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7.9: Prokaryotes

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    What are the most numerous organisms on the planet?

    Bacteria. And all it takes is one to quickly grow, under just the right conditions, into millions and billions. Luckily, we know how to control bacteria when necessary. But bacteria do serve many important purposes. In fact, we could not survive without them.

    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. Prokaryotes are the simplest organisms. The first cells and organisms to evolve would be classified as prokaryotic.

    Prokaryotes are currently placed in two domains. A domain is the highest taxon, just above the kingdom. The prokaryote domains are Bacteria and Archaea (see Figure below). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells.


    It’s not clear how the three domains are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share (see Table below). How can this be explained? One hypothesis is that Eukarya arose when an Archaean cell fused with a Bacterial cell. The two cells became the nucleus and cytoplasm of a new Eukaryan cell. How well does this hypothesis fit the evidence in Table below?

    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


    • Prokaryotes include Bacteria and Archaea. An individual prokaryote consists of a single cell without a nucleus.
    • Bacteria live in virtually all environments on Earth.
    • Archaea live everywhere on Earth, including extreme environments.


    1. What are prokaryotes?
    2. What are two major differences between Bacteria and Archaea?
    Image Reference Attributions
    201412291419893311926757_93062f7e10c13d4bcb6fbd291ec96c97-201412291419895578959692.jpg [Figure 1] Credit: User:Adenosine/Wikimedia Commons;Zachary Wilson
    Source: CK-12 Foundation
    License: CC BY-NC 3.0
    201412291419893311955305_f89d0272537c062cf747a6790c711f96-201412291419895579145697.png [Figure 2] Credit: Zachary Wilson
    Source: CK-12 Foundation
    License: CC BY-NC 3.0

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