Metabolic pathways are defined for groups of substances (
metabolites) that are interconverted by a corresponding group of
enzymes. Conceptually, they start with one substance (
glucose, say) and end with another (
carbon dioxide). However, there is no reason why they shouldn't be connected in a circle rather than an arrow, as is the case for the
TCA cycle.
A -> B -> C -> D -> E pathway
F <- E <
/ \
A D
\ /
B -> C
cycle
Naturally, the whole of the cell is part of one large metabolic net consisting of many pathways that intertwine and mix. This is the basis behind the concept of autopoesis - that the cell is catalytically closed. One large cycle.