What is Nitrogen assimilation, transamination?

Ammonium enters organic matter almost entirely through glutamate: glutamate dehydrogenase adds it to $\alpha$-ketoglutarate, and glutamine synthetase adds a second to glutamate’s side chain (one ATP), making glutamine. From these two donors, transaminases (aminotransferases, with a vitamin B$_6$ coenzyme) move the amino group onto any $\alpha$-keto acid, building amino acids from the carbon skeletons of the central pathways: the glutamate family from $\alpha$-ketoglutarate, the aspartate family from oxaloacetate, alanine and serine from pyruvate and 3-phosphoglycerate. Plants and bacteria make all twenty; animals have lost the long pathways to nine of them, the essential amino acids, and take them from food. The same transaminases, run backward, strip nitrogen from surplus amino acids for excretion as ammonia (fish), urea (mammals) or uric acid (birds, insects).