The Three Domain System

The Three Domain System, developed by Carl Woese in 1990, is a system for classifying
biological organisms.

Before Woese's discovery of archaea as distinct from bacteria in 1977, scientists believed there
were only two types of life: eukarya and bacteria.

The highest ranking previously used had been "kingdom," based on the Five Kingdom system
adopted in the late 1960s. This classification system model is based on principles developed by
Swedish scientist Carolus Linnaeus, whose hierarchical system groups organisms based on
common physical characteristics.

The Current System

As scientists learn more about organisms, classification systems change. Genetic sequencing has
given researchers a whole new way of analyzing relationships between organisms.

The current Three Domain System groups organisms primarily based on differences in
ribosomal RNA (rRNA) structure. Ribosomal RNA is a molecular building block for ribosomes.

Under this system, organisms are classified into three domains and six kingdoms. The domains
are

• Archaea
• Bacteria
• Eukarya

The kingdoms are

• Archaebacteria (ancient bacteria)

• Eubacteria (true bacteria)
• Protista
• Fungi
• Plantae
• Animalia

Archaea Domain

This Archaea domain contains single-celled organisms. Archaea have genes that are similar to
both bacteria and eukaryotes. Because they are very similar to bacteria in appearance, they
were originally mistaken for bacteria.

Like bacteria, archaea are prokaryotic organisms and do not have a membrane-bound nucleus.
They also lack internal cell organelles and many are about the same size as and similar in shape
to bacteria. Archaea reproduce by binary fission, have one circular chromosome, and
use flagella to move around in their environment as do bacteria.
Archaea differ from bacteria in cell wall composition and differ from both bacteria and
eukaryotes in membrane composition and rRNA type. These differences are substantial enough
to warrant that archaea have a separate domain.

Archaea are extreme organisms that live under some of the most extreme environmental
conditions. This includes within hydrothermal vents, acidic springs, and under Arctic ice.
Archaea are divided into three main phyla: Crenarchaeota, Euryarchaeota, and Korarchaeota.

• Crenarchaeota include many organisms that are hyperthermophiles and

thermoacidophiles. These archaea thrive in environments with great temperature
extremes (hyperthermophiles) and in extremely hot and acidic environments
(thermoacidophiles.)
• Archaea known as methanogens are of the Euryarchaeota phylum. They produce
methane as a byproduct of metabolism and require an oxygen-free environment.
• Little is known about Korarchaeota archaea as few species have been found living in
places such as hot springs, hydrothermal vents, and obsidian pools.

Bacteria Domain

Bacteria are classified under the Bacteria Domain. These organisms are generally feared
because some are pathogenic and capable of causing disease.

However, bacteria are essential to life as some are part of the human microbiota. These bacteria
preform vital functions, such as enabling us to properly digest and absorb nutrients from the
foods we eat. Bacteria that live on the skin prevent pathogenic microbes from colonizing the
area and also aid in the activation of the immune system.

Bacteria are also important for the recycling of nutrients in the global ecosystem as they are
primary decomposers.

Bacteria have a unique cell wall composition and rRNA type. They are grouped into five main
categories:

• Proteobacteria: This phylum contains the largest group of bacteria and includes E.coli,
Salmonella, Heliobacter pylori, and Vibrio. bacteria.
• Cyanobacteria: These bacteria are capable of photosynthesis. They are also known as
blue-green algae because of their color.
• Firmicutes: These gram-positive bacteria include Clostridium, Bacillus, and
mycoplasmas (bacteria without cell walls.)
• Chlamydiae: These parasitic bacteria reproduce inside their host's cells. Organisms
include Chlamydia trachomatis (causes chlamydia STD) and Chlamydophila
pneumoniae (causes pneumonia.)
• Spirochetes: These corkscrew-shaped bacteria exhibit a unique twisting motion.
Examples include Borrelia burgdorferi (cause Lyme disease) and Treponema
pallidum (cause syphilis.)
Eukarya Domain

The Eukarya domain includes eukaryotes or organisms that have a membrane-bound nucleus.
This domain is further subdivided into the kingdoms

• Protista
• Fungi
• Plantae
• Animalia

Eukaryotes have rRNA that is distinct from bacteria and archaeans. Plant and fungi organisms
contain cell walls that are different in composition than bacteria. Eukaryotic cells are typically
resistant to antibacterial antibiotics.

Organisms in this domain include protists, fungi, plants, and animals. Examples
include algae, amoeba, fungi, molds, yeast, ferns, mosses, flowering plants, sponges, insects,
and mammals.

