Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular such as consisting of a single cell; including bacteria or multicellular including plants and animals. While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (1013) cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres. All organisms are composed of one or more cells that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cell. Cells emerged on Earth at least 3.5 billion years ago.