Nitrogen Cycle
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2
NH4+ & NO3-
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All life requires nitrogen-compounds, e.g., proteins and nucleic acids.
Air with 79% nitrogen gas (N2), is the major reservoir of nitrogen. But most organisms
can not use nitrogen in this form.
Plants must fulfill their nitrogen in "fixed" form, i.e., incorporated in compounds such as:
nitrate ions (NO3-), ammonia (NH3), urea (NH2)2CO
Animals fulfill their nitrogen (and all other) compounds from plants (or animals that have
fed on plants).
In general, the nitrogen cycle has five steps:
1.
2.
3.
4.
5.
Nitrogen fixation (N2 to NH3/ NH4+ or NO3-)
Nitrification (NH3 to NO3-)
Assimilation (Incorporation of NH3 and NO3- into biological tissues)
Ammonification (organic nitrogen compounds to NH3)
Denitrification (NO3- to N2)
Industrial Fixation
• Under great pressure, at 600°C, and with the use of a catalyst,
atmospheric nitrogen and hydrogen (usually derived from natural
gas or petroleum) can be combined to form ammonia (NH3).
• Ammonia can be used directly as fertilizer, but most of its is further
processed to urea and ammonium nitrate (NH4NO3).
Natural Fixation
Atmospheric Fixation
Biological Fixation
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during lightning, nitrogen and
oxygen combine to form nitric
oxide.
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The nitric oxide then reacts with
more oxygen to form nitrogen
dioxide dissolve in rain, forming
nitric acids.
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This nitric acid reacts with
minerals of the soil to form soluble
nitrates.
Atmospheric nitrogen fixation
probably contributes some 5-8%
of the total nitrogen fixed.
The ability to fix nitrogen is found only
in certain bacteria.
Some live in a symbiotic relationship
with plants of the legume family
(e.g., soybeans, alfalfa).
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Some establish symbiotic
relationships with plants other
than legumes (e.g., alders).
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Some nitrogen-fixing bacteria
live free in the soil.
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Nitrogen-fixing cyanobacteria
are essential to maintaining the
fertility of semi-aquatic
environments like rice paddies.
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Decomposing
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Dead organism  source of organic
matter
Decomposer (microorganisms) will
break down the molecules in excretions
and dead organisms into ammonia 
ammonification
Ammonification : N organic  NH4+ , at
low temperature, various pH
Ammonia can be taken up directly by
plants - usually through their roots.
However, most of the ammonia
produced by decay is converted into
nitrates.
Nitrification : NH4+ will be oxidized to form
nitrates , at warm humid soil  pH 7
Bacteria Nitrosomonas oxidize
NH4 to nitrites (NO2-).
Bacteria Nitrobacter oxidize
the nitrites to nitrates (NO3-).
Nitrate assimilation in plants
Two steps :
1.
Reduction of nitrat to nitrit (cytosol; nitrate reductase (NR))
nitrate reductase (NR)  2 identical polypeptide  each contain
prosthetic groups : FAD, heme, pterin (complex Mo-organic molec)
in cytosol leaf (photosynthetic) or root (non photosynthetic)
nitrate, light, carbohydrate affect NR activities
2.
Reduction of nitrite to NH4+ (kloroplas/plastid → nitrit reduktase)
donor electron in leaf: ferredoxin derive from photosynthetic electron
transport
donor electron in non green tissue (root): NADPH derive from
oxidative pentose phosphate
Reduksi nitrit menjadi ammonium
• Di daun membutuhkan 6 e- dari H2O pd fotosistem non
siklik kloroplas
• Cahaya mendorong pengangkutan e- dari H2O ke
Feredoksin (Fd)
• Fd tereduksi menyediakan 6 e- utk mereduksi NO2menjadi NH4+
• Fereduksin tereduksi : donor elektron utk nitrit reduktase
di daun
Pengubahan amonium menjadi bahan
organik
• NH4+ tidak ditimbun di suatu tempat di tumbuhan
• NH4+ bersifat toksik : menghambat pembentukan ATP di
mitokondria / kloroplas
• NH4+ diubah menjadi gugus amina pada glutamin
• Selanjutnya membentuk: glutamat, asam aspartat,
asparagin
Pengubahan amonium menjadi bahan
organik
PEP karboksilase
Asparagin sintetase
Dari feredoksin di kloroplas
atau NADH/NADPH di
proplastid sel non
fotosintetik
Glutamin sintase (GS)
Glutamat sintase
GOGAT
Bentuk penyimpan nitrogen utama
Protein, klorofil, asam nukleat, dll.
