What synthesis of simple food molecules into more complex units to help an organism build new tissue is a function of what system?

The synthesis of simple food molecules into more complex units to help an organism build new tissue is primarily a function of the metabolic system, specifically anabolism. Anabolic processes involve the conversion of nutrients into macromolecules like proteins, lipids, and nucleic acids, which are essential for growth and tissue repair. This function is closely associated with the digestive and endocrine systems, which facilitate the breakdown of food and regulate metabolic processes, respectively.