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.

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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

Summary

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.

Review

What are prokaryotes?
What are two major differences between Bacteria and Archaea?

Image	Reference	Attributions
	[Figure 1]	Credit: User:Adenosine/Wikimedia Commons;Zachary Wilson Source: CK-12 Foundation License: CC BY-NC 3.0
	[Figure 2]	Credit: Zachary Wilson Source: CK-12 Foundation License: CC BY-NC 3.0