UNIVERSITY OF BATH FOUNDATION YEAR

BIOLOGY MODULE

SEMESTER 2

ASSIGNMENT 1

PRESENTATION

NITROGEN CYCLE

Prepared by

Merdiye Mavis

Nitrogen

Nitrogen is the 7th element in the periodic table.

Nitrogen gas occupies 78.1% of the atmosphere. It is the most common gas in the atmosphere.

In living organisms, nitrogen can be found in the structure of proteins, amino acids, nucleic acids (DNA and RNA), etc.

Importance of Nitrogen For Plants and Animals

Plants and animals need nitrogen however they can not take nitrogen directly from atmosphere.

They need bacteria to convert it into nitrogen compounds.

As a result the nitrogen cycle can be used to investigate how nitrogen is converted into a usable form and then passed on between different living organisms and the non-living environment.

NITROGEN CYCLE

The nitrogen cycle includes food chains and four different processes that involve bacteria.

Nitrogen Fixation Ammonification (Decay) Nitrification Denitrification

Nitrogen Fixation

Nitrogen gas is converted into nitrogen-containing compounds by microorganisms in the process of nitrogen fixation.

There are two types of microorganisms:

Free-living nitrogen fixing bacteria: They reduce gaseous nitrogen to ammonia, which they

use to manufacture amino acids and they do not require plants to live with.

Nitrogen rich compounds are released from them when they die and decay.

In aquatic environments, blue-green algae (cyanobacteria) are important free-living nitrogen fixers.

Mutualistic nitrogen- fixing bacteria:

They live in nodules on the roots of leguminous plants such as peas and beans.

Certain bacteria which are placed in the genus Rhizobium, are the only organisms that fix nitrogen through metabolic processes.

Nitrogen fixing bacteria often form symbiotic relationships with host plants.

They obtain carbohydrates from the plants which acquire amino acids from the bacteria.

Root nodules on the roots of white clover, Trifolium repens, caused by the nitrogen-fixing bacteria Rhizobium trifolii.

Root nodules of white clover

Other ways for nitrogen fixation

There are more ways to restore the ammonium ions in the soil;

Atmospheric Fixation

The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth.

Industrial Fixation

Under great pressure, at 600°C, and with the use of a catalyst, atmospheric nitrogen and hydrogen can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of it is further processed to urea and ammonium nitrate (NH4NO3).

AMMONIFICATION

Production of ammonium from organic ammonium containing compounds.

These ammonium containing compounds include urea (from breakdown of excess amino acids), proteins, nucleic acids and vitamins (found in faeces and dead organisms).

Saprobiotic microorganisms, mainly fungi and bacteria, feed on these materials releasing ammonia which then they form ammonium ions in the soil.

This is where nitrogen returns to the non-living components of ecosystem.

NITRIFICATION

It is when ammonium compounds in the soil are changed into nitrogen compounds that can be used by plants.

This process is carried out by free- living soil microorganism called nitrifying bacteria.

This process occurs in two stages: First nitrifying bacteria called Nitrosomonas change

ammonium compounds into nitrites. Then other nitrifying bacteria called Nitrobacter

change nitrites into nitrates.

DENITRIFICATION

It is when nitrates in the soil are converted into nitrogen gas by denitrifying bacteria.

They use nitrates in the soil to carry out respiration and produce nitrogen gas.

This process occurs under anaerobic conditions (where there is no oxygen).

When soil becomes waterlogged, type of microorganisms present changes because of short of oxygen.

Fewer aerobic nitrifying and nitrogen fixing bacteria are found and there is an increase in anaerobic denitrifying bacteria.

These convert soil nitrates into gaseous nitrogen. This reduces the availability of nitrogen-containing compounds for plants.

For land to be productive the soils on which crops grow must therefore be kept well aerated to prevent the build up of denitrifying bacteria.

NITROGEN CYCLE

ONE OF THE MAJOR EFFECTS OF HUMAN ACTIVITIES ON NITROGEN CYCLE

Nitrates leached from fertilised fields stimulate the

growth of algae in ponds and rivers. This is called Eutrophication.

Large amounts of algae prevent light from reaching the plants at the bottom of the ponds and rivers.

Plants can not photosynthesise because of lack of light so they die.

Bacteria feed on this dead plant matter. The increased number of bacteria reduces the oxygen

concentration in the water because they carry out aerobic respiration.

Fish and other aquatic organisms die because there is not enough dissolved oxygen.

Question

Explain how the activities of decomposers and nitrifying bacteria affect the recycling of the fallen leaves for re-use by trees.

ANSWER

Nitrogen compounds in plants

Atmospheric Nitrogen

Nitrates NitritesAmmonium compounds

Nitrogen compounds in animals

Nitrogen fixation (by nitrogen fixing bacteria)

Feeding

Ammonification (by decomposers)

Denitrification (by denitrifying bacteria)

Nitrobacter Nitrosomonas

Nitrification

Bibliography CGP, A2-Level Biology The Revision Guide, 2009, Coordination Group

Publications Ltd., Chapter 3: Energy Flow and Nutrient Cycles, Pg:40 Toole, G. and Toole, S., A2 Biology, Nelson Thornes-Publisher, 2008,

Chapter 6: Nutrient Cycle, Pg: 96-97 http://www.mpsciences.com/index-2.html (protein image) http://textbookofbacteriology.net/Impact_2.html (nitrogen fixing

bacteria image) http://scienceblogs.com/startswithabang/2010/04/

volcanic_lightning_eyjafjallaj.php (lightning image) http://www.physicalgeography. net/fundamentals/9s.html ( nitrogen

cycle image) http://www.lss-lab.bme.buaa.edu.cn/enWWT.html (eutrophication

image) http://t3.gstatic.com/images?

q=tbn:ANd9GcS_0NwBTrgvUgm1osudewcZ4n23WVshsj6c2ASnbhchk1VsYzN5 (fallen leaves image)

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