In biological classification, there are three fundamental groups that are key to understanding the diversity and function of life on earth: The first three domains are Plantae, Animalia and Viruses. Plantae is a higher plant kingdom containing chlorophyll cells that are responsible for oxygen and gross equilibrium. Animalia contains those heterotrophic organisms that have many organ systems necessary for interdependence within the environment and those significant in human life.
Viruses can neither be classified under the definition of an organism, but are heavily implicated in disease mechanisms, genetics, and evolution. A review of the previous classifications will be useful to comprehend the laws of biology, the interaction of living organisms with their environment, and its influence on human health. A general overview of these classifications, their implication in scientific study and the organisation to disclose their characteristics, changes, and processes in later sections of this article is given.
Kingdom Plantae comprises all multicellular, photosynthetic, eukaryotic organisms, also known as plants. These organisms are characterised by having chlorophyll in chloroplasts and cellulose cell walls through which photosynthesis can be utilised as a source of energy using sunlight. These organisms in this kingdom keep the environment in balance by the excretion of oxygen and serve as the source of most food chains. Kingdom Plantae encompasses a diverse range of forms from the unicellular algae to the higher flowering plants and is commonly grouped into divisions like algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
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Members of the Kingdom Plantae are living things that are present on both land and produce their food with sunlight, water, and carbon dioxide by the process of photosynthesis. They have fixed positions and grow all their life. They possess various parts, such as roots, stems, and leaves, that aid them in surviving in their environment. The basic characteristics are discussed below:
Plants in the Kingdom Plantae are classified into several major groups based on their evolutionary relationships and distinguishing characteristics. All of them are discussed below in the table:
Group | Characteristics | Reproduction |
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Bryophytes |
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Pteridophytes |
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Gymnosperms |
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Angiosperms |
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Kingdom Animalia includes all multicelled, eukaryotic heterotrophic organisms, thus relying on other organisms for nutrition. Animals lack cell walls, thus are not rigid and have the capacity to develop special tissues and organs. Animals are capable of movement at some stage in their life and have complex digestion, reproduction, and nervous control systems. Animals are sexually reproductive and have an enormous variety of body structures from simple sponges to complex mammals. Animals consist of millions of species and are crucial in ecosystems as consumers, pollinators, decomposers, etc.
The basic characteristics of the Kingdom Animalia are discussed below:
Heterotrophic nutrition: Organisms in the Kingdom Animalia are heterotrophic. This means that they get their food from other organic materials. They depend on the absorption of food rather than manufacturing food from inorganic compounds through the process of photosynthesis.
Cell structure: Animal cells do not possess cell walls, which makes them versatile in their shape and size of the shape. It has highly developed cells grouped into tissues, organs and organ systems performing given activities like digestion, circulation, etc or coordinating nervous systems.
Reproduction: Most animals are known to reproduce both sexually and asexually. Sexual reproduction entails the formation of new organisms by the combination of gametes. These are sperm and egg cells, and the practice occurs due to specially adapted reproductive organs. Some species exhibit the process of asexual reproduction through budding, fragmentation or asexual reproduction as in the case of parthenogenesis.
Development stages: Animal embryos go through embryonic development, and they go through a series of developmental stages, including fertilisation, cleavage, gastrulation and organogenesis. This occurrence results in the creation of specialised tissues, organs, and body structures in the developing organism.
Animalia is divided into two categories based on their structure. These are :
Phylum | Group | Key Characteristics | Examples |
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Porifera | Invertebrates | Aquatic, porous body, no true tissues or organs, filter feeders | Sponges |
Cnidaria | Invertebrates | Radial symmetry, stinging cells (cnidocytes), single digestive opening | Jellyfish, Coral, Sea anemone |
Ctenophora | Invertebrates | Radial symmetry, comb jellies, use cilia for movement, and bioluminescent | Comb jellies |
Platyhelminthes | Invertebrates | Flatworms, soft-bodied, have bilateral symmetry, simple organ systems | Planaria, Tapeworms |
Rotifera | Invertebrates | Microscopic, wheel-like ciliated structures near the mouth, mostly freshwater | Rotifers |
Nematoda | Invertebrates | Roundworms, unsegmented, complete digestive tract (mouth and anus), pseudocoelomate | Ascaris, Hookworms |
Annelida | Invertebrates | Segmented worms with true coelom, well-developed organ systems | Earthworm, Leech |
Arthropoda | Invertebrates | Segmented body, jointed limbs, exoskeleton made of chitin; the largest phylum | Insects, Crabs, Spiders |
Mollusca | Invertebrates | Soft-bodied, often with a calcium carbonate shell, a muscular foot and mantle | Snails, Octopus, Clams |
Echinodermata | Invertebrates | Marine, pentaradial symmetry (adults), water vascular system, endoskeleton | Starfish, Sea urchins, Sea cucumbers |
Hemichordata | Invertebrates | Marine, worm-like with a notochord-like structure, considered a link to chordates | Acorn worms |
Chordata | Vertebrates & Some Invertebrates | Presence of notochord, dorsal nerve cord, pharyngeal slits at some life stage | Fish, Amphibians, Reptiles, Birds, Mammals, Tunicates, Lancelets |
Viruses are a micro infectious agent that fall somewhere between living and nonliving objects. It is unable to grow, reproduce, or perform metabolic processes by itself and needs to find a living host cell to replicate itself. Viruses consist of genetic material, either DNA or RNA with in a protein coating, and occasionally an outer lipid coat. They infect a variety of organisms, ranging from animals, plants, fungi, to even bacteria (bacteriophages). Although best known to cause sicknesses such as flu, COVID-19, and HIV, viruses are also employed in scientific research, vaccines, and gene therapy.
