______ is the acquisition of chemical substances by organisms for use as an energy source or as building blocks of cellular structures.

The term you're looking for is "Nutrition."

Nutrition refers to the acquisition and utilization of chemical substances, which are essential for organisms to derive energy and to build cellular structures. This process involves the intake of various nutrients, including carbohydrates, proteins, fats, vitamins, and minerals, which are necessary for growth, maintenance, and overall health. Organisms can be categorized as autotrophs, which produce their own energy (like plants through photosynthesis), or heterotrophs, which obtain energy by consuming other organisms (like animals). Proper nutrition is crucial for sustaining life and supporting metabolic activities within a biological system.