What does metabolism refer to in living organisms?

Metabolism refers to the sum of all chemical reactions that occur within living organisms. This includes both anabolic reactions, which build complex molecules from simpler ones, and catabolic reactions, which break down complex molecules into simpler ones, releasing energy in the process. These metabolic processes are crucial for maintaining life, as they provide the energy necessary for growth, reproduction, and response to the environment.

The other options do not capture the comprehensive nature of metabolism. Reproduction is a specific biological function, responding to stimuli is part of an organism's interaction with its environment, and the organization of cells and tissues pertains to the structural aspect of living organisms rather than their biochemical processes. Understanding metabolism is fundamental for studying how organisms transform energy and nutrients to sustain life.