Classification of Organisms
Organisms and Environments
Background
Organisms
Biologists have identified more than 1.5 million different
species on Earth. This is only a fraction of what
scientists believe the total number could be – anywhere
from 5 million to 100 million. Because of this abundance
and diversity, scientists organize species with similar
characteristics into groups based on their structure,
function, and relationships. This is known as taxonomy
or taxonomic classification.
The current system of classification is based on the
hierarchical system of Swedish scientist Carolus
Linnaeus. Linnaeus grouped organisms based on
common physical characteristics. However, with the
ability to sequence genes, scientists are now able to
bring classification to a whole new level.
Organisms can be classified into groups based on their cellular structure, whether they have
defined nuclei or not (eukaryotes versus prokaryotes) or if their entire body is made up of one cell
(unicellular) or many cells (multicellular). Scientists can also look at how organisms function to
help classify them. For example, organisms that make their own food are known as autotrophs,
while organisms that need to consume other organisms in order to get the nutrients they need to
survive are known as heterotrophs. How an organism reproduces is another way scientist
separate organisms into smaller groups; asexually, in which only one parent passes a copy of its
genes on to its offspring, or sexually, in which two parents combine their genes and the
combination is passed on to their offspring.
Gene sequencing played a big part in organizing the three domains. In 1990, Carl Woese used
ribosomes to prove that all organisms can be placed into one of three Domains: Bacteria,
Archaea, and Eukarya. The Bacteria and Archaea Domains consist of prokaryotes. This means
that on the cellular level they do not have defined nuclei – that is, their nuclei have no cell
membrane or wall. Many members of the Domain Archaea live in extreme environments (high
heat, salt, or acid), such has hot springs, while members of the Domain Bacteria are found all
over Earth and are what we think of when we hear the word bacteria. The Domain Eukarya
(sometimes also spelled Eucarya) consists of all the eukaryotes. They have defined nuclei with
cell membranes and are unicellular or multicellular.
Please continue to the next page.
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Classification of Organisms
Organisms and Environments
Background, continued
The three Domains are currently divided into six smaller groups
known as Kingdoms.
Domain
Bacteria
Domain
Archaea
Domain
Eukarya
• Kingdom
Eubacteria
• Kingdom
Archaebacteria
• Kingdom Protista
• Kingdom Plantae
• Kingdom Fungi
• Kingdom Animalia
Kingdom Archaea – Prokaryotes
that often live in extreme
environments.
Kingdom Bacteria – Prokaryotes
that cannot live in extreme
environments.
The Domain Bacteria has only one kingdom, the Kingdom
Eubacteria.
The Domain Archaea has only one kingdom, the Kingdom
Archaebacteria.
The Domain Eukarya consists of four kingdoms: Protista, Plantae,
Fungi, and Animalia.
Kingdom Protista – uniceullar
and simple multicellular
eukaryotic organisms
•  Kingdom Protista consists of eukaryotic organisms that are
primarily unicellular and other, simple multicellular relatives, such
as slime molds or algae.
•  Kingdom Plantae are eukaryotes that are multicellular and are
autotrophs (make their own food). This includes mosses, trees,
and flowering plants.
Kingdom Fungi – Fungi are
eukaryotic heterotrophic
multicellular
•  Kingdom Fungi are eukaryotes that are heterotrophs (get food from other sources) and get their
nutrients by decomposing organic matter, like mushrooms or molds.
•  The Kingdom Animalia consists of multicellular, heterotrophic, eukaryotes. This group includes
invertebrates (animals with no spinal chord) such as insects or worms, and vertebrates (animals
with a spinal chord) such as fish, amphibians, reptiles, birds, and mammals.
Continue to the next page.
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Classification of Organisms
Organisms and Environments
Background, continued
The first classification system had only had two kingdoms, Kingdom Plantae and Kingdom
Animalia. Kingdoms were the beginning or most basic category.
Kingdom Plantae Plants are autotrophic
eukaryotic multicellular
organisms.
