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What Does "Acellular" Mean?

Meg Higa
Meg Higa

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.

Plant cells were first observed through a microscope in 1665.
Plant cells were first observed through a microscope in 1665.

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.

Frequently Asked Questions

What does the term "acellular" refer to in biology?

"Acellular" in biology describes structures or entities that are not made up of cells. This can include substances like viruses, which consist of genetic material encased in a protein coat, and do not have the cellular machinery necessary for independent life processes that characterize cellular organisms.

How do acellular organisms like viruses reproduce if they are not made of cells?

Viruses, which are acellular, cannot reproduce on their own. Instead, they must infect a host cell and hijack its machinery to replicate their genetic material and produce new virus particles. This process often results in damage or destruction of the host cell.

Are there any medical applications for acellular materials?

Acellular materials have significant medical applications, particularly in tissue engineering and regenerative medicine. Acellular dermal matrices, for example, are used in wound healing and reconstructive surgery to promote tissue regeneration without the risk of immune rejection, as they lack cellular components that can trigger an immune response.

Can acellular components trigger an immune response?

While acellular components generally have a lower risk of provoking an immune response due to the absence of cellular antigens, some acellular substances, like vaccines that contain inactivated viruses or bacterial toxins, are designed to elicit an immune response to confer immunity without causing disease.

What is the difference between acellular and multicellular organisms?

Acellular organisms lack cells entirely, while multicellular organisms are composed of multiple, specialized cells that work together. Multicellular life forms, such as humans, plants, and animals, have complex structures and systems that arise from the organization and interaction of various cell types.

Do acellular entities have any role in ecosystems or the environment?

Acellular entities like viruses play crucial roles in ecosystems by influencing population dynamics and genetic diversity. They can control the population of certain species, thereby maintaining ecological balance, and can also drive evolution through horizontal gene transfer, introducing new genetic material into organisms.

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    • Plant cells were first observed through a microscope in 1665.
      By: snapgalleria
      Plant cells were first observed through a microscope in 1665.