Metabolism encompasses all the chemical reactions that occur within a living organism to sustain life. It's divided into two main categories:

* Anabolism: This is the building up process. Anabolic reactions use energy to synthesize complex molecules from simpler ones. Examples include protein synthesis, bone growth, and the storage of carbohydrates.

* Catabolism: This is the breaking down process. Catabolic reactions release energy by breaking down complex molecules into simpler ones. Examples include the digestion of food, the breakdown of glucose for energy, and the recycling of cellular components.

In essence, metabolism is the continuous cycle of building and breaking down molecules that is essential for life.