The<unk> is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular (consist of a single<unk> . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014<unk> ; a typical<unk> size is 10 µm; a typical<unk> mass is 1 nanogram.) The largest known<unk> is an unfertilized ostrich egg<unk> 
In 1837 before the final<unk> theory was developed, a Czech Jan Evangelista Purkyně  observed small "granules" while looking at the plant tissue through a microscope. The<unk> theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more<unk> . All<unk>  come from preexisting<unk> . Vital functions of an organism occur within<unk> , and all<unk>  contain the hereditary information necessary for regulating<unk> functions and for transmitting information to the next generation of<unk> .
The word<unk> comes from the Latin<unk> la, meaning, a small room. The descriptive name for the smallest living biological structure was chosen by Robert Hooke in a book he published in 1665 when he compared the cork<unk>  he saw through his microscope to the small rooms monks lived in.
Cell 
Blood cell
Tissue 
Stem cell
Nucleosol
Blood vessel
Cancer Cell
Human body