Nitrogen Uptake

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Nitrogen Uptake

• Nearly 80 of the atmosphere is nitrogen
• Very few living organism can utilize molecular
  nitrogen (N2)
• A few bacteria, cyanobacteria and Rhizobium sp
  can fix atmospheric N2
• Where does most nitrogen come from?

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Nitrogen Sources

• Nitrogen recycling
• Industrial fixation
• Haber Process
• heat pressure
• N2 3 H2 2NH3
• Lightning
• N2 O H20 HNO3

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Nitrogen Cycling

• Without fertilization most nitrogen is via
  nutrient recycling
• Ammonification
• conversion of organic N to NH4
• soil microbes
• Nitrification
• conversion of NH4 to NO2-

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Nitrification

• 2NH3 3O2 ? 2NO2- 2H 2H2O
• It is an energy yielding process for the
  Nitrosomonas sp
• Nitrite is toxic to plants
• It is converted by Nitrobacter sp to nitrate
• 2NO2- O2 ? 2 2NO3-
• Energy releasing reaction

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Nitrogen Uptake

• Plants take up nitrogen as either the ammonium
  ion, nitrate ion or urea.
• (or as preformed protein)
• Nitrate is easily leached from soil
• major source of agriculture water pollution
• Ammonium binds to clay particles
• Urea is a slow release form of ammonium

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Nitrate Uptake

• Both nitrate and ammonium are taken up by active
  transport
• Nitrate uptake is via a high affinity symport
• It is taken up along with 2 protons (linked to
  mitochondrial respiration)
• The Km for the transporter is 1 mM nitrate

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Active Transport

• Active Transport requires expenditure of cellular
  energy
• It can be coupled with energy released or it can
  be indirect
• Coupled - the transport of a molecule is directly
  linked to the hydrolysis of ATP

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Active Transporter

• Transmembrane proteins -change confirmation to
  transport molecules

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Active Transporters

• Antiports - molecules transported in opposite
  directions
• Symport - molecules transported in same direction
• Uniport - 1 molecule in 1 direction

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Nitrogen transporter

• Nitrate transporters are classic 66 arrangement
  of most plant ion transporters
• There are both high and low affinity transporters
  in roots
• Activity regulated by ion availability
• Ammonium uptake by diffusion, passive active
  transport

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Nitrate Assimilation

• Needs to be reduced to ammonium for most
  biological functions
• Energy expensive process
• Tied in with most metabolic pathways in plants
• Most central metabolic process in biology

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Nitrate Assimilation

• Assimilation can occur in root or leaves
• Because of energy cost most nitrate is exported
  from roots to leaves (except in newly emerged
  seedlings)
• Unloading from xylem is energy dependant
• If excess nitrate is taken up by roots, it is
  stored in vacuoles
• Little storage in leaves

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Nitrate Reduction

• Nitrate Reduction occurs in cytoplasm in both
  roots and leaves
• Reaction is catalyzed by nitrate reductase (NR)
• NO3- NADH H ? NO2- H2O NAD
• NR has 2 identical subunits
• Contains a cytochrome b557
• Requires Mo

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Nitrite Reduction

• Nitrite is toxic and is rapidly transported to
  the leucoplast (roots) or chloroplast
• NR also reduces chlorate to chlorite
• Reduction of nitrite to ammonium requires the
  transfer of 6 electrons
• The reaction is catalyzed by nitrite reductase
  (NiR)

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Nitrite Reduction

• Electron source for reduction is ferridoxin in
  the leaves
• Ferridoxin is reduced by the transfer of
  electrons in photosystem I
• In roots, the reducing power is provide by NADPH
• NADPH is provide by the oxidation of G-6-P to
  ribulose -5-P

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Nitrite Reductase

• Monomeric protein
• Contains FE-S clusters and a Siroheme
• Contains a bound FAD
• NiR has a low Km, assures the complete conversion
  of nitrite
• Both NR and NiR are regulated by the presence of
  nitrate