Download Functional Anatomy of the Prokaryotic Cell

Functional Anatomy of the
Prokaryotic Cell
Prokaryote means, “before the nucleus”.
Prokaryotic cells are simpler cells than
eukaryotes, but they are still able to carry
on life processes.
Basic Characteristics of
prokaryotes
• DNA in a prokaryote is not membrane bound. In
other words, there is no nucleus.
• Unlike eukaryotes, which generally have multiple
linear chromosomes, prokaryotes have one
circular chromosome.
• Prokaryotes do not have membrane-bound
organelles.
• The cell walls of prokaryotes almost always
contain structure called peptidoglycan.
• Prokaryotes divide by binary fission.
Structures External to Cell Wall
• Prokaryotes have three different types of
appendages that Prokaryotes can have.
– 1. Flagella for motility
– 2. Fimbriae for attachment
– 3. Pili for DNA transfer
• Prokaryotes use only flagella for motility.
However, keep in mind that not all
prokaryotes are motile.
• Flagella can be arranged 4 different ways in
Prokaryotes.
• 1. Monotrichous, meaning single polar flagellum.
• 2. Amphitrichous, meaning tuft of flagella at each
end of cell
• 3. Lophotrichous, meaning two or more flagella at
one pole (or end of the cell)
• 4. Peritrichous, meaning the flagella are distributed
over entire cell, much like cilia would be on a
eukaryotic cell.
– Flagella in prokaryotes move in a circular
motion like a propellar instead of the whip like
motion used by eukaryotic flagella.
• Motility in Prokaryotes consists of “runs” and
“tumbles” (See Fig. 4.5 in your textbook for an
explanation of this concept.)
– Motility enables the microbe to move towards favorable
conditions or away from unfavorable conditions.
– Don’t be fooled though. Microbes are not “thinking”
entities. They move as a result of chemical messages
that the cell receives from the environment.
• Spirochetes, a particular shape of bacteria, us a
periplasmic flagella for motility.
– It is anchored at one end of the cell and rotates to
produce propelling motion similar to a cork screw or
winding telephone cord. Imagine taking a telephone
cord and winding it until it can’t wind any more. When
you let it go, it would rapidly unwind and move in
many directions. This is similar to how the periplasmic
flagellum works.
• Fimbriae are hair-like appendages used for attachment.
There are generally many of them that surround the cell.
• Again, not all prokaryotes have fimbriae.
– Neisseria gonorrhoeae uses fimbriae to colonize mucous
membranes. It is the organism responsible for gonorrhoea.
• Pili: (sometimes fimbriae and pili used interchangeably in
some textbooks)
– Pili are usually longer than fimbriae and number only 1 or 2 per
cell.
– Pili are long hollow tubes that can attach one cell to another for a
brief period of time.
– Join bacterial cells in preparation to transfer DNA from one cell to
another (also referred to as sex pili)
– The Pilus is the actual tube through which the DNA is transferred.
• Like in eukaryotes, prokaryotes can have a
glycocalyx. Again the glycocalyx is a sticky
substance that surrounds the cells and is used for
attachment.
– For example, streptococci that coat your teeth adhere to
the teeth using the glycocalyx.
– There are two forms the glycocalyx can take.
• 1.Capsule
• Slime layer
– A capsule is a sugar coat that is thick and gummy that
surrounds the cell. A capsule is generally an indicator
of virulence in bacteria and aids in attachment and
colonization of the host.
– A slime layer is a loose shield around the bacteria that
helps prevent water and nutrient loss. Slime layers
also help form biofilms (layers of bacteria that are
impenetrable by antibiotics and other chemicals).
Cell Wall
• Almost all organisms have cell walls
– (Mycobacteria is an exception)
• The function of the cell wall in prokaryotes is to
prevent the cell from rupturing when the pressure
inside cell is greater than pressure outside cell.
• The cell wall is composed of 2 main molecules:
– N-acetylmuramic acid (NAM)
– N-acetylglucosamine (NAM)
– Also known as peptidoglycan.
Cell Wall Cont.
