The smallest unit of life that can replicate is the cell. Cells are often referred to as the basic building blocks of life because every living organism is composed of one or more cells. They perform essential functions, including growth, metabolism, response to stimuli, and reproduction. Cells can be categorized into prokaryotic cells, found in bacteria and archaea, and eukaryotic cells, which make up plants, animals, fungi, and protists. Each type of cell exhibits complex machinery and structures that enable it to carry out its life-sustaining processes and reproduce to form new cells.