Which of the following describes a series of defined steps that alter a specific molecule?

The correct answer is a metabolic pathway, which refers to a series of defined, sequential chemical reactions within a cell that convert a specific substrate through various intermediates into a final product. These pathways are essential for cellular processes, as they facilitate the transformation of molecules, allowing cells to build up and break down biomolecules efficiently. Each step in a metabolic pathway is catalyzed by specific enzymes, which ensure the reactions occur in a controlled manner.

This concept is fundamental in biology because it helps explain how nutrients are processed, energy is generated, and metabolic regulation is maintained within an organism. For example, the catabolic pathway, which is another option, is indeed a type of metabolic pathway that specifically breaks down molecules to release energy but does not encompass all metabolic transformations. Bioenergetics deals more broadly with the flow of energy through living systems, rather than a specific sequence of reactions. Exocytosis, on the other hand, is a process of transporting substances out of the cell, rather than describing a series of alteration steps on a molecule. Thus, metabolic pathways encompass all these reactions and processes in a structured way.