Schleiden and Schwann are credited as the founders of The Cell Theory  All living things are made of cells. Cells exhibit the fundamental characteristics of life: Obtain energy—photosynthesis, chemosynthesis, or digestion Use energy—respire Excrete Waste Show Order Homeostasis Evolve Reproduce Move Respond to changes in environment Grow and repair damage to self
The development of the Cell Theory involved the input of numerous individuals from various nationalities over many years
Rudolph Virchow stated “omnis cellula a cellula ” [all cells arise from pre-existing cells based on observations of dividing cells 
The Cell Theory was modified to incorporate Virchow’s work All organisms are made of cells Cells are the smallest structural and functional unit of an organism [that carries out the life processes] All cells come from pre-existing cells
BUT, exceptions remain: 1. Single-celled organisms ARE cells, not composed of cells; that is, they are a-cellular 2. Many cells are unique in composition—muscle cells and fungal hyphae are multi- nucleated (i.e., more than one nucleus)
And, what about viruses? Are not composed of cells but consist of a nucleic acid (DNA or RNA) in a protein coat Yet conduct all of the cell processes (although with help, not independently)
SO: Could there be an organism that does not contain a single cell? Might there be some other fundamental unit of life? At least, in some cases? Are the life processes sufficient criteria to distinguish between the living and the non-living?
Microscopes Max magnification – X1000 Max Resolution 0.2um Max magnification X1000,000 Max Resolution 0.5nm