Nurul Afina ShafieYear 10Sc1, 2011

Sayyidina Otman Secondary School, Tutong

80 % of the atmosphere is made up of nitrogen gasnitrogen gas.

Nitrogen gas is very unreactive unreactive and very few organisms can extract it from the air.

Nitrogen is essential for all organisms as it is needed to make proteinsproteins (e.g meat, fish, milk, peas)

Some terms you need to know first:

N2 Nitrogen

NH4+ Ammonium

NH3 Ammonia

NO2- Nitrite

NO3- Nitrate

NO2Nitrogen oxide

Plants can easily make carbohydrates like sugars and starches

NO3-+

Aminoacid

nitrate ions need to be added to carbohydrates to make amino-acids

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

The amino-acids are joined together in chains to make polypeptides

But where does the nitrate come from?

Certain bacteria are able to take atmospheric nitrogen and “fix” it into nitrate ions

N2

These nitrogen fixing bacteria are either free living in the soil...

NO3-

Or live in special nodules in the roots of plants

NO3-

The nitrate in the soil or in the root nodules can then be used by the plant to make proteins

Bacteria also help to recycle the nitrogen that has been made into protein

After death...

And of course the production of waste compounds containing nitrogen

Bacteria break down these proteins and waste products to release ammonia NH3

Bacteria break down these proteins and waste products to release ammonia NH3

protein NH3

The ammonia is further broken down by nitrifying bacteria to release nitrates

NH3 NO3-

Unfortunately some of the nitrates in the soil end up back as nitrogen in the atmosphere...

N2

Nitrates

Denitrifying bacteria use nitrates and release nitrogen into the air

Putting it all together into a cycle...

N2

Nitrogen fixing bacteria in soil and nodules of plants

Nitrates are made into proteins in plants

NO3-

Animals get their protein by eating

Death and decay

Bacteria break down the protein into nitrates

NO3-

Which can be taken back up by plants and made into proteins

Denitrifying bacteria use nitrates and release nitrogen into the air

Lightning and the addition of fertlisers also adds nitrates to soil

Stages in N cycle

Nitrogen fixation Use of Nitrates by plants Assimilation by animals Decomposition and ammonification Nitrification Denitrification

Nitrogen Fixation The process of converting gaseous nitrogen (N2) into ammonia (NH3).

Certain bacterial species, both aerobic and anaerobic, carry out this conversion.

This process can be done in four ways:1. Free living nitrogen fixing bacteria (Rhizobium) found in the soil.

2. Nitrogen fixing bacteria (Rhizobium) found in the roots of some plants (leguminous). The rhizobium use an enzyme called nitrogenase which converts N2 gas into ammonium ions NH4+. They can only do this if they have 1). a supply of N2, 2). A supply of ATP 3).anaerobic conditions (no oxygen).

3. Fixation in the atmosphere -lightning. The energy from lightning causes the N2 and O2 to react to form nitrogen oxides. These dissolve in the rain and fall onto the ground.

4. Fixation by Harber process. This is when N2 and H2 gases are reacted together to produce ammonia. This ammonia is converted into ammonium nitrate, which is the most widely used inorganic fertiliser.

Use of nitrogen fixation in plants

In legumes the fixed nitrogen produced by the Rhizobium in their root nodules is used to make amino acids. This is distributed to all areas of the plant to make proteins.

Some plants take up nitrate ions (NO3-) from the soil by active transport. This is then converted to nitrite (NO2-), then ammonia, and then amino acids e.g.

NO3- NO2- NH3 amino acids

(Nitrate) (nitrite) (ammonia) (amino acids)

So plants use nitrates to make amino acids.

Assimilation of nitrogen Animals get their nitrogen from the proteins they eat. Proteins are broken down into amino acids during

digestion. The amino acids are then built up again into proteins

in cells during protein synthesis.

Protein amino acids proteins

Excess amino acids (any surplus protein in our diet) is deaminated in the liver and is excreted in urea as urine.

Excess amino acids urea urine

Decomposition and Ammonification

Decomposers gradually break down the protein in dead plants and animals into amino acids – decomposition.

Decomposers include bacteria and fungi which produce protease enzymes.

They use some of the amino acids for their own growth.

The rest is broken down into ammonia. Ammonia is also produced from the urea in animal

urine. The production of ammonia is called

ammonification.

Nitrification

The ammonia in the soil is then converted into nitrite ions (NO2-) and then nitrate (NO3-) ions by nitrifying bacteria. (Nitrosomanus and Nitrobacter).

Denitrification Dentrifying bacteria reverse the nitrogen fixation

process. They convert nitrates back into nitrogen gas. Found in sewage treatment, compost heaps and

wet soils.

What is the role of these bacteria in the N cycle?

1. Nitrogen fixing (Rhizobium).Convert Nitrogen gas into organic

nitrates in soil and plants. 2. Nitrifying bacteria - Nitrosomanus Convert ammonium into nitrites 3. Nitrifying bacteria - Nitrobacter.Convert nitrites into nitrates 4. Dentrifying.Convert nitrates into Nitrogen gas.