“The task of the excellent teacher is to stimulate apparently ordinary people to unusual effort. The tough problem is not in identifying winners: it is in making winners out of ordinary people.”
Sep 28, 2013
Sep 22, 2013
Nitrogen cycle
Atmospheric nitrogen cannot be absorbed
directly by plants (to make proteins).
Plants absorb nitrogen from the soil in
the form of ammonium, nitrites or nitrates ions. The process of
converting atmospheric nitrogen into ammonium compounds is called nitrogen
fixation.
Nitrogen fixation is carried out by nitrogen – fixing
bacteria :
i) Nostoc sp. (cyanobacteria) & Azotobacter sp. that live freely in soil
ii) Rhizobium sp. that live in root nodule of
leguminous plants.
Both use atmospheric nitrogen to make
ammonium compounds –(NH3
– ammonia & NH4+ - ammonium)
Nitrifying bacteria convert ammonium compounds into nitrites
& nitrates.
The process is called nitrification.
i) Ammonia is oxidised into nitrites (NO2-)
by Nitrobacter sp.
ii) Nitrites are oxidised into nitrates (NO3-)
by Nitrosomonas sp.
Nitrates in the soil are absorbed by
plants & converted into plant proteins.
When animals eat plants, the proteins are
transferred into the body of the animals & become animal protein.
Dead plants, animals & waste
materials are decomposed by decaying bacteria & fungi/decomposers into
ammonium compounds.
The cycle is balanced by returning of
nitrogen to the atmosphere by denitrifying bacteria.
The process is called denitrification.
Denitrifying bacteria break down nitrates
into gaseous nitrogen & oxygen.
Oxygen – used by bacteria; nitrogen –
returned to the atmosphere.