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The first living organisms on Earth were bacteria. These small organisms still exist today and are responsible for many things. In this lesson, we will explore both ancient bacteria and true bacteria.

Characteristics of Prokaryotes

Cellular structure of prokaryotes
Prokaryote Structure Diagram

While prokaryotes greatly affect our lives, we hardly ever think of these tiny living things. These organisms are responsible for things like making yogurt and cheese, but also for sometimes making us sick. Let’s look at the basic properties of prokaryotes before we look at how these organisms have evolved over time.

Prokaryotes are organisms whose cells lack a nucleus and membrane-bound organelles. They are small and unicellular, meaning they are only made of one cell and not many cells like we are. These one-celled organisms have cell walls that provide protection from the outside world. While they don’t have a nucleus, they do have genetic information that’s contained in a nucleoid – which means ‘nucleus-like’ and is a region in the cell that contains the genetic information.

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Prokaryotes have one of three distinct shapes
Prokaryote Shapes

All prokaryotes also have a simple structure in common. They can be one of three different shapes: spherical, rod-shaped or spiral. Ones that are spherical are called cocci.

You may have heard of strep throat. This is caused by a prokaryote called Streptococcus, which is partially named because of its spherical shape. Rod-shaped prokaryotes are called bacilli. Bacillus anthracis, which is the prokaryote that causes anthrax, is an example of a rod-shaped prokaryote.

Spiral prokaryotes are spirochetes, such as the prokaryote that causes Lyme disease.

Lyme disease is caused by spiral-shaped bacteria
Lyme Disease Bacteria

The last major similarity of prokaryotes we need to address is how they get food. Autotrophic prokaryotes make their own food. These organisms are either photoautotrophs, meaning they use light energy to make food via photosynthesis, or they are chemoautotrophs, meaning they use chemicals to make food via chemosynthesis.

Domain Archaea

Prokaryotes are divided into two domains: archaea and bacteria.

We’ll first look at archaebacteria. Archaebacteria were the first prokaryotes and live in extreme environments. Evolutionarily, they have some things in common with bacteria and some things with eukaryotic organisms (like us). While they are the first known living organisms on Earth, they are still around, and we continue to learn more about these amazing organisms that live in environments we generally consider to be uninhabitable. Archaea are divided into three categories based on the environments in which they live.

  • Thermophiles are heat-lovers and live in places such as deep sea thermal vents and hot springs.

    In Greek, the term ‘therm’ means ‘heat,’ such as in ‘thermometer’ and ‘thermal underwear,’ and ‘philos’ means ‘lover.’

  • The next group of archaebacteria are called halophiles and they are salt lovers. In Greek, the word ‘halo’ means ‘salt,’ and we already know that ‘philos’ means ‘lover.’
  • The last group is the methanogenes, which use carbon dioxide and hydrogen to make methane. They are found in marshes, swamps, sewage treatment facilities and even in the guts of cows.

Domain Bacteria

The second group of prokaryotes is that with which you are more familiar.

The kingdom eubacteria are true bacteria. They have countless roles, including decomposition and recycling of nutrients, digestion and disease.Eubacteria are often involved in symbiotic relationships with other organisms. These are close interactions between two different species. Examples of these symbiotic relationships include the bacteria that live in our intestines and help us get the nutrients we need as well as the bacteria that fix nitrogen from the atmosphere so that plants can use it.A negative impact that bacteria have on our lives is that they are responsible for about half of all human diseases. This is because some bacteria produce toxins that can harm other organisms.

Fortunately, we can use antibiotics to treat bacterial infections. Antibiotics work by killing bacteria. However, they must be specialized for certain types of bacteria and used properly. Because bacteria can evolve quickly, they can sometimes become immune to an antibiotic. This antibiotic resistance is why some antibiotics that were commonly used are no longer used – they are no longer effective at killing bacteria.

Examples of diseases caused by bacteria include Lyme disease, cholera and strep throat.

Lesson Summary

While both archaebacteria and eubacteria are prokaryotic, they are evolutionarily different. All prokaryotic cells are unicellular, have a cell wall and lack both a nucleus and membrane-bound organelles. Some use light energy to make food and are photoautotrophs, while others use chemicals to make food and are chemoautotrophs.The most ancient form of life on Earth belonged to the domain archaea, which contains the kingdom archaebacteria. These organisms live in extreme environments that most other life forms find uninhabitable. The three groups are thermophiles, halophiles and methanogenes.

Archaebacteria have evolutionary similarities to both eubacteria and eukaryotic organisms, such as humans.The domain bacteria contains the kingdom eubacteria and is known as true bacteria. This domain contains the bacteria that are in our digestive systems, used to make food items, such as yogurt, and sometimes cause diseases. We can use antibiotics to kill bacteria and to treat diseases caused by these prokaryotes. While prokaryotes may seem unimportant because of their size and simplicity, both archaebacteria and eubacteria play vital roles in our lives.

Learning Outcomes

After this lesson, you’ll be able to:

  • Describe the characteristics of prokaryotes
  • Distinguish between archaebacteria and eubacteria
  • List the three groups of prokaryotes in domain archaea
  • Explain the functions of prokaryotes in domain bacteria

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