The NITROGEN CYCLE
Chemistry KS4 science GCSE/IGCSE/O level Revision Notes
PART E A description of the
complexity NITROGEN CYCLE
GCSE/IGCSE/US grade 8-10 Chemistry Revision
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The nitrogen cycle is described in terms of the
function of nitrifying bacteria, lightning and production of nitrogen oxides,
function of denitrifying bacteria, action of nitrogen-fixing bacteria, part
played by leguminous plants (legumes) in nitrogen fixation, bacteria
decomposers, action of soil bacteria, plants absorbing nitrates from the soil.
These revision notes on the nitrogen cycle, should prove useful for the new AQA
GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st
Century) GCSE (9–1), (9-5) & (5-1) science courses.
PART E 6.
The Nitrogen Cycle
the gaseous element N2(g)
- Nitrogen is an extremely
important element for all plant or animal life! It is found in
important molecules such as amino acids, which are combined to
form proteins. Protein is used everywhere in living organisms from muscle
structure in animals to enzymes in plants/animals.
- Nitrogen from the
- Action of nitrifying bacteria,
e.g. they function in the root nodules of certain plants like peas/beans (the
legumes), can directly convert atmospheric nitrogen into nitrogen
compounds in plants e.g. nitrogen => ammonia => nitrates
which plants can absorb.
- However, most plants can't do
this conversion from nitrogen => ammonia, though they can all
absorb nitrates, so the 'conversion' or 'fixing' ability might be introduced into other
plant species by genetic
- The nitrogen from air is
converted into ammonia in the chemical industry, and from this
artificial fertilisers are manufactured to add to nutrient
deficient soils. However, some of the fertiliser is washed out of
the soil and can cause pollution.
- The energy of lightning
causes nitrogen and oxygen to combine and form nitrogen oxides which
dissolve in rain that falls on the soil adding to its nitrogen
- N2(g) + O2(g)
==> 2NO(g), then
- then 2NO(g)
+ O2(g) ==> 2NO2(g)
- NO2(g) +
water ==> nitrates(aq) in rain/soil
- Incidentally, reactions 1.
and 2. can also happen in a car engine, and NO2 is
acidic and adds to the polluting acidity of rain as well as
providing nutrients for plants!
- Nitrogen recycling
apart from the atmosphere:
- Nitrogen compounds, e.g.
protein formed in plants or animals, are consumed by animals higher
up the food chain and then bacterial and fungal decomposers
break down animal waste and dead plants/animals to release nitrogen
nutrient compounds into the soil (e.g. in manure/compost)
which can then be re-taken up by plants.
- Nitrogen returned to the
- However, the action denitrifying
bacteria will break down proteins completely and release
nitrogen gas into the atmosphere.
Diagram of the nitrogen cycle
- More 'biological detail' of the NITROGEN
CYCLE with reference to the above diagram so you can show an understanding of how nitrogen is recycled.
in the air (78%, ~4/5th) cannot be used directly by
most plants and all animals.
- No animals and only a few specialised plants
can directly use the very unreactive nitrogen from air, but all
plants nitrogen in some form to synthesise amino acids and proteins for
growth and maintenance and for DNA in cell reproduction.
- However, nitrogen can be changed into
nitrogen compounds like nitrates which the plants can use.
- Animals rely on plants or other animals in
the food chain for their source of nitrogen compounds e.g. protein in grass,
crops or other animals.
Action of Nitrogen-fixing
bacteria living in root nodules of plants or in the soil, their function is
to fix nitrogen gas from the atmosphere into a chemical form the plant can
- Leguminous plants like peas, lentils, clover
and beans can absorb nitrogen from the air via their root nodules (swellings
on the root surface) which contain enzymes capable of converting ('fixing')
atmospheric nitrogen into soluble nitrate - a nutrient essential for amino
acids, proteins and therefore plant growth.
- Legumes and their root nodule bacteria are
an example of mutualism (see section 3.19 b) because the plant root supplies
the bacteria with carbohydrate food and minerals and the bacteria supplies
the plant in the form of the nitrate ion.
- The process of converting nitrogen in air
into nitrogen compounds is sometimes called 'nitrogen fixation'.
The action of
lightning can convert nitrogen gas into nitrates.
- The very high electrical energy discharges
from lightning activates nitrogen and oxygen molecules to react and form
nitrogen oxides. These dissolve in rain to form nitrates which end up in the
soil when rainwater trickles into the soil.
Action of decomposers: Their
function is to break down dead
animals and plants
- Decomposers, e.g. various organisms like
bacteria, fungi or worms can break down dead animals or plants. They break
down proteins to amino acids.
Action of soil bacteria: Their
function is to convert proteins and urea into ammonia
or ammonium ions.
- Decomposer bacteria in the soil can change
proteins from dead plants/animals and urea in animal urine/droppings into
ammonia/ammonium ion compounds.
- d) plus e) is sometimes called putrefaction
by putrefying bacteria.
Action of nitrifying bacteria: Their function is to convert this ammonia to nitrates - the process of
- Nitrifying bacteria oxidise ammonia/ammonium
ions from the decayed material to form nitrates, the nitrate ion can be
absorbed by plants through their root systems.
Plants absorb nitrates
from the soil.
- Plants absorb nitrates (soluble in water) in
the moisture that the roots absorb from the surrounding soil.
- Plants can use the nitrate ion in forming
amino acids from which the plant can make its proteins.
Nitrates are needed by plants to make proteins for growth.
- Nitrates are an essential nutrient for
plants to synthesis amino acids and hence proteins.
Nitrogen compounds pass along a
food chain or web of food chains.
- All food chains involve the passing of
carbon compounds e.g. sugars, carbohydrates, fats and proteins up to
the next trophic level i.e. the consecutive eating along a food chain (and
waste produced on the way).
- e.g. grass ==> cow ==> human
- Plants make their own protein from nitrates,
but animals must obtain it from plants or other animals. In fact the protein
is broken down in digestion to amino acids and each animal makes its own
proteins from these amino acid residues.
Action of denitrifying bacteria:
Their function is to
convert nitrates to nitrogen gas.
- Particular bacterial organisms can remove
the oxygen from nitrate compounds to form the element nitrogen gas.
- These denitrifying bacteria live in
anaerobic conditions like waterlogged soils and use the nitrate ion to
- This is the opposite function of the
nitrogen-fixing bacteria (b).
A Reversible Reactions * B
Reversible reactions and Equilibrium
Haber Synthesis of ammonia * D(a) The Uses of ammonia-nitric acid-fertilisers
The nitrogen cycle (this page)
Foundation tier (easier) multiple choice QUIZ on ammonia,
nitric acid and fertilisers etc.
Higher tier (harder) multiple choice QUIZ on
ammonia, nitric acid and fertilisers etc.
Advanced A Level Notes - Equilibrium
Advanced A Level Chemistry Notes p-block nitrogen & ammonia
nitrogen cycle, water cycle, decomposition - decay investigation, biogas
gcse biology revision notes
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