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Prokaryote Classification

Prokaryotes comprise two of the three domains of life, archaea and bacteria.

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Prokaryote Domains

What are the most numerous organisms on the planet?

Bacteria are the largest group of organisms on the planet, and all it takes is one bacterium to quickly grow, under just the right conditions, into millions and billions more. Luckily, growth of bacteria is controlled many different ways in nature, however it is important to remember that without them we could not survive.

Evolution of Prokaryotes

No doubt you’ve had a sore throat before, and you’ve probably eaten cheese or yogurt. If so, then you’ve encountered the fascinating world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny and sometimes bothersome, but they are the most numerous organisms on Earth. Without them, the world would be a very different place.

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Prokaryotes are currently placed in two domains. A domain is the highest taxon in classification, just above the kingdoms. The prokaryote domains are Bacteria and Archaea (see Figure above). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells.It’s not entirely clear how the three domains are related. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share (see Table below). Some suggest the eukaryotic cells arose from Archaea (compare the table below and the figure above).

Characteristic Bacteria Archaea Eukarya
Flagella Unique to Bacteria Unique to Archaea Unique to Eukarya
Cell Membrane Unique to Bacteria Like Bacteria and Eukarya Unique to Eukarya
Protein Synthesis Unique to Bacteria Like Eukarya Like Archaea
Introns Absent in most Present Present
Peptidoglycan (in cell wall) Present Absent in most Absent

The Prokaryotic Domains

Domain: Bacteria

Bacteria are the most diverse and abundant group of organisms on Earth. They live in almost all environments. They are found in the ocean, the soil, and the intestines of animals. They are even found in rocks deep below Earth’s surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. It’s estimated to be 5 × 1030, or five million trillion trillion. You have more bacteria in and on your body than you have body cells!

Bacteria called cyanobacteria are very important. They are bluish green in color (see Figure below) because they contain chlorophyll. They make food through photosynthesis and release oxygen into the air. These bacteria were probably responsible for adding oxygen to the air on early Earth. This changed the planet’s atmosphere. It also changed the direction of evolution. Ancient cyanobacteria also may have evolved into the chloroplasts of plant cells.

Cyanobacteria Bloom. The green streaks in this lake consist of trillions of cyanobacteria. Excessive nutrients in the water led to overgrowth of the bacteria.

Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species can be classified on the basis of various traits: structure, shape, arrangement, metabolism, motility (movement), and the structure of the cell wall.

  Domain Archaea

Scientists still know relatively little about Archaea. This is partly because they are hard to grow in the lab. Many live inside the bodies of animals, including humans. However, none are known  to cause disease.

Archaea were first discovered in extreme environments. For example, some were found in hot springs. Others were found around deep sea vents. Such Archaea are called extremophiles, or “lovers of extremes.” Figure below describes three different types of Archaean extremophiles. The places where some of them live are thought to be similar to the environment on ancient Earth. This suggests that they may have evolved very early in Earth’s history.

Extremophile Archaea. Many Archaea are specialized to live in extreme environments. Just three types are described here.

Archaea are now known to live just about everywhere on Earth. They are particularly numerous in the ocean. Archaea in plankton may be one of the most abundant types of organisms on the planet. Archaea are also thought to play important roles in the carbon and nitrogen cycles. For these reasons, Archaea are now recognized as a major aspect of life on Earth.


  • Bacteria live in virtually all environments on Earth.
  • Archaea live everywhere on Earth, including extreme environments.


Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology Biology for AP* Search: Two Domains: Bacteria and Archaea
  1. How do eubacteria and archaebacteria differ? Describe these differences.
  2. Describe the extreme environments of archaebacteria.


1. Distinguish between Gram-positive and Gram-negative bacteria, and give an example of each.

2. Summarize the evolutionary significance of cyanobacteria.

3. What are extremophiles? Name three types.

4. Compare and contrast Archaea and Bacteria.

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  1. [1]^ License: CC BY-NC 3.0

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