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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.