The basic unit of function and structure in living organisms is the cell, so the stored answer "Cells" is correct. Cells are the smallest units generally considered alive and are responsible for carrying out the chemical processes that sustain organisms.
Cell theory, developed in the 19th century by Matthias Schleiden and Theodor Schwann, established that organisms are made of one or more cells and that the cell is the fundamental unit of biological structure and function. Rudolf Virchow later added the principle that cells arise from pre-existing cells.
Robert Hooke first described and named cells in 1665 after observing the tiny compartments in cork under a microscope. Modern biology recognizes two broad cellular organizations: prokaryotic cells, such as bacteria and archaea, and eukaryotic cells, found in animals, plants, fungi, and many single-celled organisms.
Cells differ greatly in size and specialization, but all must regulate their internal environment and process genetic information. A bacterium may consist of one cell, while a human body contains many specialized cell types working together in tissues and organs.