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Prokaryotic & Eukaryotic Cell Structure

Life is found in two very different cell types.  The first is termed "prokaryotic", meaning first or primitive nucleus.  These were the first cells to appear on earth and are found in only one type of living organism - bacteria.  

Another form of cell structure appeared more recently.  This has a much more complicated structure and is termed "eukaryotic", which means "true nucleus".  All other forms of life have this cell structure - protozoans, algae, fungi, plants, and animals.

As their names imply, one key anatomical difference between prokaryotes and eukaryotes lies in the structure of the nucleus.  


We are taught in biology classes that cells consist of a cell membrane, a nucleus (with a nuclear envelope, nucleopores, nucleolus, and string-like chromosomes), mitochondria, ER, Golgi bodies, etc.  Plant cells have cell walls and chloroplasts on top of all of these other structures.  If you talked about cilia and flagella and the spindle apparatus, you most likely heard of a "9+2 arrangement of microtubules".  So, when you think of what you have learned in biology classes about cell structure, realize that you have learned about eukaryotes.  A quick search of YouTube for eukaryotic cell structure will bring up many to choose from. Here is one that gives a nice review of eukaryotic cell structure.

Prokaryotes (the bacteria, Monerans) are far less complex.  They lack a true nucleus; instead they have a closed circle of DNA for their genome found in the cytoplasm of separated by a nuclear membrane.  They lack mitochondria and chloroplasts, instead getting their energy through reactions taking place in their cell membrane.  There is no ER, Golgi, or other membrane-bound organelle inside.  The cell membrane is unlike those of plants or animals in its composition, but not in its function.  The bacterial cell wall is based on peptidoglycan, a unique substance in nature, to which other materials are added to create Gram positive and Gram negative cell walls. Their flagella are a single microtubule instead of 9+2 and they do not have a spindle apparatus.  So, quite clearly bacterial/prokaryotic cells are unlike cells of anything else on earth. This YouTube video gives an introduction to bacteria - the prokaryotes.  Here's another one for good measure.

One other thing to consider is how complexity translates into size.  Less complex bacteria (prokaryotes) are much smaller than their eukaryotic counterparts.  Size is limited by something called the surface:volume ratio.  As a cell grows, volume increases much more rapidly than surface area.  When you consider that everything necessary for life must pass through that surface (the cell membrane), growing too large can mean not enough nutrients can enter to support the faster growing volume.  Likewise, waste has to make its way out of the cell through that same surface.  Because bacteria lack internal means for transporting materials, s:v ratio governs the size cells can grow.  On the other hand, eukaryotes, with their numerous organelles and internal membranes, can divide the labor and effectively transport nutrients and waste, thus allowing them to grow much larger in size. This YouTube podcast explains this and also gives an additional review of prokaryotes and eukaryotes.

Having a different cell structure has its curses and blessings.  Unique properties of bacterial cells allow them to cause disease - things we call toxins or virulence factors can cause reactions in our bodies that make us sick.  But also, because their cell structure is different, it is possible to find chemicals that will selectively kill bacteria without harming human cells by targeting the differences between the two.  Such antibiotics are the "magic bullets" that enable healthcare professionals to successfully treat bacterial infections.

After watching the resources above, you may be asked to take a quiz over the basics of prokaryotic and eukaryotic cells.

In the sections to follow, we will take a look at microbial anatomy - prokaryotic microbes AND eukaryotic microbes - and delve deeper into the structure of microscopic cells.