Figure 1
Simplified pathway for nitrogen incorporation from soil to animal protein.
Arrows represent: uptake (dashed line), chemical transformations (solid line) and metabolic processes (solid curves). (a) Plant NO3– uptake. (b) NO3– converted to glutamine31. (c) NH4+ uptake. (d) NH4+ converted to glutamine by attachment to glutamate31. (e) Glutamine supplies nitrogen for synthesis of other amino acids. The associated shift in δ15N values depends on the specific amino acid, plant part and plant type31. (f) Consumption of amino acids by the animal. (g) Source amino acids are minimally involved in metabolic processes, undergoing small changes in δ15N values (e.g. increases in δ15NPhe values from diet to consumer tissue are commonly ≤ 2‰20). (h) Trophic amino acids are heavily involved in metabolic processes, undergoing enrichment in 15N (e.g. increases in δ15NGlu values from diet to consumer tissue are commonly 6–7‰20). Katherine Allan drew the images of grass and mammoth in Figure 1.
