About 78 percent of the atmosphere is made up of "free" nitrogen, or
nitrogen that is not combined with other elements.
All living things need
nitrogen to build proteins and certain other body chemicals.
However, most organisms --
including plants, animals, and fungi -- cannot get the nitrogen they need from the free nitrogen
in the air.
They can use only nitrogen that is combined with other elements in
But how are these nitrogen-containing compounds
Nitrogen transferred from the non-living portion of the environment into living
Certain kinds of bacteria are able to use the free nitrogen in the air
to make nitrogen compounds through a process known as nitrogen
Most of the nitrogen fixation on Earth occurs as a result of the activity of
Some of these bacteria live in the
Others grow inside special structures on the roots of certain plants,
including beans, clover, alfalfa, peas, and peanuts.
One family of nitrogen
compounds produced by nitrogen-fixing bacteria consists of substances called
Nitrates can be taken from the soil by plants.
Inside the plants, the nitrogen
in the nitrates is used to make compounds such as proteins.
The compounds made by the
plants can be used by animals, fungi, and other organisms that cannot use nitrates
Look at the illustration below and trace the steps of the nitrogen cycle from the
free nitrogen in the air to the nitrogen in the bodies of animals.