Viruses are small infectious particles that are not cell-borne and cannot grow or replicate on their own. They must infect the body of a living host, i.e., a human, animal, plant, or even a bacterium, to replicate. Viruses do not survive outside a host. Viruses consist of genetic material, either DNA or RNA, covered by a protein coat. Viruses are responsible for causing numerous diseases, but are also employed in genetic studies and vaccine production. Some of the common characteristics are discussed in detail below:
The classification of viruses is done based on the genetic material they carry. These are:
DNA Viruses: These viruses have their genetic material in the form of deoxyribonucleic acid, or DNA for short. Some of the examples of DNA viruses are herpesviruses, adenoviruses and poxviruses. DNA viruses replicate their genomes with the help of enzymes of the host cell and can become integrated with the host genome (e.g., retroviruses).
RNA Viruses: Some viruses contain RNA as their genetic material. These types of viruses are called RNA viruses. They are also subdivided into positive-sense RNA viruses-which are easily translated into proteins, negative-sense RNA viruses-require RNA polymerase to synthesise mRNA and retroviruses-which use reverse transcriptase to synthesise DNA. Examples include Influenza viruses such as A and B, Hepatitis C viruses, and HIV.
There are basically three types of hosts. All of them are discussed below:
Type of Virus | Target Host | Key Characteristics | Examples |
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Bacteriophages | Bacteria | Infectious bacterial cells, composed of a protein coat and genetic material, may follow a lytic or lysogenic cycle | T4 phage, Lambda phage |
Plant Viruses | Plants | Cause plant diseases, spread by insects or contact, and hinder agriculture and crop yields | Tobacco mosaic virus (TMV), Potato virus Y (PVY) |
Animal Viruses | Animals (including humans) | Infecting animal cells may cause mild to fatal diseases, transmitted via air, contact, or vectors | Influenza virus, HIV, SARS-CoV-2 |
Viruses have a life cycle which outlines how they replicate and move within the host cells. These are discussed below in the table:
Stage | Lytic Cycle | Lysogenic Cycle |
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Attachment | A virus attaches to specific receptors on the host cell membrane. | The virus attaches to the host cell and injects genetic material. |
Entry | Viral DNA or RNA enters the host cell. | Viral DNA or RNA enters the host cell. |
Integration | Not applicable. | Viral genome integrates into host DNA as a prophage (bacteria) or a provirus (animal). |
Replication & Transcription | Viral genes are replicated and transcribed using the host's machinery. | Integrated viral DNA replicates along with host DNA without harming the host. |
Assembly | New viral components are synthesised and assembled into complete virus particles. | No new viruses are formed at this stage. |
Release | The host cell bursts (lysis), releasing new virions. | No release occurs unless the virus is induced into the lytic cycle. |
Induction | Not applicable. | Under stress or signals, the virus may enter the lytic cycle. |
Effect on Host Cell | The host cell is destroyed. | The host cell remains alive and functions normally until induction occurs. |
Other useful resources
Difference Between DNA and RNA viruses | Kingdom Fungi |
Difference Between Virus And Bacteria | Amoebiasis |
Differences between Lichens and Mycorrhizae | Five Kingdom Classification |
Members of Plantae are complex and multicellular machinery. Their cell walls are made of cellulose.
They are mostly autotrophic, and they carry out photosynthesis using the chlorophyll compound.
These include sexual and asexual reproduction and the plants go through generations called the alternation of generations.
They possess organs like roots for anchorage, stems for the transportation process and leaves for the process of photosynthesis.
The animals of the Animalia kingdom belong to this system depending on their relation to evolutions and body structure.
There are several large phyla of parasitic worms and these are porifera, cnidarians, platyhelminthes, nematoda, annelida, arthropod, mollusca, echinodermata, and chordata.
Classification is by their body plan, whether they possess tissues or organs, and reproductive strategies.
The virus is made up of genetic material either DNA or RNA and this material is protected by an outer layer known as capsid.
They do not possess a cell structure and they are unable to perform metabolic activities independently.
Viruses have to get into a host to reproduce being obligate intracellular parasites that lead to plant, animal and bacterial diseases.
Plants as much as animals belong to the category of satopod organisms, that is, organisms that synthesise their foods.
Animals: AnimaAnimalsthe same as animals they are heterotrophic and acquire nutrients through ingestion of other organisms or organic substrates.
Plants spread chlorophyll and other pigments to lay on light energy, on the other hand, animals consume food through digestion to get energy.
Some common viral diseases in humans include:
Influenza (Flu): By the flu viruses that give respiratory manifestations.
COVID-19: Induced by the SARS-CoV-2 virus, entails a multitude of respiratory and systemic manifestations.
HIV/AIDS: This is due to the human immunodeficiency virus (HIV) which impacts the immunological system.
Hepatitis: Infectious diseases which affect the liver- Hepatitis A, B, C, etc.
Herpes: Herpes simplex virus that results in cold sore, genital herpes, and other related disorders.
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