Cell (biology)

The cell is the basic structural and functional unit of all forms of life. A cell consists of cytoplasm enclosed within a membrane; many cells contain organelles, each with a specific function. The term comes from the Latin word cellula meaning 'small room'. Most cells are only visible under a microscope. Cells emerged on Earth about four billion years ago. All cells are capable of replication, protein synthesis, and motility.

Cells are broadly categorized into two types: eukaryotic cells, which possess a nucleus, and prokaryotic cells, which lack a nucleus but have a nucleoid region. Prokaryotes are the single-celled archaea and bacteria, whereas eukaryotes can be either single-celled, such as amoebae, or multicellular, such as some algae, plants, animals, and fungi.

Eukaryotic cells contain membrane-bound organelles such as the cell nucleus, and mitochondria, which provide energy for cell functions, and chloroplasts, in plants that create sugars by photosynthesis. Other organelles may be proteinaceous such as ribosomes. A unique membrane-bound prokaryotic organelle the magnetosome has been discovered in magnetotactic bacteria.

Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells in a monastery. Cell theory, developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all organisms, and that all cells come from pre-existing cells.