The cell is the basic structural, functional and biological unit of all known living organisms. It is the smallest unit of life that is classified as a living thing (except virus, which consists only of DNA/RNA covered by protein and lipids), and is often called the "building block of life".
It consists of a protoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including most bacteria) or multicellular (including plants and animals).
While the number of cells in plants and animals varies from species to species, Humans contain about 100 trillion (1014) cells. Most plant and animal cells are between 1 and 100 micrometres and therefore are visible only under the microscope.
The cell was discovered by Robert Hooke in 1665. The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that all cells come from preexisting cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. Cells emerged on planet Earth at least 4.0–4.3 billion years ago.