What is metabolism?

Metabolism refers to the comprehensive range of chemical reactions that occur within a living organism to maintain life. This includes processes such as the conversion of food into energy, the synthesis of necessary compounds, and the breakdown of waste products. Metabolism is crucial for organisms as it enables growth, reproduction, maintenance of cellular structures, and response to environmental changes.

This definition embraces both catabolic reactions, which break down molecules to generate energy, and anabolic reactions, which construct molecules from smaller units. Therefore, understanding metabolism is essential as it reflects how organisms obtain and utilize energy, making this choice the correct representation of metabolism in biological terms.

In contrast, the other options refer to different concepts; for instance, the notion of the total number of living organisms pertains to ecology and population studies, rather than the biochemical processes within individuals. The process of producing energy from sunlight is a specific aspect of metabolism known as photosynthesis, primarily relevant to plants and certain microorganisms. Cellular respiration is indeed a metabolic pathway, but it does not encompass the entirety of metabolic processes, making it a narrower definition. Thus, the comprehensive understanding of metabolism as the sum of all chemical reactions solidifies choice B as the most accurate answer.