Which of the following best defines metabolism?

• A. The sum of all chemical reactions in an organism
• B. The breakdown of large molecules into smaller ones
• C. The building up of molecules
• D. The control of bodily functions

Answer: (A)

Metabolism is the sum of all chemical reactions in an organism, sustaining life by transforming energy and materials to power cells and build new structures. It covers both breaking down molecules to release energy (catabolic processes) and using energy to synthesize larger molecules from smaller ones (anabolic processes). That broad view makes it the best definition, because it includes the entire network of reactions rather than just one direction. The other descriptions describe only parts of metabolism—breaking down molecules, building them up, or simply regulating bodily functions—without capturing the full set of chemical reactions that keep organisms alive.