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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

80 % of the atmosphere is made up of nitrogen gas

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nitrogen gas is very unreactive

and very few organisms can extract it from the air

Nitrogen is essential for all organisms as it is needed to make proteins

Some terms you need to know first:

• N2 Nitrogen• NH4+ Ammonium• NH3 Ammonia• NO2- Nitrite• NO3- Nitrate• NO2 Nitrogen oxide

Plants can easily make carbohydrates like sugars and starches

NO3-+

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

NO3-+

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

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

Aminoacid

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

But where does the nitrate come from?

But where does the nitrate come from?

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

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NO3-

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

NO3-

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

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


protein


protein NH3

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

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 atmosophere...

Nitrates

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Nitrates


Putting it all together into a cycle...

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Nitrates

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NO3-

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Death and decay

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Death and decay

Bacteria break down the protein into nitratesNO3

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NO3-


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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 from the air is converted into NITRATES by NITROGEN FIXATION 4 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 fixed nitrogen by 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).

• 2. Nitrifying bacteria - Nitrosomanus

• 3. Nitrifying bacteria - Nitrobacter.

• 4. Dentrifying bacteria.

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.

Stages in N cycle – breifly summarise each one:1. Nitrogen fixation

2. Use of Nitrates by plants

3. Assimilation by animals

4. Decomposition and Ammonification

5. Nitrification

6. Denitrification

Summarise the N cycle:• Make a large labelled copy of the

nitrogen cycle from page 97 on A3 paper.

• Use different colours and a key.• Write notes around the diagram

using the handouts and pages 97-100.

• Nitrogen Fixation: converts gaseous nitrogen (N2) into ammonia (NH3). Certain bacterial species, both aerobic and anaerobic, carry out this conversion.

• Nitrification: only certain bacteria, the nitrifying bacteria, can use NH3 as an energy source. The reaction occurs in two steps:

– Nitrosomonas bacteria convert ammonia (NH3) to nitrite (NO2-) – Nitrobacter bacteria convert nitrite (NO2-) to nitrate (NO3-)

• Denitrification: bacteria that can respire anaerobically will convert nitrate (NO3-) to nitrite (NO2-). Note that nitrate is now serving as an electron acceptor. Some anaerobic respirers can also use nitrite (NO2-), converting it further into nitrous oxide (NO), nitrogen dioxide (NO2), and ultimately nitrogen gas (N2).

• Assimilation: ammonia can be directly assimilated into organic compounds inside cells, producing amino groups (-NH2).

• Excretion: during excretion, fermentation, and other catabolic processes, excess amino groups (-NH2) are released, ultimately producing ammonia (NH3).

• Assimilatory Nitrate Reduction: since nitrate (NO3-) is far more common than ammonia, many organisms can only acquire nitrogen in the form of nitrate. They must reduce nitrate to form the amino groups needed for metabolism. This process, which superficially resembles nitrate reduction by anaerobic respiration, is entirely different.

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