Simplifying Microbiology One Post Every Saturday + Helpful Glossary

#4- Who are these Prokaryotes and Eukaryotes?

You are about to understand the Tree of Life but in microbiology

Good work on understanding my first 3 posts! 🙌🎉 I do try my very best to convey such complex topics in the simplest way possible, which takes a lot of time but it's worth it.

Some of you guys may have noticed that I sprinkled the terms ‘prokaryotes’ and ‘eukaryotes’ in my first 2 posts but with little explanation. Do not fret! I will be explaining them right away in this post, starting with the tree of life.

What’s the Tree of Life?

To be frank, I have never learnt about it. We just simply jumped into studying bacteria, viruses, diseases and all that science-y stuff at school and university. Sometimes, I hear this term used in books but never looked into it further, until now. Guess this is a learning opportunity for me here, so here goes nothing 😄

Life on this planet started with a simple group of single-celled, or unicellular organisms, known as prokaryotes. This category is now split into two distinct groups— Bacteria and Archaea. They were both considered as just ‘Bacteria’, but due to some analysis in their DNA and biological functions, the scientists decided to put them into two separate domains. They most likely diverged way before we showed up, therefore the differences.

With even more further analysis, researchers found out that the first eukaryote probably came out when an archaeal cell swallowed an ancient bacterium and BAM! A third split occurred, eventually creating different forms of life: plants, birds, fishes, insects, animals, microscopic animals, and us.

In conclusion, these major branches or domains known as archaea, bacteria, and eukaryotes, shaped the entire world as it is and formed the tree of life.

This tickles your fancy and now you want to know more? Check out this cool website!

Figure 1- The tree of life, based on the sequencing and comparison of nucleotides from ribosomal RNA (rRNA) from several species. It shows that both Archaea and Bacteria sprung from a common ancestor cell before evolving further into distinct entities. Here, eukaryotes emerged from Archaea. Source- The Molecular Biology of the Cell, by Alberts et al., 6th Edn.

This post shall only look into the structural and organelle differences between these large groups of living beings.

Now let's go check out some prokaryotes!

Prokaryotes: The Living Predecessors

First things first, prokaryotes are usually unicellular organisms without a well-defined nucleus, meaning, it does not have its genome enclosed in a nuclear membrane (See Post #2). Its namesake is derived from Greek — pro is “before” + karyon is “nut, kernel”, where the ‘nut/kernel’ refers to the nucleus.

As mentioned earlier, it contains bacteria and archaea. These classes of organisms can survive the craziest of environments where humans wouldn’t dare to set foot! This can range from arctic cold, acid waste, nearby volcanoes, deepwater hydrothermal vents and so on. While others choose the simple life, like, staying in soil and being part of the human body, and other not-so-crazy places.

Figure 1- A transmission electron micrograph (TEM) of Deinococcus radiodurans, a polyextremophile seen as a tetrad. Source- By Credit: TEM of D. radiodurans acquired in the laboratory of Michael Daly, Uniformed Services University, Bethesda, MD, USA. http://www.usuhs.mil/pat/deinococcus/index_20.htm — Copy at en:Image:Deinococcus.jpg, uploaded by en:user: Statkit1, taken from www.ornl.gov/ORNLReview/v34 The Oak Ridge National Laboratory (Higher version, current from: http://genome.gsc.riken.go.jp/hgmis/graphics/slides/images/YGG-00-0076_web.jpg), Public Domain, https://commons.wikimedia.org/w/index.php?curid=157172

That’s one tough cookie!: Deinococcus radiodurans (See Fig. 1), a bacterium that has earned its spot in The Guiness Book of World Records due to its great ability to survive in the most extreme environments, which are, cold, dehydration, vacuum, acid, and serious levels of ionisation radiation.

Now let’s see the structure of a regular prokaryote.

Figure 2- A typical structure of a prokaryote. Source- This vector image is completely made by Ali Zifan — Own work; used information from Biology 10e Textbook (chapter 4, Pg: 63) by: Peter Raven, Kenneth Mason, Jonathan Losos, Susan Singer · McGraw-Hill Education., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44194140

After being familiar with the structure and organelles in a eukaryotic cell in my second post, you will see that a prokaryote doesn't have much in it (See Fig. 2). It mainly lacks membraned organelles, such as mitochondria, Golgi bodies, ER, etc. The most noticeable feature is the absence of a true nucleus to contain its genome, but instead, it's floating in a designated region in the cytoplasm.

