What is the process called that involves the breakdown of complex molecules into simpler ones?

The process that involves the breakdown of complex molecules into simpler ones is called catabolism. This is a crucial part of metabolism, which encompasses all chemical reactions occurring in the body. Catabolic reactions release energy by breaking down larger molecules, such as carbohydrates, fats, and proteins, into smaller units, like glucose and amino acids. The energy released in these reactions can then be used for various cellular functions and processes.

Anabolism, in contrast, is the process of building larger molecules from simpler ones, which typically requires energy input. Metabolism refers to the entire range of biochemical processes, including both catabolic and anabolic pathways, while biotransformation usually refers to the chemical modification made by an organism on a chemical compound, often related to drug metabolism or detoxification processes. Therefore, catabolism specifically denotes the breakdown processes, making it the correct answer.