Functional Anatomy of the Prokaryotic Cell

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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.

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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.

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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.

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• 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.

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• 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.
• Dont 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
  cant 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.

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• 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.

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• 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).

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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.

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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.)

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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. Thats 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.

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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

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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.

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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

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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.

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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.

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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

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• 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.

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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 its cell wall.
  Interestingly, archaea are more closely related
  to eukaryotes than to prokaryotes.

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• 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.

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• 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.

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• A Prokaryotic Species is a population of cells
  with similar characteristics (notice, they dont
  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.
• Lets 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.

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• 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.