Claim: Cells are what organisms are composed of.

The cell -LRB- from Latin cella , meaning `` small room '' -RRB- is the basic structural , functional , and biological unit of all known living organisms . A cell is the smallest unit of life that can replicate independently , and cells are often called the `` building blocks of life '' . The study of cells is called cell biology .   Cells consist of cytoplasm enclosed within a membrane , which contains many biomolecules such as proteins and nucleic acids . Organisms can be classified as unicellular -LRB- consisting of a single cell ; including bacteria -RRB- or multicellular -LRB- including plants and animals -RRB- . While the number of cells in plants and animals varies from species to species , humans contain more than 10 trillion -LRB- 1012 -RRB- cells . Most plant and animal cells are visible only under a microscope , with dimensions between 1 and 100 micrometres .   The cell was discovered by Robert Hooke in 1665 , who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery . 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 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 cells . Cells emerged on Earth at least 3.5 billion years ago .