Comparison of Classification Systems

Systems for classifying organisms change with new discoveries made over time. The earliest
systems recognized only two kingdoms (plant and animal.) The current Three Domain System
is the best organizational system we have now, but as new information is gained, a different
system for classifying organisms may later be developed.

Here is how the Seven Kingdom System compares to the Three Domain System, which has six
kingdoms:

Seven Kingdom System:

• Monera- is a kingdom in biology that comprises prokaryotes, which are single-celled

organism that have no true nucleus.
• Protista- Protists are simple eukaryotic organisms that are neither plants nor animals or
fungi. Protists are unicellular in nature but can also be found as a colony of cells.
• Fungi- is any member of the group of eukaryotic organisms that includes microorganisms
such as yeasts and molds, as well as the more familiar mushrooms.
• Plantae- includes all the plants. They are eukaryotic, multicellular and autotrophic
organisms. The plant cell contains a rigid cell wall. Plants have chloroplast and
chlorophyll pigment, which is required for photosynthesis.
• Animalia- All animals are members of the Kingdom Animalia, also called Metazoa. This
Kingdom does not contain prokaryotes (Kingdom Monera, includes bacteria, blue-green
algae) or protists (Kingdom Protista, includes unicellular eukaryotic organisms).
• Monera- Monera is a kingdom in biology that comprises prokaryotes, which are single-
celled organism that have no true nucleus. Monera is divided into archaebacteria and
eubacteria.
• Chromista- Chromista is a proposed but seemingly polyphyletic biological kingdom
consisting of single-celled and multicellular eukaryotic species that share similar features
in their photosynthetic organelles (plastids).

Level Of Classification
Taxanomic classification divides species in a hierarchical system beginning with a
domain and ending with a single species.

Taxonomists classify all organisms into a hierarchy, and give them standardised names,
that are often Latin or Greek, or derived from other languages and even people's
names.These specialised groups are collectively called the classification of living things.
There are seven main levels of classification in the hierarchy. They are, from the most to
the least inclusive:

Kingdoms is the most basic classification of living things.

Currently there are five kingdoms-Animal, Plant, Fungi, Protist and Monera
(unicellular). Living things are placed into certain kingdoms based on how they obtain
their food, the types of cells that make up their body, and the number of cells they contain.

Phylum is the next level following kingdom in the classification of living things. It is an
attempt to find physical similarities among organisms within a kingdom. These physical
similarities suggest that there is a common ancestry among those organisms in a
particular phylum.

Classes are way to further divide organisms of a phylum. Organisms of a class have even
more in common than those in an entire phylum.

Order Organisms in each class are further broken down into orders. A taxonomy key is
used to determine to which order an organism belongs. A taxonomy key is a checklist of
characteristics that determines how organisms are grouped together.

Families Orders are divided into families. Organisms within a family have more in
common than with organisms in any classification level above it. Because they share so
much in common, organisms of a family are said to be related to each other.

Genus is a way to describe the generic name for an organism. The genus classification is
very specific so there are fewer organisms within each one. For this reason, there are a
lot of different genera among both animals and plants. When using taxonomy to name
an organism, the genus is used to determine the first part of its two-part name.

Species are as specific as you can get. It is the lowest and most strict level of classification
of living things. The main criteria for an organism to be placed in a particular species is
the ability to breed with other organisms of that same species. The species of an organism
determines the second part of its two-part name.
 Domain Eukarya
Did you know that humans are closely related to plants than they are to bacteria? One of the
main reasons is that both humans and plants belong to the same domain known as Eukarya.
Eukarya is a domain or a large taxonomy group that is made up of organisms that contain a
nucleus within their cells. A nucleus is a type of organelle found in a cell that contains the
genetic information of that cell. Let's take a look at the Eukarya domain as well as its
characteristics and some examples of eukaryotic organisms.

Kingdom Plantae
Kingdom Plantae includes all the plants. They are eukaryotic, multicellular and autotrophic
organisms. The plant cell contains a rigid cell wall. Plants have chloroplast and chlorophyll
pigment, which is required for photosynthesis.

Gumamela

The gumamela is a fast-growing evergreen shrub with dark

green and glossy leaves with dicot flowers. The leaves are
oval in shape, and they measure between 3 inches to 4.2
inches long. The edges of the leaves often are slightly
serrated. They arrange themselves in a spiral pattern and
alternate around a long stalk.

Domain: Eukarya
Kingdom: Plantae
Phylum :
Class: Magnoliopsida
Order: Malvales
Family: Malvaceae
Genus: Hibiscus
Species: rosa-sinensis

Ecological Importance : The gumamela flower attracts butterflies and moths and is a food
source for their larvae. Hummingbirds are also pollinators for the plant, and they are
attracted by the bright colors.
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