Chapters
In this article, we will discuss the classification of organisms in the Eukarya domain into the taxonomic hierarchy of kingdom, phylum, class, order, family, genus, and species. Moreover, we will also discuss the characteristic features of the kingdoms Protoctista, Fungi, Plantae, and Animalia. In the end, we will describe how viruses are classified.
Classification of Organisms
In biology, the hierarchical classification system of organisms is employed to organize and group similar organisms together. There are many taxonomic ranks in a system and the species is the lowest taxonomic rank and the domain is the highest one.
- In a genus, similar species are grouped together
- In a family, similar genera are grouped together
- In order, similar families are grouped together
- In a class, similar orders are grouped together
- In a phylum, similar classes are grouped together
- In a kingdom, similar phyla are grouped together
- In a domain, similar kingdoms are grouped together
Classification of Organisms in the Eukarya Domain
Similar to other domains, the Eukarya domain contains the taxonomic hierarchy of kingdom, phylum, class, order, family, genus, and species.
Examples
- A wolf is an example of an organism that belongs to the taxonomic groups listed below:
- Domain: Eukarya
- Kingdom: Animalia
- Phylum: Chordata
- Class: Mammalia
- Order: Carnivora
- Family: Canidae
- Genus: Canis
- Species: lupus
- Another example of an organism that belongs to the Eukarya domain is the colourful flowering plant Hibiscus rosa-Sinensis. This plant belongs to the taxonomic groups listed below:
- Domain: Eukarya
- Kingdom: Plantae
- Phylum: Angiospermae
- Class: Dicotyledonae
- Order: Malvales
- Family: Malvaceae
- Genus: Hibiscus
- Species: rosa-Sinensis
In the next section of the article, we will discuss the characteristic features of the kingdoms Protoctista, Fungi, Plantae, and Animalia.
Kingdoms in Domain Eukarya
The domain Eukarya can be divided into the following four kingdoms:
- Protoctista
- Fungi
- Plantae
- Animalia
Organisms that belong to the above four kingdoms show different characteristics and features, however, they have many things in common like:
- Their cells have membrane-bound nuclei that separate genetic material from the cytoplasm
- They have compartmentalization within their cells due to the presence of other organelles
Kingdom Protoctista
All organisms belonging to the kingdom Protoctista are eukaryotic. It involves all eukaryotic cells that are not included in the other three eukaryotic kingdoms. Organisms that belong to this kingdom exhibit a lot of diversity in every aspect of life such as life cycle, structure, feeding and trophic levels, and modes of locomotion. These organisms are either single-celled organisms or a group of similar cells.
A group of Protoctista called protozoa has cells that resemble animal cells as they do not contain cell walls. Another group of Protoctista called algae has cells that resemble plant cells because they have chloroplasts and their cell walls are composed of cellulose. An example of a protoctist includes Stentor roseli which contains flagella all over its body that assist it in feeding and movement.
Kingdom Fungi
The oldest known organism in the world is a fungus. All fungi are eukaryotic cells and the cells of fungi have the following features:
- Their cell walls are not made up of cellulose rather they are usually composed of the polysaccharides chitin and glucans
- They do not contain cilia
Energy
Fungi are heterotrophs as they have the following characteristics:
- They utilize organic compounds made by other organisms to get energy and molecules for metabolism
- They get this energy and carbon either by digesting dead or decaying matter extracellularly or from parasites on living organisms.
Reproduction
They reproduce through spores that disperse on the nearby ground.
Body structure
They have a simple body structure and have the following features:
- They can be unicellular and some of them have long threads known as hyphae that grow from the main fungus body (mycelium)
- Larger fungi contain fruiting bodies that release numerous spores
Example
Rhizopus nigricans is an example of a fungus that is present on bread.
Kingdom Plantae
Plants are multicellular eukaryotic organisms, and their cells possess the following characteristics:
- All plant cells have cell walls made up of cellulose
- They have large, usually permanent vacuoles that offer structural support to the cell
- These cells can differentiate into specialized cells to create tissues and organs
- They contain chloroplasts that make photosynthesis possible (remember that not every plant cell contains chloroplasts)
- Sometimes, they also have an additional structure known as flagella
Autotrophs
They are autotrophs which imply that they can synthesize their organic compounds and molecules to use them as energy and build biomass from inorganic compounds.
Body structure
They have complicated body structures as they contain branching systems above and below the ground.
Example
The example includes Bristlecone pines found in the USA, which are estimated to be 3000 years old.
Kingdom Animalia
Like plants, animals are also multicellular eukaryotic organisms and their cells possess the characteristics listed below:
- They can differentiate into several different specialized cell types that can form organs and tissues
- They contain small temporary vacuoles, for instance, lysosomes
- They sometimes contain cilia
Heterotrophs
They are considered heterotrophs because they possess several feeding mechanisms.
Body structure
They possess diverse body forms and the communication inside their complicated body structure occurs through the nervous system and chemical signaling.
Example
The example includes blue whales which are the largest living animal species.
In the next section of the article, we will discuss the classification of viruses.
What are Viruses?
Viruses refer to microorganisms that can only be observed under an electron microscope. Viruses possess no cellular structure, and they attack the host cells by hijacking their DNA replication machinery. They obtain the energy required for replication through reparation in the host cell organisms, hence they do not belong to any of the three-domain classification systems.
Scientists are still struggling to reach a consensus as to whether these organisms should be classified as living or non0living because they are unable to carry out defining characteristics of life outside the host cells.
Classification of Viruses by their Genetic Material
Viruses can be classified based on the following two characteristics:
- Kind of nucleic acid, i.e. RNA or DNA their genome is composed of
- Either they are single-stranded or double-stranded
In cellular organisms such as animals and plants, DNA is always double-stranded, whereas RNA is always single-stranded. However, DNA and RNA in viruses can be either single-stranded or double-stranded. Consequently, the following four groups of viruses exist
- As a result, there are four groups of viruses that exist:
- DNA single-stranded viruses
- RNA single-stranded viruses (the example of this type of genome includes SARS-CoV-2 which was responsible for the COVID-19 pandemic)
- DNA double-stranded viruses
- RNA double-stranded viruses
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