How Prokaryotic Structure & Function Relate to Behavior & Identification

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  • 0:01 Prokaryotes
  • 1:18 Morphology
  • 2:40 Motility
  • 3:33 Reproduction
  • 4:40 Metabolic Diversity
  • 6:09 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Prokaryotes are the most numerous and most diverse assortment of living organisms on the planet, so how do identify one? In this lesson we will examine four major factors that can be used to identify specific prokaryotes.


Take a look at this rock; those dark areas are fossilized bacteria that existed roughly two billion years ago. How cool would it have been to have see these things alive? Well to do this, you could invent a time machine and travel back to the early days of Earth's history or you could just book a trip to South America. In deep-sea muds off the coast of Chile, living bacteria, almost identical to these fossils, have been discovered.

Now bacteria are members of a wide group of generally single-celled organisms without distant organelles called prokaryotes. The other members of this group being the Archaea. The oldest fossilized prokaryotes are roughly 3.5 billion years old, so about a billion years younger than the Earth itself, and many have barely changed because they are so well adapted to certain environments.

So, this means they don't have much diversity right? Wrong. Prokaryotes display the greatest amount of diversity on the planet amongst living things and as anyone who's heard of the bubonic plague can attest, each species can have a major impact. So how do we know which is which? Well let's take a look at some common ways to identify our prokaryotes.


Ok, let's start finding ways to narrow down the many species of prokaryotes to identify specific samples. Now bacteria and archaea are very similar in appearance and cellular function, so off the bat I'm going to make this easier and help us identify this specimen as a bacterium. So how do we start identifying a specific species? One great way is through morphology, or the structure of organisms.

At first, many prokaryotes kind of look similar to us but there are ways to illuminate differences. One of the most basic methods is the gram stain test, in which dye is used to detect the molecule peptidoglycan. Simple prokaryotes have simple cell walls with lots of peptidoglycan, which we call gram-positives and others have more complex cell walls with little peptidoglycan, called gram-negatives. So that is a simple way to start identifying the kind of bacteria in a sample.

Of course, we can also just look at its shape. There are three basic prokaryotic shapes found in both bacteria and archaea. They are coccus, or spherical, bacillus, or rod-shaped, and helices, or twisted. If you can identify the shape, you are on the right path to identifying your mystery bacteria.


Now generally, we can identify most prokaryotes using stain tests and morphology alone but let's be extra sure here. Another factor we can look at is motility, the ability to control directional movement. Look at this bacteria; this is E. coli, a nasty bacteria, that can be pretty harmful. See those long, hair-like fibers? Those are flagella, and they are whipped back and forth to help the bacteria cell move. Many times they are also used to help the bacteria come together and form larger colonies. E. coli here has pretty good motility since it has multiple flagella. About half of all prokaryotes are motile to some degree, mostly used to escape danger. But our sample here shows no signs of motility, so we can cross off the moto-prokaryotes from the list of possible suspects.


OK, from here, let's look at reproduction. Most prokaryotes reproduce asexually, without a sexual partner through a process called binary fission. Basically, the organism replicates its genetic material and then divides itself into two cells of the same size, essentially cloning itself unless mutations occur, allowing for new adaptations.

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