The smallest unit of life in biology is the cell. Cells are the basic building blocks of all living organisms, serving as the fundamental unit of structure and function. They can exist as unicellular organisms, such as bacteria, or as part of multicellular organisms, including plants and animals. Cells perform various essential functions, including metabolism, energy production, and reproduction. They consist of various components, such as the nucleus, mitochondria, and cell membrane, each with specific roles that contribute to the overall functioning of the organism.