Time to see this in a bit of detail, starting from the outside and working our way in:

• Pilus (plural: pili)- Fine, hair-like appendages on the surface of most bacteria and archaea
• Flagellum (plural: flagella)- Long tail-like structures that aid in movement
• Capsule- Outermost firm shell made of complex proteins and carbohydrates seen in some prokaryotes
• Cell wall- A covering that offers protection and shape to most bacteria
• Plasma membrane- Phospholipid bilayer responsible for regulating the movement of molecules in and out of the cell
• Cytoplasm- Cellular jelly, mainly composed of water
• Ribosomes- Complex structures that take part in protein synthesis
• Nucleoid- A region in the cytoplasm where the prokaryote’s single DNA molecule sits

Extra Fact: The outermost layer of some types of prokaryotes is called the glycocalyx and it comes in 2 forms — the firm capsule (as mentioned above), and a loose slime layer.

So now you know the basic features of a prokaryote, especially the nucleoid which gave this massive group of diverse organisms a common name.

Some additional points to remember:

• Smaller in size
• Have a single chromosome which is usually circular
• RNA and protein synthesis happen simultaneously (aka coupled)
• Ribosomes have 2 subunits- 50S and 30S to give a whole 70S ribosome (not a typo!)
• Not many cellular structures
• Flagella made of flagellin protein
• Undergo simple cell division (binary fission)

Next stop, eukaryotes!

Eukaryotes: Nucleus Show-Offs

You guys have already seen the structure of a classy eukaryote, but if some of you are new here or have forgotten (it’s ok, I forget, too), here is the link to the second post to see the characteristics of both animal and plant cells.

Figure 3- 3D Anatomy of a eukaryotic cell showing all the membraned organelles. Source- Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002–4436

The term eukaryote is derived from Greek as well — eu is “well, good” + karyon is “nut, kernel”. It is pretty obvious since they all have a well-defined nucleus complete with 2 membranes, and various proteins working and maintaining it.

Other than that, eukaryotes comprise a vast multitude of organisms from fungi, slime moulds, aquatic animals, reptiles, birds, mammals, amphibians, shellfish, tardigrades, more microscopic animals, insects, etc. In other words, eukaryotes either come as unicellular or multicellular.

Another interesting tidbit is the presence of two organelles that possess their own genomic DNA — mitochondria and chloroplast (dedicated post coming soon!).

More points on these guys:

• Cells are definitely larger
• Have multiple chromosomes (not circular) + histones
• RNA and protein synthesis in nucleus and cytoplasm, respectively
• Their larger 80S ribosomes have 60S and 40S subunits (not a typo again!)
• Well structured and has a cytoskeleton
• Cilia and flagella made of microtubules; cell crawling with actin filaments
• Can undergo mitosis and meiosis

Summary

Figure 4- Summarising the typical structure of a eukaryote (left) and prokaryote (right). Source- By Science Primer (National Center for Biotechnology Information). Vectorized by Mortadelo2005. — This file has been extracted from another file: Celltypes.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2145991

To summarise, eukaryotes are a bit more complex due to the number of organelles and the various forms they can occur in when compared to prokaryotes (See Fig. 4). But, keep in mind, one group is not superior to the other, they all have amazing features, abilities, and various important roles to play.

Once again, thank you for making it through this very detailed post! Now you know a little about the origin of life and some things on them which is great on your part, even if you understood only some of it 🙌

Do send a comment over, give me a follow and subscribe to get more simplified microbiology every Saturday! 😄👍

Glossary

Unicellular- Means single-celled; plural: multicellular

Bacterium- Singular version for Bacteria

Transmission electron micrograph (TEM)- Image obtained from a transmission electron microscope

Polyextremophile- An organism capable of living in many extreme environments

Tetrad- A cluster of four

Tardigrades- Multicellular microscopic organisms that can be found in lichen, moss to extreme environments like arctic tundra, deserts, etc.; known as water bears or moss piglets

Sources

• Part I: Introduction to the cell, Chapter 1: Cells and Genomes. From the textbook Molecular Biology of the Cell, by Alberts et al., 6th Edn.
• Me remembering my school and university lecture material