Acellular Organisms

The cell is the basic unit of structural organisation in all living organisms. The living world also contains certain non-cellular or acellular components that lack a discrete cell. Here, let’s see the meaning of acellular organisms with examples.

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Acellular – Meaning

The cell is the basic unit of structural organisation in all living organisms. It has a set of units that is vital to permit its own growth and reproduction from simple nutrients. Different biologists defined cells in different ways. But all of them excluded this one component called ‘viruses’. Viruses are acellular organisms that lack cellular structures.

What are Acellular Organisms?

Acellular tissues or organisms are those that are not made up of discrete cells or cannot split into cells, such as the hyphae of certain fungi.

It refers to something that is organic and lacks cells. These particles, in fact, lack virtually most of the fundamental components that characterise life. Prions, viruses and certain biological products like vaccines are examples of acellular particles. Most descriptive definitions of life have assumed that an organism must be formed of one or more cells; however, this is no longer regarded as required in contemporary criteria.

Acellular does not imply unicellular. A biological creature made up of a single cell is referred to as unicellular. Bacteria, archaea and protozoa are examples of single-celled (unicellular) organisms. Whereas acellular refers to ‘non-cellular’ structures.

Acytota and Cytota

There are many DNA and RNA-containing entities that reproduce within and propagate across cells throughout all kingdoms of life, despite lacking their own cells. These entities are the only ones with distinct genetic identities that are not represented in any classic tree of life. Often, the taxonomic term “Acytota” is used to denote these unique life forms. It is a taxonomic domain for all non-cellular life forms. But Cytota denotes cellular life forms. Both Cytota and Acytota together form the Biota.

Viruses

Viruses are obligate intracellular parasites. Their reproduction is solely dependent on the host cell’s synthetic machinery. Thus they are neither considered a cell nor a complete organism. Viruses are significantly smaller than bacteria and are seen with the help of an electron microscope. These submicroscopic biological entities lack a proper cellular organisation. But possess their own genetic material. They are either RNA or DNA components but never both.

Virus – Structure and Components

They contain an infectious virus particle known as a virion. It is composed of an RNA/DNA core that is wrapped by a protective coat of protein termed capsid. The capsid has numerous capsomeres that have a few structural units made up of polypeptide chains. These capsomeres play a vital role in determining the shape of a virion.

Viral structure

See more: Difference between Virus and Bacteria

Viroids

In 1971, Diener used the term viroid to designate a novel family of subviral agents distinguished by the absence of a virion or extracellular dormant phase. It has a genome that is substantially smaller than that of viruses.

Here, the infective agent is a protein-free pathogen. They have no capsid and exist like naked RNA molecules. They replicate autonomously despite the fact that they do not code for any protein. It was first discovered in the potato spindle tuber disease. Viroids are known to trigger certain plant diseases as well as some animal and human illnesses.

Prions

Another unusual virus-like agent is the prion. It is a rod-shaped proteinous particle that causes a number of diseases like the Scrapie disease (affects the central nervous system of sheep and goats) and Scrapie-like Creutzfeldt-Jakob disease (affects the nervous system of humans). Prions are contagious proteinaceous particles. They are small particles without detectable nucleic acid and are resistant to heat, UV radiation, nucleases, and proteases. It has been proposed that they can also cause certain other chronic neurological degenerative illnesses in humans.

Keep exploring BYJU’S Biology to learn more such exciting topics.

Also Check:

Difference between Viroid and Prions

Difference between Virus and Virion

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Frequently Asked Questions

Q1

What are virusoids?

They are single-stranded circular RNA that rely on viruses to replicate and encapsidate. Virusoids are viroids that have been encased in a helper virus coat protein. They are therefore similar to viroids in their replication method and absence of genes, but they vary in the fact that viroids lack a protein coat.
Q2

What is a virion?

The virion is the extracellular infectious viral particle. It is a simple viral particle that may infect a host cell. It is made up of a protein and genophore (a kind of nucleic acid). Furthermore, the arrangement of capsomeres in the capsid (protein coat) determines the shape of a virion.
Q3

What is the difference between acellular and unicellular?

A biological creature made up of a single cell is referred to as unicellular. Bacteria, archaea and protozoa are examples of single-celled (unicellular) organisms. Whereas acellular entities are non-cellulars and do not have even a single cell. Prions and viruses are examples of acellular entities.
Q4

Are viruses cellular creatures?

Viruses do not easily fit the parameters of a true cell. They lack a plasma membrane and metabolic machinery for protein and energy production. But, like any other cellular organisms, they have a genetically determined macromolecular organisation. They have RNA/DNA and can auto replicate. Moreover, they can reproduce only inside the host cells (plant, bacteria or animal cells). Thus it is difficult to classify viruses as cellular entities.
Q5

What are acellular vaccines?

The two main types of vaccines are – acellular vaccines and whole cell vaccines. Acellular vaccines contain cellular components but not whole cells. They specifically have antigenic or allergenic cell components. Example – Pertactin is an outer membrane protein (OMP) antigen present in all virulent strains of Bordetella pertussis. It is included in acellular pertussis vaccines.
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