Ammonia  Assimilation 

The incorporation of reduced inorganic nitrogen (ammonia) into biomolecules  is ammonia assimilation. the biomolecules generated by ammonia assimilation are amino acid, glutamate and glutamine. both these amino acids serve as important amino donors in biosynthesis of a variety of nitrogen containing compounds. 

The assimilation of ammonia is a two step process catalyzed by glutamine synthetase (GS) and glutamine 2-oxoglutarate aminotransferase (GOGAT) or glutamine synthesis. in the first step glutamine is formed from glutamate the reaction consumes ATP and is essentially irreversible under physiological condition. it proceeds via an enzyme bound y-glutamyl phosphate intermediate 

Glutamic acid +ATP 

(Y-glutamyl +nitrate

Glutamine +pi

The enzyme has high affinity for ammonia and is therefore an effective route for ammonia assimilation under conditions when ammonia production is less, for instance during nitrogen fixation.

The next enzyme is GOGAT which transfers the amide nitrogen of glutamine to 2-oxoglutarate with concomitant reduction of the carbonyl group.

Another enzyme that can assimilation ammonia by reductive amination of 2- oxoglutarate  is glutamate dehydrogenase (GDH). it becomes relevant only when the concentration of ammonia is higher .it is less important in nitrogen fixing organism.

in plant ammonia assimilation includes primary nitrogen assimilation reassimilation of photorespiratory ammonia  released in mitochondria and reassimilation of recycle nitrogen. primary nitrogen assimilation in nitrogen fixing root nodule of legumes involving the direct fixing of nitrogen by the bacterial partner followed by ammonia assimilation catalyzed by cytosolic GS isozyme and NADH -GOGAT. in non legumes nitrate is absorbed by roots which is either reduced to ammonia by concerted action of NR and NIR or transported to the leaves for nitrate assimilation. in either case ammonia is assimilated by different isozyme of GS /GOGAT  in the roots and leaves. GOGAT is ferredoxin dependent in chloroplasts.

you are aware of enormous photorespiratory loss of carbon dioxide in c3 plant initiated by the oxygenase activity of rubisco. along with carbon dioxide there is also released of ammonia in the mitochondria but unlike carbon dioxide ,ammonia  is reassimilated by chloroplast GS/GOGAT. finally ammonia recycling occurring at all times in plant such as amino acid deamination .on two occasions (germination and senescence ) massive amounts of ammonia is reassimilated to glutamine or glutamate for transport.