Through illustrations and interactives you will explore the major divisions (domains, kingdoms) of life. Before you get started, don’t forget to print out your OnTRACK Biology Journal.

TEKS Standards and Student Expectations

B(8) The student knows that taxonomy is a branching classification based on the shared characteristics of organisms and can change as new discoveries are made. The student is expected to:

B(8)(C) compare characteristics of taxonomic groups, including archaea, bacteria, protists, fungi, plants, and animals

Learning Objectives

Describe how taxonomists classify living organisms.

Describe the three-domain classification system and the six-kingdom classification system.

Describe common characteristics of organisms grouped into each of the six kingdoms.

Essential Questions

How do the three domains of life differ from each other?

How are organisms classified into kingdoms?

What is the difference between a prokaryote and a eukaryote?

What is the difference between an autotroph and a heterotroph?

Vocabulary

• Kingdom
• Archaebacteria
• Eubacteria
• Protista
• Fungi
• Plantae
• Animalia
• Domain
• Bacteria
• Archae
• Eukarya
• Prokaryote
• Eukaryote
• Autotroph
• Heterotroph

Carolus Linnaeus was the first scientist to begin to classify organisms. During Linnaeus' time, the only known differences among living things were the characteristics that separated animals from plants. As biologists learned more about living organisms, they soon realized that two kingdoms did not reflect the diversity of life. The classification system has continually changed since Linnaeus' work in the 1700's as scientists learn more and more about the diversity of life on Earth. Today all living organisms are classified into one of six kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, or Animalia. The chart below shows how the kingdoms have changed over time.

As scientists began to understand more about DNA, evolutionary biologists established a new taxonomic category—the domain. A domain is a larger and more inclusive category than a kingdom. There are three domains—Bacteria, Archaea, and Eukarya.

Directions: Watch Classification - Domains to learn more about the three domains.

Archaebacteria and eubacteria are both single-celled prokaryotes (i.e., cells without nuclei or internal organelles). Archaebacteria are known as ancient bacteria and are usually found in extreme environments like hot springs, deep sea vents, or high salt environments like the Dead Sea or Great Salt Lake. 

Eubacteria are far more common than archaebacteria and are typically found in human daily life. E. coli and salmonella are examples of eubacteria.

The National Science Foundation's Tree of Life project estimates that there could be anywhere from 5 million to 100 million species on the planet, but science has only identified about 2 million of them.

This is an incredible number of organisms, with many more yet to be discovered. Scientists want to study these organisms to better understand how they are similar or different, as well as what makes them unique. These scientists (taxonomists) group organisms according to their characteristics and then name them. Taxonomy is the branch of biology that deals with classifying and naming living organisms.

There are six major kingdoms that fall under the three domains that organisms can be grouped into. Each of these kingdoms has distinct characteristics. Classification starts with these criteria:

1. Number of Cells (Unicellular or Multicellular).
2. Type of Cells (Prokaryote or Eukaryote).
3. Method of Nutrition (Autotroph or Heterotroph).

Here is a diagram that illustrates the relationship between the domains and the kingdoms.

Watch this video to see Angela compare and contrast the characteristics of the six different kingdoms.

Here is the Spanish version of the video.