The Latin prefix a- means “without.” The Latin word cellula means “a small room.” Acellular is a term most recognized in the biological sciences to mean living things without conventional cell structures and functions.
Plant cells were first observed through a microscope in 1665. By 1840, the Cell Theory had established the atomistic principle that the cell is the basic unit of life, the smallest living biological organism. The metabolic functions of a cell — respiration, growth, reproduction — were adopted as the requisite definition of life itself. The egg is the quintessential model of a cell, an ostrich egg being the largest known specimen of a single cell.
Human beings are differentiated multicellular organisms with approximately 100 trillion co-dependent, but individual, cells. Difficult as it is to fathom, by a very large margin, the majority of the Earth's total biomass is unicellular life — organisms such as bacteria consisting of just one cell. By the turn of the 21st century, advances in technology and microbiology had discovered acellular organisms such as viruses without one or more of the defining characteristics of a cell.
As with the ostrich egg, one traditionally indispensable key characteristic of a cell is its enclosure, its shell, the cell membrane. A single complex cell was assumed to have the capacity for life because all of the many functional ingredients necessary were self-contained and accessible. The unicellular protozoa called ciliates furthermore have tiny, vibrating hair-like growths projecting off their cell membranes and enabling them to move. This motility, interpreted as purposeful, represented the deterministic principle of life. The most common substitute meaning of acellular is apparent life not enclosed by a cell membrane.
Most scientists, certainly virologists, have come to believe that viruses are alive, despite being mostly just strands of their particular genetic material sheathed in a protective coat of specific proteins that help them identify and infect other host cells. They had been originally thought to be, in a sense, inert, with their sometimes harmful effects merely the metabolic changes of their living host organisms. In 2003, a class of unusually large viruses called mimivirus was discovered to be capable of creating proteins on their own, without need of a host. This led to the speculative assumption that there might be many other viruses with abilities that had once been reserved for cellular life.
An acellular entity is, by definition, smaller than a cell. Such organisms are difficult to find, much less study, without the help of imaging equipment such as electron microscopes. Nevertheless, science continues to discover not only new viruses, but also other unique acellular entities, such as prions and fosmids, that might be candidates for the moniker “living organism.” An entirely separate classification name for them, Acytota, was proposed. Whether acellular organisms truly constitute life, the long-standing Cell Theory as well as the accepted definitions of life are under scientific challenges.