Lecture 7: Assimilation of nitrogen by plants Flashcards
why is nitrogen needed in plants
needed for amino acids, proteins and other cellular constituents.
nitrogen deficiency results in
stunted growth & chlorotic leaves
what is generally the most limiting nutrient to plant growth
Nitrogen
N2 is an extremely ____ molecule
unreactive
The industrial Haber-Bosch process: equation + use of global energy
N2 + 3H2 –> 2NH3 requires high temperatures (300-600 degrees C ), high pressures (20-80MPa) and has a low yield
- uses 1-2% of global energy
_ tonnes of oil are needed to produce 1 tonne of fertiliser
2
____ is the point of entry of nitrogen into organic combination
Ammonia
Ammonia is generated in two ways:
1) reduction of N2 by micro-organisms, often in symbiotic association with other plants
2) reduction of nitrate (the predominant form of soil in N) to ammonia, catalysed by nitrate and nitrite reductase in roots or shoots
nitrate reduction accounts for about __ Mt per annum, compared with N2 fixation at ___ Mt
nitrate reduction = 2 X 10^4 Mt
N2 fixation = 200 Mt
pollutants such as NO2 are also
assimilated via the pathway of nitrate reduction
how many species micro-organisms (bacteria & cyanobacteria) accomplish nitrogen bacteria with the aid of nitrogenase
over 250
EQUATION:
N2 + 16ATP +8e +10H+ –> 2NH4+ + H2 + 16ADP + 16Pi
There are three problems facing organisms which reduce nitrogen:
- reaction very sensitive to oxygen (which inactivates the nitrogenase)
- side reaction - the reduction of 2H+ to H2
These two features are reminiscent of Rubisco
- requires a very large input of energy in the form of ATP and reductant
Gloeocapsa sp. in the cyanobacterium when does photosynthesis + N2 fixation occur
photosynthesis, and accompanying O2 evolution, occurs during the day, and N2 fixation at night
explanation of nitrogen-fixing symbioses
Although nitrogen fixation is confined to micro‐organisms, there is a wide range of symbiotic associations of these organisms with, e.g., lichens, diatoms, bryophytes, pteridophytes (Azolla), cycads, Alnus, and legumes (peas, beans, clover, many tropical trees).
in legumes for every 100 carbon atoms fixed in the leaves, ___ go to the nodules
1/3.
5 C atoms for growth and maintenance of the nodule.
photosynthate
12 C atoms for respiratory production of ATP and NADH (to fix nitrogen).
15 C atoms to provide carbon substrates for the assimilation of ammonia (these largely return to the shoot).
Darkened plants fix N2 at very ____
low rates
CO2 enrichments affect on N2 fixation
CO2 enrichment, which inhibits photorespiration, enhances N2 fixation.
Oxygen supply in legume nodules: O2 is maintained in nodules at a ___
low and buffered concentration by an O2 carrier - leghaemoglobin
leghaemoglobin facilities the diffusion of
O2 within the nodule, but the free concentration of O2 is about 10 nM in soybean nodules, compared with 280 microM in air-saturated water
oxygen supply in legume nodules allows
rapid rates of respiration and hence production of ATP and reductant, but reduces the danger of inactivation of nitrogenase by O2
What is the first organic product of ammonia assimilation
Glutamine
Nitrogen assimilation:
Nitrogen assimilation is the formation of organic nitrogen compounds like amino acids from inorganic nitrogen compounds present in the environment
how is ammonia assimilated into amino acids?
(cycle)
Glutamate + ATP + NH3 –> Glutamine (under glutamine synthesise) ADP released
glutamine (under glutamate synthase) + 2-oxoglutarate produce glutamate [ferredoxin(red) goes to ferredpxin(ox) in this step]
RECYCLE
Glutamate + glutamine can lead on to form a.a.
amides + ureides are characterised by
a high N:C ratio
Ureides are
tropical legumes
other ways of acquiring nutrients =
carnivore in N-poor environments
venus fly trap etc
