A metabolic pathway is a connected sequence of enzyme-catalyzed chemical reactions in metabolism, with a reaction product of one step serving as a substrate for another. A pathway boundary is a choice about which transformations to count: glycolysis ends at pyruvate, whereas subsequent fermentation or cellular respiration changes the final material balance. Adding the individual chemical equations cancels internal intermediates and exposes the net stoichiometry; it does not determine the reaction rate.
Some notable examples:
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