Biological Nitrogen Fixation
• Conversion of nitrogen into
compounds by combining with
carbon, hydrogen and oxygen
before it can be absorbed by the
plants.
• It requires a complex set of
enzymes and a huge expenditure
of ATP. This is known as nitrogen
fixation.
• Although the first stable product
of the process is ammonia, this is
quickly incorporated into protein
and other organic nitrogen
compounds.
The micro-organisms which
can fix atmospheric nitrogen
can be classified into two main
groups
i.
Symbiotic micro-organisms.
The symbiotic bacterium
Rhizobium (saprofit !) is
found in association with the
root nodules (acts as a site
of Nitrogen fixation) of the
leguminous plants
ii.
Free living micro-organisms
Fiksasi N2
Tumbuhan dapat
memfiksasi N2
Bakteri sianobakter
Tumbuhan tidak
dapat memfiksasi N2
Bakteroid, sel-sel korteks
Sumber N utama NO3- dan NH4+,
NH4+ segera dioksidasi menjadi
NO3- → nitrifikasi
Proliferasi sel, bintil akar
Asimilasi nitrat
bakteroid, fiksasi N
Menghasilkan NH4+
NH4+ diubah menjadi senyawa organik/transaminasi
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Pembentukan nodul/bintil akar
Sinyal kimiawi dari akar
Dikenal bakteri
Bakteri merombak dinding sel, bakteri
masuk ke dalam sel bulu/rambut akar
Membentuk benang infeksi dari
membran plasma, bakteri
membelah di benang infeksi
Sitoplasma : leghemoglobin,
mengangkut O2 ke bakteroid,
mengatur jumlah O2
Benang infeksi ; sel- korteks
Bakteri dilepas dlm sitoplasma
sel tetraploid (sel diploid ?)
Sel tetraploid membelah
proliferasi jaringan bintil akar
Bakteri membesar : bakteroid,
berkelompok dikelilingi membran
peribakteroid
Formation of root nodules
• When a root hair of a leguminous plant comes in contact with
the bacterium-Rhizobium, it curls or becomes deformed.
• At the site of curling, the rhizobia (bacteria) invade the root
tissue.
• Some of the bacteria within the root tissue enlarge to become
membrane bound structures called bacteroids. These cannot
divide, while some bacteria remain untransformed to facilitate
further infection.
• The plant responds to this invasion by forming an infection
thread made up of plasma membrane that grows inward from
the infected cell of the host, separating the infected from the
rest of the plant.
• Cell division in the infected tissue leading to nodule formation.
• The nodule thus formed establishes a direct vascular
connection with the host for the exchange of nutrients.
Fiksasi N2 oleh bakteroid
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di bintil akar:
N2 + 8 e- + 8 H+ + 16 ATP  2 NH3 + H2 + 16 ADP + 16 Pi
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Energi ATP dan elektron diperoleh dari oksidasi KH/sukrosa
Oksidasi KH pd bakteroid menyebabkan : reduksi
NAD + → NADH atau
NADP + → NADPH
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NH3- / NH4+ diangkut keluar bakteroid menuju sitosol
Sitosol : NH4+ diubah menjadi bentuk utama nitrogen (glutamin, asam
glutamat, asaparagin) yang diangkut dari bintil akar ke bagian
tumbuhan yang lain
glutamin, asam glutamat, asaparagin: bentuk utama nitrogen yang
diangkut ke sel perisiklus (dekat berkas pembuluh yg mengelilingi bintil)
→ xilem → daun → floem → akar dan bagian lain
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Faktor-faktor yang mempengaruhi
fiksasi N
• Laju fotosintesis :
Kelembaban tinggi, suhu hangat, sinar matahari cukup,
[CO2] tinggi → fiksasi N 
• Genetis (mikroba) :
Proses pengenalan spesies bakteri – tumbuhan,
kemampuan nitrogenase mereduksi H+
• Tahap pertumbuhan :
tertinggi setelah pembungaan, saat perkembangan
reproduktif (kebutuhan N di biji dan buah yang sedang
berkembang )
Nitrogen Cycle - Nitrogen
Assimilation
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