• Many disease causing bacteria can be grouped into one of two
categories based on their cell wall structure.
• These two groups are called gram positive (G+) and gram negative
(G-).
• G + bacteria have a very thick and rigid cell wall it is composed of a
very thick layer of peptidoglycan
– Peptidoglycan is made up of repeating units of NAG and NAM bound
together and layered on top of each other. (See fig.4.13 b)
• G- bacteria have a more complex cell wall structure. It is composed
of an outer memb. (lipid bylayer) that contains LPS
(lipopolysaccharide) on the outside of the outermembrane and
phospholipids on the inside of the outer membrane.
– Next is a thin layer of peptidoglycan that is loosely attached to the inside
of the outermembrane.
– Surrounding the peptidoglycan on the top and bottom is the periplasmic
space
– See fig. 4.14 (We will revisit this concept over and over throughout this
class so make sure you understand it.)
Structures Internal to Cell Wall
• Cell Membrane (cytoplasmic memb)
– Encloses the cytoplasm of cell. Like eukaryotes, the cytoplasm
contains a lot of water, unlike eukaryotes it does not have
microtubules or microfilaments.
– It is a typical phospholipid bilayer. Therefore, what is the charge
on this membrane?
– It also contains proteins like those discussed with eukaryotes.
Keep in mind though that the protein structure is different between
eukaryotes and prokaryotes. That’s an important concept when
talking about drug treatments for disease.
• Fluid Mosaic Model is what we use to describe the cell
membrane.
– It state that the membrane is vicous or fluid. It allows the proteins
within the membrane to move throughout the phospholipids freely.
• The function of the cell membrane is to selectively allow
materials such as nutrients and wastes to enter and exit.
– The cell membrane is also site of cellular respiration.
Structures Internal to Cell Wall
• Cytoplasm: substance w/in cell membrane.
– It is 80% water.
– It contains the nuclear area (remember there are
no membrane bound organelles inside a
prokaryotic cell).
– It contains ribosomes.
– It contains inclusions
Structures Internal to Cell Wall
• Nuclear area
– The nuclear area contains the single circular chromosome found in
prokaryotes.
– Sometimes plasmids are also found in the nuclear area.
• Plasmids are small, circular pieces of DNA that contain extra genes
that the cell can use.
• Sometimes they carry genes for antibiotic resistance or for toxins
• They are also used very frequently for genetic engineering because
they are easily manipulated and can be easily moved into a cell.
• Ribosomes
– Ribosomes in prokaryotes are free.
– Like ribosomes in eukaryotes, they are the site for protein
synthesis
– There is a difference in the structure and mass of the ribosomes in
eukaryotes and prokaryotes. This is important to know because it
helps to treat bacterial infections.
Structures Internal to Cell Wall
• Inclusions
– Storage for reserve nutrients
•
•
•
•
Polysaccharide granules: store glycogen and starch
Lipid inclusions
Sulfur granules
Carboxysomes: store enzymes for carbon fixation from carbon
dioxide
• Gas vacuoles: control buoyancy to receive sufficient oxygen,
light and nutrients
• Magnetosomes: store iron oxide, act like magnet to reach
attachment sites
Endospores
• Some bacteria, such as Bacillus and Clostridium, form
resting cells called endospores upon depletion of nutrients
in the environment.
• Endospores are essentially highly durable dehydrated cells,
similar to a plant seed. It contains all the genetic
information of the original cell and will form a living cell
again once the environmental conditions are right.
• Endospores are formed inside the cell and then released
into the environment when the cell dies.
• Upon release, endospores can survive extreme heat, lack of
water, exposure to many toxic chemicals, and radiation.
• 25 million year old endospores trapped in amber
germinated when placed in nutrient media.
Endospores
• Sporulation is the process of endospore formation.
• Endospores return to a vegetative state (living cell) by a process called
germination.
• Sporulation does not increase the number of cells but rather preserves
the genetic information of the parent cell until conditions are right for
it to grow again.
• Endospores are important to the food industry. They are can be
responsible for food contamination.
