III' UPTAKE AND ASSIMILATION

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III. UPTAKE AND ASSIMILATION

• Main driving force for Ion uptake
• Electrochemical potential (E) across plasma
  membrane.
• Generated by the HATPase

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• - HATPase pumps H out of the cell
• - establishes an electric potential of -100 to
  -200 milliVolts across plasma membrane
• - attracts cations

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Types of Transport

• Uniport - 1 molecule, one direction (e.g., H
  out)
• Antiport - 2 molecules, 2 directions
• Symport - 2 molecules, same direction
• Electrogenic net change in charge across
  membrane

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Active Transport

• Against concen. gradient
• Requires energy
• Two types
• Primary directly coupled to chemical or
  photochemical rxn
• Secondary driven by E, setup by HATPase

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Transporters

• proteins that facilitate the crossing of
  specific molecules across a membrane
• 3 types
• Translocators
• Channels
• Pores

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Translocators

• Very specific
• Show saturable kinetics ?
• Bind substrate, then form pore-like structure

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Ion Channels and Porins

• Ion channels
• Unlike translocators, open at both ends
• Transport much faster than translocators
• Often open transiently
• Respond to electropotential across membrane
• Also regulated by Ca2 and phytohormones
• Porins
• Larger openings than ion channels
• Filled with water molecules
• Relatively non-specific (anything lt30,000
  Daltons)
• Found in outer membranes of mitochondria and
  chloroplasts

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ASSIMILATION OF ANIONS

• Nitrate
• reduced to NH3 in 2 steps
• occurs in roots and leaves
• 2e1-
• Step (1) NO31- -------gt NO21-
• nitrate
  nitrite
• Catalyzed by Nitrate Reductase (NR), a
  cytoplasmic enzyme
• e1- come from NADH

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• 6e1-
• Step (2) NO21- -------gt NH3
• nitrite ammonia
• Catalyzed by Nitrite reductase, a chloroplast
  enzyme
• e1- come from Ferredoxin (in leaves)
• Rate-limiting step is (1), NR is highly
  regulated
• ammonia and amino acids
• nitrate, light and sugars

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Nitrogen assimilation (cont.)

• NH3 quickly added onto glutamate
• NH3 ATP Glu ----gt Gln ADP
• Glu - glutamate Gln - glutamine
• catalyzed by Glutamine synthetase (GS)

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• NH3 is transferred to other molecules by
  specific aminotransferases

• Nitrate assimilation is expensive as much as
  25 of reducing power from photosynthesis may
  be used for nitrate reduction

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Herbicides that block Amino Acid Synthesis
Glyphosate - Roundup
Heldt, 1997
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Nitrogen Fixation

• Some plants get reduced Nitrogen from N- fixing
  bacteria that live symbiotically with roots
• Rhizobium-Legume symbiosis
• - Rhizobia live in root nodules (intracellular),
  fix N2 to NH3, which the plant uses
• - plant provides reduced carbon sources low O2
  environment in nodules

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Rhizobium nodules on Soybean (Glycine max) roots
Salisbury Ross, 1992
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Bacteroids in soybean (Glycine max) nodules
4-6 bacteroids in each peribacteroid space PBM
peribacteroid membrane
Salisbury Ross, 1992
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Nodular bacteroid metabolism
Heldt, 1997
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Nitrogen Fixation Chemistry

• reduction of atmospheric N2 into ammonia
• 6e1-
• N2 ------gt NH3
• N2 12ATP 6e1- ---gt 2NH41 12ADP
• Rxn catalyzed by Nitrogenase
• O2 strongly inhibits enzyme!

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How is a low free O2 obtained in root nodules?

• Legume root nodules have hemoglobin, called
  leghemoglobin!
• Located in cytoplasm of host cell
• Binds up free O2

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Nitrogen recycling

• Alot of nitrogen redistribution and recycling in
  plants
• Examples
• Nitrogen content drops drastically in leaves
  prior to abscission
• Nitrogen content decreases in vegetative tissues
  as it rises in seeds
• Recycling involves
• proteinases that degrade proteins
• transport of amino acids out of senescing tissue

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Sulfate Phosphate assimilation

• SO42- incorporated initially into cysteine
  within chloroplasts requires ATP e-1
• 8e1-
• SO42- -------gt R-S-H (cysteine)
• Phosphate assimilation is mainly by
  photosynthesis
• H2PO41- ADP ------gt ATP