ABSTRACT
For purposes of this chapter, we define a pathway as a description of the mechanism of the typical process by which a cellular or organismic function is realized. This definition makes two points. First, a pathway is not a mechanism, but a description of mechanisms, that is, it exists as a model subject to our investigative needs and not as an entity itself. Second, a pathway must have functional interpretations, that is, it must correspond to a goal or need we can identify in the cell or organism. Thus this definition does not provide a purely objective criterion but tosses the problem into the laps of people who want to define “function” at the cellular or organismic level. It also removes from consideration any sequence of biochemical interactions that do not have such an interpretation.
