What are Prokaryotes?
In this video were going to take a basic view of the Prokaryotic cell.
First, let's begin by looking at the Phylogenetic genetic tree or are the tree of life.
Here you will see all living organisms fall within three
Branches or what we refer to as domains.
There's the bacteria domain in blue, the Archaea domain in red and the Eukarya domain in brown.
Prokaryotes belong to the first two domains of bacteria and Archaea.
Those prokaryotes in the Archaea domain live in extreme environments and are billions of
years old.
The domain of bacteria contains the prokaryotes in which you are most familiar such as E-coli,
streptococcus, staphylococcus and more.
And that will be the focus of this video.
So what are prokaryotes?
Prokaryotes are ubiquitous meaning that they're going to be found everywhere, they're going
to be found in large numbers and even in those places in which other organisms can't live.
Prokaryotes are tiny, on average there about 10 times smaller than an eukaryotic cell.
Prokaryotes are cellular in nature, and although they may seem simple in structure they're
actually quite complex.
Looking at the structure at the prokaryotic cell you willl probably see that it is missing
organelles.
There is no nucleus, or any other membrane-bound structures that you would see in an eukaryotic
type cell.
So let's look at the individual structures and their functions of the prokaryote cell.
When we do this we're going to divided into three groups, the appendages, the external
structures and the internal structures.
Let's start with the appendages there are two main appendages in the prokaryotic cell.
First is the flagella.
The flagella is used for motility to move the bacteria around in its environment.
The bacteria will vary in the number of flagella and the location of the flagella.
However in all bacteria the flagella are going to be protein in nature and they will move
in a circular motion, clockwise and counterclockwise in order to propel the cell.
The second appendage is what we refer to as a pili or pilus for singular.
There are two main types of pili.
The one that is shown on this bacterial structure is the fimbriae.
In that case these protein structures are numerous around the cell and they're going
to be used to help the cell adhere to other cells or to structures in the environments.
There's also a conjugation pill in which those bacterium will only form a few conjugation
pili to connect to bacteria, and where those bacteria can then exchange a specific piece
of DNA referred to as a plasmid.
Let's look a little bit on the external structures.
The external structures, we will start with the outermost which is the glycocalyx, and
work toward the innermost structure of the plasma or cell membrane.
The glycocalyx is also referred to as the sugar coat because it is comprised of polysaccharides.
The glycocalyx can be used for many things within a bacteria; it can help prevent phagocytosis,
it can help the bacteria to maintain or to adhere to nutrients, to prevent it from drying
out; it can help with many things that help it with survival or with virulence.
What its exact functions will be, will be dependent upon the type the structure.
There are two looks or forms for the glycocalyx.
In this particular picture the glycocalyx is firmly attached to the underlying cell
and has a defined shape.
And therefore we refer to it as a capsule.
However in some bacteria the glycocalyx does not have a distinct shape and it is loosely
attached to the underlining bacterium and we refer to that as a slime layer.
In some bacteria is there is a fourth layer, not shown here on this model, but it would
fall in between the red and the yellow ,and it's called an outer membrane.
The outer membrane is found exclusively in gram-negative type bacteria.
It is a phospholipid bilayer that has unique structures for the gram-negative bacteria.
The next structure is the cell wall.
A majority of bacteria have the cell wall, as shown here in yellow.
The cell wall helps with two main features for the bacterium.
It helps the bacteria maintain its shape and it helps prevent the bacterium from lysing
when it should get into an environment in which it is unfamiliar.
The cell wall is also a defining characteristic for bacterium, meaning we can use it clinically
based upon is structure and the composition to help identify the bacteria.
We'll talk a little bit more about the typical bacterial cell wall in the next slide.
Let's move on then to the plasma or the cell membrane shown here in green.
This is a traditional cell membrane meaning that is going to be a phospholipid bilayer
with embedded proteins, etc. and it is going to regulate what enters and leaves the cell.
Now back to the cell wall for just a moment.
This is generally an area in which students have difficulty understanding the composition.
Therefore there is a separate video describing bacterial cell walls, and I will refer you
to the video.
However there is, for most bacteria, going to be a wall composed of what is referred
to as peptidoglycan, and the thickness of the peptidoglycan helps us to identify two
main groups a bacteria: gram-positive and gram-negative.
Gram-positive bacteria will have multiple layers of peptidoglycan sheets and so therefore
their cell wall will be very thick.
Gram-negative bacteria have only one or two layers of peptidoglycan and therefore they
will have a thin cell wall.
So let's move to the internal structures, again notice there are no organelles or membrane-bound
structures here.
But we do have several structures that we need to talk about.
The first being the largest one that you see here and that is the actual DNA which is going
to be circular in nature and it is going to be compressed, and it is going to be found
in a general area within the cell.
And that area or location is referred to as the nucleoid.
There are also other pieces of DNA, they're going to be small circular piece in DNA only
containing a very few number of genes, 10 or less, is the general rule.
Those genes are going to be non-essential for
growth or survival of the cell.
Instead they will be used to help with virulence, often times they are associated with antibiotic
resistance, the production of a toxin and etc.
And this this is the structure in which that conjugation pilus will then use to transfer
from one bacterial cell to the other.
Bacterium do you have ribosomes, just like eukaryotic cells, and the function of the
ribosome is the same, for protein synthesis.
The only difference between prokaryotic and eukaryotic ribosomes are their sizes.
The bacterial ribosome has 2 subunits, one is (excuse me) 30 S and
50 S and the overall unit is seventy (70).
Bacteria cells also have something referred to as inclusions.
Inclusions are just going to be areas in which certain nutrients or products are stored or
supplied within the cell.
And not shown in this picture, but a couple of species a bacteria, in particular the genus
of Bacillus and Clostridium, will form endospores.
And endospores are survival methods for certain bacteria when they get into environments in
which all of a sudden it makes it difficult to live.
So we've been through all the bacterial structures and their functions, now we need to, as a
recap, go through and define which of those structures are found on all bacteria, and
which one you will see on most or a few.
To begin with all bacterial cells will have the following structures.
All are going to have a cell or plasma membrane, all bacteria must have cytoplasm and ribosomes.
Bacteria are going to have the cytoskeletal structures and DNA.
One thing to point out about the DNA is that although it is circular and coiled, it does
not use histones, such as eukaryotic cells do.
And again to point out no bacterial cell will contain organelles or those membrane-bound
structures.
Most bacteria will possess a cell wall, and the majority of those that you will see will
have a cell wall composed of peptidoglycan.
Again there is a separate video talking about the cell wall structure giving specifics and
I will refer you to that video.
Most bacteria will also have the sugar coat or the glycocalyx, as either a capsule or
as a slime layer .
Some bacteria, but not all , will have the flagella or the pill.
The gram negative only will have the outer membrane.
Some bacteria will have plasmids or inclusions and the Clostridium and Bacillus Genus will
have endospores.
So with that, let's talk just a moment about bacterial shapes.
There are three basic bacterial shapes, the bacillus,
meaning that it's rod shade; coccus, meaning that it is round or sphere, and a spirillum
shape, which means that the bacteria is going to have a twist or be spiral in shape.
For the bacillus, the rod shape, it's often going to be found to be either single or double
or it can be in a chain which we refer to a streptobacillus.
The cocci or coccus, which is the round one, will have large number of arrangements.
It too can be either single; it can have a double, refer to as the diplococci; it can
even be in a cluster, referred to as staphylococcus; or as a chain in streptococcus.
Sometimes it will be found in a cube, sarcinae, or even just as 4 as a tetra.
With this we will conclude our video on prokaryotic cells.