The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. An organism's essential function is to consume (ingest) energy and molecules in the foods we eat, convert some of it into fuel for movement, sustain our body functions, and build and maintain our body structures. There are two types of reactions that accomplish this: anabolism and catabolism.

Anabolism is the process of combining smaller, simpler molecules into larger, more complex substances. Anabolic reactions require energy in the form of ATP to combine monosaccharides to form polysaccharides, fatty acids to form triglycerides, amino acids to form proteins, and nucleotides to form nucleic acids. These reactions are also called biosynthesis reactions, as they can create new molecules, cells and tissues and revitalize organs.

Catabolic reactions break down large organic molecules into smaller molecules, releasing the energy contained in the chemical bonds. The complex molecules found in foods are broken down so the body can use their parts to assemble the structures and substances needed for life. These energy releases (conversions) in catabolic reactions are not 100 percent efficient. The amount of energy released is less than the total amount contained in the molecule. Approximately 40 percent of energy yielded from catabolic reactions is directly transferred to the high-energy molecule adenosine triphosphate (ATP), the energy currency of cells. It can be used immediately to power molecular machines that support cell, tissue, and organ function, which includes building new tissue and repairing damaged tissue. ATP can also be stored to fulfill future energy demands. The remaining 60 percent of the energy released from catabolic reactions is given off as heat, which tissues and body fluids absorb.

Metabolism is the sum of all anabolic and catabolic reactions that take place in the body. Both anabolism and catabolism occur simultaneously and continuously to keep us alive.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 1.3: Functions of Human Life and Openstax, Anatomy and Physiology 2e, Section 24.1: Overview of metabolic Reactions