Here's why:

* Metabolism encompasses all the chemical reactions that occur within a living organism to sustain life. These reactions include:

* Anabolism: Building up complex molecules from simpler ones (e.g., protein synthesis)

* Catabolism: Breaking down complex molecules into simpler ones (e.g., cellular respiration)

All living cells need to carry out these reactions to:

* Obtain energy: Cells need energy to perform their functions.

* Synthesize essential molecules: Cells need to build proteins, nucleic acids, lipids, and other molecules for growth, repair, and communication.

* Maintain homeostasis: Cells need to regulate their internal environment and respond to changes in their surroundings.

While other activities like reproduction, growth, and response to stimuli are important to living organisms, they are not essential for the survival of individual cells.

Therefore, metabolism is the fundamental activity that defines all living cells.