1. Leghaemoglobin (as the name implies) sequesters oxygen in root nodules. This is important as Rhizobium bacteroids cannot fix nitrogen in the presence of oxygen, because nitrogenase is rapidly destroyed by oxygen.
2. Nitrogen gas consists of N₂ molecules. Although fixation is exergonic (−ve ∆G) the triple bond between the N atoms is extremely strong, and the activation energy is enormous. Plants lack a suitable catalyst (nitrogenase), except those in symbiosis with nitrogen-fixing bacteria.
3. Glutamine synthetase is allosterically regulated, so it must be multimeric.
4. GDH has a much higher K_m for ammonia than GS. Most organisms would die if exposed to a sufficient ammonia concentrations to make GDH run at a significant rate.
5. Links between urea and Krebs cycle.
   • Assimilation: α-ketoglutarate supplied by Krebs to glutamate synthase.
   • Transamination: keto acids supplies by Krebs to transaminases.
   • Degradation: urea cycle requires aspartate (from oxaloacetate) and produces fumarate. These are Krebs intermediates.