– Endospores can survive boiling water for several hours
– Endospores can germinate and produce toxins when conditions are right.
– For example, botulism is caused by an endspore forming bacteria,
Clostridium botulinum. This organism grows in environments without
oxygen. So during the canning process if endospores are introduced to the
food and correct cooking temperatures and times are not observed the
endospores can survive. Once the cans or bottles are sealed an
oxygenless, or anaerobic environment is created. The endospore can now
germinate and form toxins. The toxins are responsible for the symptoms
and illness caused in botulism.
Shape and Arrangement of
Bacterial Cells
• Bacterial cells come in all kinds of shapes and
sizes and arrangements.
• Coccus, single ball shaped cell (cocci, many ball
shaped cells)
–
–
–
–
Diplococci, two ball shaped cells connected together.
Streptococci, chains of ball shaped cells.
Tetrads, four ball shaped cells bound together.
Sarcinae, groups of eight ball shaped cells connected
together
– Staphylococci, balled shaped bacteria connected
together in what look like grape clusters
• Bacillus (bacilli) is rod shaped bacteria
–
–
–
–
Diplobacilli, two rods connected together.
Streptobacilli, chains of rods connected end to end.
Coccobacilli, short fat rods that look similar to cocci.
(Bacillus has to meanings. Bacillus, the genus name of
bacteria and bacillus, the cellular shape of a bacterium.)
• Other shapes
– Vibrio, comma shape.
– Spirilla, spirochetes and others. See figure 4.22 for
great diagrams of each shape.
Taxonomy
• A new classification system now divides all
life into three domains instead of two.
• This classification system is based on the phylogeny of
each group of organisms. Phylogeny is the evolutionary
history of a group of organisms. In other words, how are
they related?
• New Classification system consists of the 3 Domains:
– 1. Eukarya: animals, plants, fungi, protists
– 2. Bacteria: pathogenic, nonpathogenic, photoautrophic
– 3. Archaea: prokaryotic type single celled organism without
peptidoglycan in it’s cell wall. Interestingly, archaea are more
closely related to eukaryotes than to prokaryotes.
• Archaea generally live in extreme environments
– Methanogens (produce methane from carbon dioxide and
hydrogen)
– Extreme halophiles (live in high salt concentrations)
– Hyperthermophiles (hot, acidic environments)
– These are all examples of archaea.
• The current scientific theory is that the 3 domains
arose from one single organism called the
universal ancestor. (See fig. 1.15)
• Classification of each organism is important
because it provides us with valuable information,
such as cellular shape, metabolic functions,
growth characteristics, etc. about each one.
• This is a brief review of the classification system
used for all organisms. (I will never ask you to
reproduce this list but it will help you to
understand how prokaryotes are classified and
named.)
– Domain
– Kingdom
– Phylum
– Class
– Order
– Family
– Genus
– Species
• A Eukaryotic Species is a group of closely related org.
that breed among themselves.
– Dogs are a great example of this.
• A Prokaryotic Species is a population of cells with similar
characteristics (notice, they don’t breed with each other
like the eukaryotic species.)
– Staphylococcus epidermidis and Staphylococcus aureus are cells
that belong to the same genus, Staphylococcus but the species are
different. That means that they might look different from each
other when grown on a plate or they might make different enzymes
or one might cause disease more frequently than the other.
• A Prokaryotic Strain is a member of same species that is
not exactly identical to the rest of the species.
– Let’s say that our Staphylococcus aureus develops resistance to
Penicillin. It still has all of the same characteristics of Staph.
Aureus but it now has an additional characteristic that sets it apart
from the rest of the species.
• Bacterial organisms are named using both Genus
and Species names.
– Sometimes organisms are named using the name of
scientist who discovered the organism.
• For examaple, Eschericia coli discovered by a scientist who
had the last name Eschericia.
• When writing the name of an organism it is proper to capitalize
the genus name and italicize both the genus and species names.
If the names are hand written then it is proper to underline both
name.
• Now your ready to complete Homework 3.
Homework 3 is due Monday, Sept. 11 by
midnight.