Kingdom Animalia Animals are eukaryotic
heterotrophic multicelluar
organisms.
As more and more organisms were discovered and scientists were able to make more detailed
examinations of the organisms, new Kingdoms were added to the classification system. In 1969,
R. H. Whittaker used the characteristics of body structure, reproduction cycle, and cell structure
to classify all known organisms into five Kingdoms: Monera, Protista, Plantae, Fungi, Animalia.
Kingdom Monera included what is today classified as Kingdom Eubacteria and Kingdom
Archaebacteria.
Advances in technology allowed scientists to explore environments once considered too harsh to
support life. C.R. Woese discovered that prokaryotic organisms that lived in very extreme
conditions were different from other prokaryotic organisms. At first these organisms were
considered to be bacteria and were classified in the Kingdom Monera, however as the ability to
examine the genetic make up of organisms emerged, Dr. Woese determined these organisms
were not typical of bacteria and had some characteristics found in eukaryotes. He named the
organisms as Archaea and proposed the new three Domain classification system. The Kingdom
of Monera was eliminated and two new kingdoms were formed: Kingdom Eubacteria and
Kingdom Archaebacteria.
This area of study shows how science is fluid and constantly changing. With the increasing
discoveries of new organisms and the continued advancement of technology, scientists are able
to gather more and more information about living things. As history indicates, the classification
system of today may be made obsolete with the discoveries of tomorrow.
Sources:
http://www.biology.iupui.edu/biocourses/N100/2k23domain.html retrieved 11/16/2012
http://biology.about.com/od/evolution/a/aa041708a.htm retrieved 11/16/2012
Complete the Background questions in your Student Journal.
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Classification of Organisms
Organisms and Environments
Part I: In What Domain Do I Belong?
Scientists sort life forms into categories based on their characteristics. First, scientists look at the
types of cells an organism has to identify whether the organism is a prokaryote or a eukaryote.
Cells in a prokaryote:
•  Lack a nucleus
•  Lack structures with membranes
inside the cell
Cells in a eukaryote:
•  Have a nucleus
•  Have membranes that enclose complex
structures, including the nucleus
Scientists also look at whether an organism likes living in an extreme environment. An extreme
environment might mean an extremely hot place, a very acidic one, or a very alkaline one.
Considering this additional characteristic, scientists sort organisms into the three largest
categories, called Domains. The three Domains are:
Archaea – a prokaryote; many thrive in extreme environments
Bacteria – a prokaryote that cannot live in extreme environments
Eukarya – any eukaryote
Cut out the In What Domain Do I Belong cards. Paste the cards in Part I of your Student
Journal.
Complete Part I in your Student Journal.
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Classification of Organisms
Organisms and Environments
Part II: In What Kingdom Do I Belong?
Scientists sort organisms into smaller categories than Domains, called Kingdoms. The six
Kingdoms are: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia.
CELL COMPOSITION
•  Unicellular – composed of a single cell
•  Multicellular – composed of multiple cells
EATING
•  Autotrophic – makes own food
•  Heterotrophic – eats other organisms
REPRODUCTION
•  Asexual – offspring arise from a single parent
•  Sexual – offspring arise from the mating of two parents
Cut out the “In What Kingdom Do I Belong” cards. Paste the cards in Part II of your Student
Journal.
Part III: Domain and Kingdom Slide Show
1.  Search around your home, school, neighborhood, and the Internet for examples of the three
Domains and six Kingdoms.
2.  Use a digital camera to take pictures or find images from the Internet for each Domain and
Kingdom. Collect at least two examples for each domain and kingdom.
3.  Write a description of each Domain and Kingdom.
4.  Create a slide show. The slide show should include a title page and at least one page per
Domain and Kingdom with appropriate descriptions of each.
5.  Place your photographs and descriptions on the appropriate slides.
6.  Share your slide show with your class.
Complete Parts II and III and the Reflections and Conclusions questions in your Student Journal.
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