Acellular Organisms - Meaning, Types and Function
Acellular organisms are not made of cells. These are life forms that lack cellular structure. Acellular organisms cannot perform independent life processes and require a host for survival and reproduction. Examples include viruses, viroids, and prions. Understanding acellular organisms is crucial for Class 11 Biology and NEET exam preparation.
This Story also Contains
- What are Acellular Organisms?
- Discovery of Cells and Acellular Organisms (Robert Hooke)
- Difference between Cellular and Acellular Organisms
- Acytota vs Cytota: Classification of Acellular Organisms
- Viruses
- Viroids
- Prions
- Importance of Acellular Organisms in Biology
- Acellular Organisms NEET MCQs
All the basic activities that happen inside the smallest living part, called the cell. The discovery of the cell was made by Robert Hooke. Life on Earth depends on both living and non-living things. Non-living things are made of tiny particles called atoms, while living things are made of cells. In biology, viruses and vaccines are called acellular organisms because they are not made of cells and are not fully considered living. These acellular organisms lack organelles and cannot perform independent processes.
What are Acellular Organisms?
Acellular organisms lack a cellular structure. The typical living organisms include 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. Hence, they rely on host cells for reproduction and metabolic activities.
Discovery of Cells and Acellular Organisms (Robert Hooke)
The cell is the basic unit of life. The term acellular means not made of cells. Acellular organisms cannot perform cell division except under specific conditions. It characterises entities like viruses, viroids, and prions, which do not have a cellular structure. These acellular organisms rely on host organisms for survival and reproduction.
Difference between Cellular and Acellular Organisms
Living organisms can be cellular (made of one or more cells) or acellular (not made of cells). The table below shows the differences between Cellular and Acellular Organisms based on structure, function, and examples:
| Cellular Organisms | Acellular Organisms |
|---|---|
Living organisms are made up of one or more cells. | The organisms that are not made up of cells. |
It has a cell structure and contains organelles like cell membranes, cytoplasm, mitochondria, etc. | Do not have any type of cell organelles; they only contain protein and genetic material for viruses. |
Consider the living organisms category | Generally not considered in the category of fully living creatures. |
Perform metabolic processes independently | Cannot perform independent metabolic processes. |
Can reproduce on their own. | Required for a host for replication, for example, a virus. |
Can easily survive independently in different environments and conditions. | Cannot survive on their own; they need a host to stay active. |
Cells have compounds like proteins and enzymes. | No cells are present, and compounds are also absent. |
Can easily perform simple to highly complex processes. | Cannot perform complex functions without entering a host cell. |
Acytota vs Cytota: Classification of Acellular Organisms
Acytota and Cytota are the two fundamental biological classifications of acellular organisms.
Acytota is made up of acellular organisms such as viruses, viroids, and prions. These acellular organisms have no cellular structure and must use a host organism for their survival.
Cytota includes all forms of cellular life. This group range from prokaryotes to eukaryotes, having different cellular structures and thus having independent metabolic functions.
This classification helps us clearly distinguish cellular life from acellular life. It shows how acellular organisms are unique compared to normal living cellular organisms.
Viruses
Viruses are tiny, infectious and acellular organisms. These organisms can reproduce only inside a living cell. They are made of genetic material (DNA or RNA) and a protein coat. They lack cellular structure. Below are a 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 acellular organisms (viruses): 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 acellular organisms. These agents are made of only a short strand of RNA. They infect plants and do not have a protein coat like viruses. The 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).
Viroids do not code for proteins. But it still disrupts 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 acellular organisms. They are infectious protein particles that contain no genetic material (no DNA or RNA).
These cause diseases by changing the shape of normal proteins in the brain.
Also, 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.
Importance of Acellular Organisms in Biology
The acellular organisms, such as viruses, viroids, and prions, play a significant role despite their simple structure. There are several important aspects of acellular organisms in biology, biotechnology and medicine -
They help scientists understand the borderline between living and non-living systems.
Viruses are widely used in genetic engineering and biotechnology, for example, in gene therapy and vaccine development.
They also serve as tools in molecular biology to study DNA replication, transcription, and protein synthesis.
They have medical importance since many viral infections (like influenza, HIV, and COVID-19) influence human health.
Acellular Organisms NEET MCQs
Q1. Viruses form _______ around their nucleic acid.
Envelope
Capsid
Capsule
Cell membrane
Correct answer: 2) Capsid
Explanation:
Viruses encase their nucleic acid in a capsid. A protein shell called the capsid shields the virus's genetic material, which can be either DNA or RNA. It is composed of capsomeres, which are smaller protein components. During infection, the capsid helps the virus's attachment to host cells.
Hence, the correct answer is option 2) Capsid.
Q2. Viroids differ from viruses in having:
DNA molecules without a protein coat.
RNA molecules with a protein coat.
RNA molecules without a protein coat.
DNA molecules with a protein coat.
Correct answer: 3) RNA molecules without a protein coat
Explanation:
Since viroids are RNA molecules without a protein covering, they differ from viruses.
Viroids are tiny and uncomplicated. They lack the protective protein covering found in viruses and are composed of a single circular RNA strand. They only infect plants, and by disrupting their regular functions, they can lead to illnesses.
Hence, the correct answer is option 3) RNA molecules without a protein coat.
Q3. The pathogenicity of a virus is due to its _____ while host specificity is due to its _______. Choose the correct option to fill in the blanks.
Nucleic acid, protein coat, respectively
Mesosome, external envelope, respectively
Glycoprotein, lipoprotein layer, respectively
Protein coat, genome, respectively
Correct answer: 1) Nucleic acid, protein coat, respectively
Explanation:
The pathogenicity of a virus is determined by its genome, which consists of nucleic acid (either DNA or RNA), while its host specificity is dictated by the viral protein coat or capsid. The protein coat contains specific receptors that enable the virus to recognise and attach to host cells, determining which organisms or tissues the virus can infect.
Hence, the correct answer is option 1) Nucleic acid and protein coat, respectively.
Frequently Asked Questions (FAQs)
Viroids are RNA strands without protein coat. Prions are infectious proteins without DNA or RNA.
Viruses lack cells and organelles. They only have genetic material and a protein coat.
Cellular organisms have cells and can live alone. Acellular organisms lack cells and need a host.
These are the organisms which cannot be divided into discrete cells or it may be a something like an organic material but not consists of individual cells.
