Reaction pathways hierarchical structure

A series of connected nodes without cycles. One node, called the root, is the starting point of all pathways and has no predecessors. Nodes that terminate the pathways are called leaves. Trees can be used to represent hierarchical structures. Thus, they can be employed to visualize the steps of a reaction or of a synthesis route. See Synthesis Tree,... 

The hierarchical character of reaction systems does not end with decomposition of reactions into steps. At the next level, many reactions taken together make up pathways. In bioprocesses, such pathways form long chains, cycles, and branching structures that accomplish biologically identifiable functions. In other processes, a pathway is used to describe the sequence of transformations that are needed to obtain the desired product from the available raw materials. One might even envision, at the next level, pathways combined to describe entire chemical plants or families of processing technologies. 

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