Document page
biology 0610printed 272 · PDF 272
328 pagesrevision 46933
printed 272 · PDF 272
Page image rendered from the stored file at 110 dpi; it sharpens as you zoom. Outlines are placed from each region’s stored position on the page (612 × 792 pt).
readable
Source of this text: embedded text layer. The better of the embedded text and OCR is kept.
272
Nitrogen Cycle
●
To make proteins, living organisms also need nitrogen. The air is 78% N2 so
there is plenty of Nitrogen around.
●
However, they are very inert so they do not normally react and are unable to
be used.
●
First, nitrogen needs to be changed to a reactive form such as NH3 or NO3-1.
This process is known as nitrogen fixation and this can happen in several ways.
●
Lightning makes some nitrogen gas in the air combine with oxygen, forming
nitrogen oxides. They dissolve in rain and are washed into the soil forming nitrates.
●
Artificial fertilizers can be made, such as ammonia. Nitrogen and hydrogen
can be used to make ammonia in the Haber’s process, which may then be used to
make nitrates.
●
Nitrogen fixing bacteria in soil or root nodules use nitrogen gas from the air
spaces in soil and combine it with other substances to make ammonium ions and
other compounds.
●
Once the nitrogen has been fixed, plants use it to make proteins. Animals eat
plants, so they eat proteins from them.
●
When an animal or plant dies, decomposers decompose the body. The
proteins are converted to ammonium ions and released.
●
A group of bacteria, called nitrifying bacteria, turns these ions into nitrates in a
process called nitrification. Nitrogen is also returned when animals excrete
nitrogenous waste material like urea, which they have produced after the deamination
of excess amino acids . This is also converted to nitrates by nitrifying bacteria.
●
A third group of bacteria, called denitrifying bacteria, undo the work done by
nitrifying bacteria. They turn nitrates and ammonia in the soil to nitrogen gas in a
process called denitrification, thus completing the nitrogen cycle.
Populations
●
A population is a group of organisms of one species living together in the
same area at the same time.
●
A community is all of the populations of different species in an ecosystem
living together.