Exception to Cell Theory

No scientific generalisation in biology is completely without exception, and cell theory is no different. Viruses, along with the related viroids and prions, are the acknowledged exceptions. The reason is structural rather than incidental: the cell theory treats protoplasm - the living material filling a cell - as the one indispensable feature that defines something as a genuine cell, and viruses simply do not have protoplasm at all. Outside a host cell, a virus is an inert particle of nucleic acid wrapped in a protein coat, with none of the living, metabolically active material that every true cell contains. Because of this, viruses, viroids and prions are described as sub-cellular, obligate parasites - entities that can only r …