Acellular Organisms - Meaning, Types and Function

All the basic activities that happen in a living body take place inside the smallest living part, called the cell. The cell was first discovered by Robert Hooke. Life on Earth depends on both living and non-living things. When we look at what these things are made of, we find that non-living things are made of tiny particles called atoms, while living things are made of cells.

This Story also Contains

1. Acellular – Meaning
2. Viruses
3. Viroids
4. Prions
5. Difference between Cellular and Acellular

Some things, like viruses and vaccines, are called acellular because they are not made of cells and are not fully considered living. Acellular Organisms are one of the most important topics in biology.

Acellular – Meaning

The term "acellular" attributes to anything which is not made of cells and can't divide into cells. It characterises entities like viruses, viroids, and prions, which do not have a cellular structure. These acellular forms rely on host organisms for survival and reproduction.

What are Acellular Organisms?

Acellular organisms lack a cellular structure, in contrast to typical living organisms that comprise one or more cells. Examples of such organisms are viruses, viroids, and prions. These do not have their own cellular machinery, like organelles or enzymes, to maintain independent life, and hence they rely on host cells for reproduction and metabolic activities.

Acytota and Cytota

Acytota and Cytota are the two fundamental biological categories. Acytota is made up of acellular entities, such as viruses, viroids, and prions, that have no cellular structure and must use a host organism for their survival. Cytota includes all forms of cellular life, ranging from prokaryotes to eukaryotes, having distinct cellular structures and thus having independent metabolic functions. This allows us to distinguish cellular life from acellular life.

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Viruses

Viruses are tiny infectious agents that can only reproduce inside living cells. They are made of genetic material (DNA or RNA) and a protein coat but lack cellular structure. Below are few characteristics of viruses:

• Viruses are non-cellular infectious agents made of genetic material (DNA or RNA) enclosed in a protein coat called a capsid.

• They are inactive outside living cells but become active and multiply only inside host cells (obligate parasites).

• They infect plants, animals, bacteria, and even fungi.

• Example: HIV, Influenza virus, Tobacco Mosaic Virus (TMV).

• The structure may be helical, polyhedral, or complex, depending on the type of virus.

• Viruses can cause diseases like AIDS, flu, smallpox, and measles.

Viroids

Viroids are the smallest known infectious agents, made of only a short strand of RNA. They infect plants and do not have a protein coat like viruses. Following are some characteristics of viroids:

• Viroids are smaller than viruses and consist of only a short strand of circular RNA, with no protein coat.

• They infect plants, causing diseases that can affect crop production.

• Viroids are replicated inside host cells using the host's enzymes.

• First discovered by Theodor Diener in 1971.

• Example: Potato spindle tuber viroid (PSTVd).

• They do not code for proteins, but can still disrupt normal cell function.

Prions

Prions are unusual infectious particles made only of proteins, with no genetic material. They can cause serious brain-related diseases in animals and humans. The characteristic features of certain Prions are given below:

• Prions are infectious protein particles that contain no genetic material (no DNA or RNA).

• They cause diseases by changing the shape of normal proteins in the brain.

• Prions are highly resistant to heat and disinfectants.

• Example: Mad Cow Disease (BSE), Creutzfeldt–Jakob disease (CJD) in humans.

• Discovered by Stanley Prusiner, who won the Nobel Prize.

• Prion diseases affect the nervous system, leading to brain damage and death.

Difference between Cellular and Acellular

Living organisms can be cellular (made of one or more cells) or acellular (not made of cells). The table below shows how they differ based on structure, function, and examples.

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