Which process involves breaking down substances into smaller molecules?

The process of breaking down substances into smaller molecules is known as catabolism. This biochemical process is crucial for energy production and metabolism. During catabolism, complex molecules such as carbohydrates, fats, and proteins are degraded into smaller units, which can then be utilized by the body for various energy-producing pathways or for the synthesis of new molecules.

In contrast, anabolism refers to the metabolic pathways that construct molecules from smaller units, effectively building larger and more complex structures, rather than breaking them down. Aerobic processes involve the use of oxygen to convert food into energy, which typically includes both catabolic and anabolic reactions. Fermentation is a metabolic process that converts sugar to acids, gases, or alcohol in the absence of oxygen, which also does not directly relate to the breakdown of substances in the way catabolism does.

Thus, catabolism specifically denotes the breakdown of complex substances into simpler, smaller molecules, highlighting its fundamental role in energy metabolism.