogen by eating proteins and other nitrogen-containing organic compounds that were originally synthesised by plants.

Decay and ammonification

Animals excrete nitrogen-containing compounds such as ammonia and urea. When they die, protein molecules in their bodies are broken down by enzymes produced by bacteria, fungi and other decomposer organisms. These processes add ammonia and ammonium ions to the soil.


Nitrifying bacteria oxidise ammonia to nitrate ions. This is done in two stages:

• Nitrosomonas oxidises ammonium ions to nitrite ions, N02-;
• Nitrobacter oxidises nitrite ions to nitrate ions, N03-.

The nitrate ions can then be taken up by plant roots.


Several different types of bacteria get their energy by converting nitrate ions to nitrogen gas. This process is called denitrification, and it returns nitrogen gas to the atmosphere.

Syllabus 2015 

(d) describe how nitrogen is cycled within an ecosystem, including the roles of nitrogen-fixing bacteria (e.g. Rhizobium) and nitrifying bacteria (Nitrosomonas and Nitrobacter);

(e) use the knowledge gained in this section in new situations or to solve related problems.
Note: An ecosystem should be studied in relation to an area familiar to the candidates.

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