Assimilation of mineral nutrients (Chapter 12)

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• Assimilation of mineral nutrients (Chapter 12)
• Incorporation of inorganic forms of essential
  elements into organic compounds necessary for
  growth and development.
• Nitrogen assimilation is quantitatively the most
• important and is energetically expensive.
• Two main ways that plants get reduced N
• Assimilation of NO3- or NH4 absorbed by roots
• Symbiotic N2 fixation

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The Nitrogen Cycle
Fig. 12.1
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Microorganisms have a central role in almost all
aspects of nitrogen availability and thus for
life support on earth some bacteria can
convert N2 into ammonia by the process termed
nitrogen fixation these bacteria are either
free-living or form symbiotic associations with
plants or other organisms (e.g. termites,
protozoa) other bacteria bring about
transformations of ammonia to nitrate, and of
nitrate to N2 or other nitrogen gases many
bacteria and fungi degrade organic matter,
releasing fixed nitrogen for reuse by other
organisms.
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The Nitrogen Cycle
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Nitrate and ammonium assimilation involves their
conversion into amino acids Nitrate assimilation
requires two reduction steps NO3- NAD(P)H H
2e- --gt NO2- NAD(P) H2O enzyme is
nitrate reductase NO2- 6 Fdred 8H 6e- --gt
NH4 6Fdox 2H2O enzyme is nitrite
reductase
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Biological Nitrogen Fixation converts N2 gas into
chemical forms usable by microbes and plants.
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Examples of N2 fixing plants of the
Southwest Mesquite, Acacia, Palo Verde
Symbionts are Rhizobium Bradyrhizobium Lupine
s - Bradyrhizobium
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Root nodules
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Nitrogenase is the key enzyme in biological N
fixation. A bacterial enzyme, not made by plants
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Biological N fixation is energetically expensive,
16 ATP/N2.
Note that Molybdenum is a cofactor
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