The Nitrogen Cycle

A2 OCR Biology

Asking questions is a sign of INTELLIGENCE

Unfortunately all questions must wait until the end of the lecture.

By the end of this session I will:• be able to describe the importance of nitrogen in

living systems.• outline the role of micro-organisms in the cycling of

nitrogen• Describe the nitrogen cycle using the appropriate

AS/A2 scientific language. • (k) describe the role of decomposers in the

decomposition of organic material;• (l) describe how microorganisms recycle nitrogen

within ecosystems. (Only Nitrosomonas, Nitrobacter and Rhizobium need to be identified by name).

Nitrogen the basics – needed for

• Nitrogen (N) is an element like carbon. All creatures need nitrogen to survive.

• Nitrogen is needed for the synthesis of proteins e.g. enzymes.

• Nitrogen is also an important component of nucleic acids

Nitrogen the basics – chemistry of

• There are huge amounts of nitrogen gas in the atmosphere, but most animals and plants have no way of using it as it is very unreactive due to the triple covalent bond.

• It needs to be fixed (put into a biologically useful compound). After it is fixed, it can then start to move through the cycles and organisms in an ecosystem.

Nitrogen the basics – sources of

• Let's start with the main sources of nitrogen. Nitrogen gas is the most abundant element in our atmosphere.

• The other main source of nitrogen is in the nitrates of soil. The nitrogen in the atmosphere cannot be used while the nitrates in the soil can be used by plants. Nitrogen can be converted into useful nitrate compounds by bacteria, algae, and even lightning. Once in the soil, the nitrogen becomes biologically accessible.

Forms of Nitrogen

• Urea CO(NH2)2

• Ammonia NH3 (gaseous)• Ammonium NH4

• Nitrate NO3

• Nitrite NO2

• Atmospheric Dinitrogen N2

• Organic N• CHEMISTS WE NOW NEED TO CORRECT

SOME OF THESE IONS!!

Nitrates are recycled via microbes

Nitrification

Nitrification

Ammonium NH4+

Ammonification

Nitrite NO2-

Soil organic nitrogen

Animal protein

Root uptake

Nitrate NO3

-

Plant protein

Importance of bacteria in the nitrogen cycle

• Nitrogen fixing bacteria

• Nitrifying bacteria

• Denitrifying bacteria

Nitrogen fixing bacteria (Mr M discuss nitrogenase here!!)

Function…….…… converts nitrogen into

nitrogen containing compounds

E.g. ….…… Rhizobium

Found in ….….. Roots nodules of

leguminous plants and soil

Mutualism

Nitrifying bacteriaFunction….….. Convert

ammonia in nitrite ions and then into nitrate ions.

E.g. …..…..Nitrosomonas

and Nitrobacter.

Found in….….well aerated

soils.

Denitrifying bacteria

Function….

…. Convert nitrates into nitrogen gas

Found in….

…. Sewage works, compost and wet soils

Nitrogen inthe air

animal protein

dead plants & animalsurine & faeces

ammonia

nitrites

nitrates

plant madeprotein

decomposition by bacteria & fungi

bacteria

(nitrifying bacteria)

nitrates absorbed

denitrifyingbacteriaroot nodules

(containing nitrogen fixing bacteria)

nitrogen fixing planteg pea, clover

bacteria

Other ways in which nitrogen can be fixed

Lightning

Haber process

Lightning

• The electrical energy of lightning causes nitrogen gas (N2) to react with oxygen (O2) in the atmosphere to produce nitrate ions (NO3-) which reach the soil dissolved in precipitation.

• Recent research suggests the amount of nitrates produced by lightning may be as high as 50%.

Haber process

Basic summary of the nitrogen cycle

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EutrophicationYet another example of pollution, eutrophication is when lakes become stagnant due to careless use of fertiliser. There are six steps:

1) Inorganic fertilisers used on fields are washed into the lake

3) This growth causes overcrowding and many plants die due to lack of enough light or food

2) The fertiliser causes increased growth in water plants

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Eutrophication4) Microorganisms and bacteria increase in number due to the extra dead material

5) These microorganisms use up the oxygen in the lake during respiration

6) The lack of oxygen causes the death of fish and other aquatic animals

Can’t…breathe…

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Eutrophication4) Microorganisms and bacteria increase in number due to the extra dead material

5) These microorganisms use up the oxygen in the lake during respiration

6) The lack of oxygen causes the death of fish and other aquatic animals