Citrate cycle carbon flows

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

Fig. 11.2 Main carbon flows during fumaric acid production by Rhizopm species. The inner circle represents the mitochondrial membrane, with citrate cycle reactions occurring in the mitochondria, while the outer circle represents the cytoplasmic membranes...

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