As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses can still be classed into “species” based on their biology and genetics, but many aspects of such a classification remain controversial. In May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described. Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted. The molecular mechanisms of cell biology are based on proteins. Most of these are… read more