Nitrogen Fixation

Plants reduce nitrate ions back to ammonia, the form in which nitrogen is incorporated into proteins.

Nitrification Denitrification

Denitrifying bacteria

Nitrifying bacteria oxidize ammonia to nitrate ions.

Denitrifying bacteria

Nitrifying bacteria oxidize ammonia to nitrate ions.

manufacture amino acids, from which the plant's proteins and all its other nitrogen-containing compounds are formed. Animals cannot reduce nitrogen, and they depend on plants to supply them with reduced nitrogenous compounds.

Bacteria called denitrifiers return nitrogen from animal wastes and dead organisms to the atmosphere as N2. This process is called denitrification (see Chapter 27). In combination with leaching and the removal of crops, denitrification keeps the level of available nitrogen in soils low.

This global nitrogen cycle is complex. It is also essential for life on Earth: Nitrogen-containing compounds constitute 5 to 30 percent of a plant's total dry weight. The nitrogen content of animals is even higher, and all the nitrogen in the animal world arrives there by way of the plant kingdom.

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Essentials of Human Physiology

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