There Are Three Distinct Domains of Life

 There Are Three Distinct Domains of Life

The many diverse forms of life that have been discovered may be organised into three primary categories, which are also referred to as kingdoms or realms. Although there is a great diversity of life, it is possible to classify it all. These groups illustrate different lines of development that can be traced back to the same ancestor (see Figures 1 for further information).

History

Respected biochemists Carl Woese, Otto Kandler, and Mark Wheelis first proposed the three-domain concept in 1990. The primary objective of this innovative framework was to efficiently classify various organisms into categories according to their most fundamental characteristics.

Classification

The classification of all living organisms can be broadly grouped into three distinct domains of life.

l Eubacteria ( Eukaryotes)

l Prokaryotes

l Archaebacteria (Archaea)

1. Prokaryotes

A prokaryote is a unicellular organism lacking a nucleus and membrane-bound organelles. Bacterial cells possess distinct characteristics that differentiate them from eukaryotic cells, namely the absence of a nucleus and other organelles. The word "prokaryote" comes from the Greek words for "before" and "nucleus," respectively. Thus, it is used to define creatures composed of cells but without a nucleus. The emergence of nuclei in eukaryotes represents a relatively recent evolutionary event, suggesting that prokaryotes likely predated eukaryotes for a significant duration.

Examples of Prokaryotes

Prokaryotes are a group of living things that include blue-green algae, bacteria, and mycoplasma. Cyanobacteria are prokaryotic creatures that can make food through photosynthesis. Because there is only one barrier around a bacterial cell, all of the cell's activities must take place inside the cytoplasm. These creatures can either live on their own or use other organisms as hosts.

Groups of prokaryotes

Prokaryotes can be broadly classified into two distinct categories based on their biochemistry.

i. Archaebacteria

(Greek arche- , “origin”)

A collection of single-celled organisms referred to as archaea. The absence of nuclei in the cells of these microorganisms leads us to categorise them as prokaryotes.

Habitats  of Archaebacteria

The biochemistry of archaebacteria is not as well studied as that of other bacteria since most of them dwell in extreme environments such as salt lakes, hot springs, extremely acidic bogs, and the depths of the ocean.

ii. Eubacteria

(again, from Greek eu, “true”)

Eubacterial cells have a single circular chromosome that encases their DNA. Eubacteria organisms are unicellular, prokaryotic microorganisms that do not have a nucleus.

Habitats of Eubacteria

Eubacteria can be found in diverse environments, including soil, surface water, and the tissues of both living and deceased organisms. Researchers have primarily focused their attention on Eubacteria, a subgroup that includes Escherichia coli. 

Figure 1: The tree of life consists of three fundamental branches. A diagram resembling a "family tree" is frequently employed to illustrate phylogenetic relationships. If the number of evolutionary divergences between two organisms is reduced, it indicates a higher degree of genetic relatedness between them.

2. Archaebacteria (Archaea)

Based on the existing body of evidence, it can be inferred that the divergence between archaebacteria and eubacteria occurred at an early stage in the evolutionary process, resulting in the formation of distinct domains known as Archaea and Bacteria.

Eukaryotic organisms, constituting the Eukarya domain, are derived from a common lineage that also gave rise to the Archaea. Consequently, archaebacteria exhibit a closer evolutionary relationship to eukaryotes than to eubacteria.

Habitats of Archaebacteria

Archaea and bacteria are classified into groupings based on their environmental preferences.

Aerobic Habitat

Organisms inhabiting environments characterised by aerobic conditions and abundant oxygen obtain their energy through the mechanism of electron transfer from fuel molecules to oxygen. This process occurs within an environment characterised by aerobic conditions.

Anaerobic Habitat

Certain microorganisms have the ability to survive in environments devoid of oxygen, known as anaerobic conditions. These microorganisms obtain energy by engaging in electron transfer processes, wherein electrons are transferred to compounds such as nitrate, resulting in the formation of N2, sulphate, leading to the production of H2S, or carbon monoxide, resulting in the formation of CH4. Many species exhibit an inability to survive in the presence of oxygen due to their evolutionary adaptation to anaerobic environments.

Classification of Organism According to Energy  

The sources of energy and carbon required for the synthesis of cellular materials are presented in Figures 2.

When considering strength, it is possible to make distinctions between:

i. Phototrophs

Phototrophs, derived from the Greek term trophe- meaning "nourishment," Exploiting and utilising solar energy.

The phototrophic organisms can be further categorised into subgroups.

l Autotrophs

Autotrophs, which are capable of acquiring all necessary carbon from carbon dioxide (CO2).

l Heterotrophs

The term "heterotroph" refers to organisms because they get their nourishment from organic sources.

ii. Chemotrophs

Chemotrophs derive their energy from the process of fuel oxidation. Chemotrophic organisms require an exogenous source of nutrients as they lack the ability to convert carbon dioxide into organic compounds.

Chemotrophs that exclusively derive their carbon atoms from CO2, known as autotrophs chemotrophs, do not exist. However, chemotrophs can be classified into distinct groups based on alternative criteria.

l Lithotrophs

The term "lithotroph" refers to the group of organisms that obtains its metabolic energy from the oxidation of inorganic substances. Unlike most creatures, which must depend on organic components for energy, lithotrophs are capable of deriving their vitality from inorganic materials including minerals, metals, and even some gases. In environments where organic matter is scarce, such hydrothermal vents in the deep sea, caverns, or severe ecosystems with high levels of acidity or sulphur, you may often find these species.

For example

Lithotrophic organisms derive energy from inorganic sources via several metabolic pathways. Chemolithotrophs employ chemosynthesis to convert hydrogen sulphide, ammonia, and iron into energy-storing molecules. "Photolithotrophs" combine inorganic compounds with sun energy to make organic molecules.

l Organotrophs

Organisms that derive their metabolic energy through the process of oxidising organic substances are referred to as organotrophs. Organotrophs, as opposed to lithotrophs, derive energy by metabolising carbon-containing compounds, such as sugars, lipids, and proteins, rather than inorganic chemicals.

Organotrophs comprise a significant proportion of microorganisms, as well as the overwhelming majority of animals and plants. The acquisition of their energy is derived from cellular respiration and fermentation, which are two biological processes responsible for the decomposition of intricate organic compounds into high-energy substances such as adenosine triphosphate (ATP).

Note

The majority of organisms engaged in photosynthesis encompass autotrophs, heterotrophs, lithotrophs, and organotrophs. In contrast, chemical oxidizers consist predominantly of lithotrophs, organotrophs, and chemical oxidizers. Prokaryotic organisms possess the capacity to obtain carbon and energy via diverse metabolic pathways. Escherichia coli can be categorised as a chemoorganoheterotroph owing to its capacity to obtain energy and carbon from organic compounds found in its immediate surroundings, serving as an illustrative example. Photolithoautotroph organisms, such as cyanobacteria, utilise solar energy to perform protein synthesis through the process of carbon dioxide fixation in the aquatic environment. Like E. coli and other chemoheterotrophic bacteria, humans are classified as belonging to the Homo sapiens species.