The macromolecule that functions in the selective acceleration of chemical reactions is called an enzyme. Enzymes are biological catalysts that speed up chemical reactions in the body by lowering the activation energy required for the reaction to occur.
Enzymes are composed of proteins, and their specific structure allows them to interact with particular substrates, which are the reactants in the chemical reactions they catalyze. Each enzyme has an active site, a unique region where substrates bind, and this specificity is what allows enzymes to facilitate only certain reactions among the many that could occur in biological systems.
In summary, enzymes are essential for life as they ensure that metabolic processes happen at a rate that is compatible with life, thus helping maintain homeostasis in organisms.