Transcript Bio426Lecture26Apr5 - NAU jan.ucc.nau.edu web server

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
1. Assimilation of NO3- or NH4+ absorbed by roots
2. Symbiotic N2 fixation
The Nitrogen Cycle
Fig. 12.1
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.
The Nitrogen Cycle
Nitrate and ammonium assimilation involves their
conversion into amino acids
Nitrate assimilation requires two reduction steps
NO3- + NAD(P)H + H+ + 2e- --> NO2- + NAD(P)+ + H2O
enzyme is nitrate reductase
NO2- + 6 Fdred + 8H+ + 6e- --> NH4+ + 6Fdox + 2H2O
enzyme is nitrite reductase
Biological Nitrogen Fixation converts N2 gas
into chemical forms usable by microbes and plants.
Examples of N2 fixing plants of the Southwest
Mesquite, Acacia, Palo Verde
Symbionts are Rhizobium & Bradyrhizobium
Lupines - Bradyrhizobium
Root
nodules
Nitrogenase is the key enzyme in biological N fixation.
A bacterial enzyme, not made by plants
Biological N fixation is energetically expensive, 16 ATP/N2.
Note that Molybdenum is a cofactor