Download 4 Prokaryote Cells

EXTERNAL COMPOSITION OF A PROKARYOTIC CELL
PLASMA MEMBRANE
CELL WALL
GLYCOCALYX
CAPSULE
SLIME LAYER
FLAGELLUM
SEX PILUS
FIMBRAE
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1. PLASMA MEMBRANE: All cells (Prokaryote and Eukaryote) have a plasma
membrane; it is a baggie-like structure that holds the organelles inside of the cell.
Any substance that can rupture the plasma membrane will kill the whole organism;
therefore this structure is carefully studied. Alcohol, soaps, and other detergents
easily rupture the plasma membrane.
The plasma membrane is semipermiable; it has pores in it that allow some
substances to come and go (oxygen and water molecules), but does not allow other
things to get inside or leave. Therefore it regulates the flow of nutrients in the cell. It
allows low molecular weight (small sized) substances (such as water) to get in and
out depending on their concentration within the cell and outside of it. This is called
diffusion, and does not require the cell to expend any energy.
Inside all living cells, there is a certain amount of salt; the cytoplasm of bacteria
contains 0.9% NaCl (salt). Water follows wherever salt is. If a cell is soaked in a salt
solution (hypertonic solution), the concentration of salt outside of the cell is higher
than the inside of the cell, so the water will follow salt out, and the cell will shrink.
Conversely, if you soak a cell in pure water (hypotonic solution), there will be more
salt inside of the cell, so water will diffuse into the cell, causing the cell to explode
(osmotic shock). When a cell dies, it is called necrosis. The cell wall of bacteria is
rigid and protects the organism from osmotic shock. Normally, there is equilibrium
inside and outside of the cell (isotonic solution).
The plasma membrane is composed of a phospholipid bilayer. This means that there
are two layers of a compound consisting of phosphates and lipids (fats). In the
diagram below, the blue circle is the phosphate and the black line is the chain of
lipids. The plasma membrane is bilayered, so there are two sets of these structures.
They face each other at the lipid chain. Therefore, the outer and inner sides of the
membrane are water soluble, and the area between is not water soluble. This gives the
membrane semipermiablity, which allows it to take in certain substances and keep out
other substances.
Embedded within the phospholipids bilayer are lipoproteins (LP), made of lipid (fat)
and proteins. These special proteins can transport larger molecules (like sugars)
directly into the cell, like allowing someone in through a revolving door. This is
called active transport. It requires the cell to spend some energy in the form of ATP.
The plasma membrane is also the site of enzymes for energy production in the cell.
Gram negative organisms have an inner and an outer plasma membrane, whereas
Gram positive organisms only have one plasma membrane. In Gram negative
organisms, the outer plasma membrane contains a special structure called a
lipopolysachharide (LPS), which means it is made of lipids (fats) and many sugars
(polysaccharides). The LPS embedded in the plasma membrane in bacteria is
recognized as a foreign element by our immune system (white blood cells: WBC’s).
This makes it an antigen (something our immune system sees as foreign and needs to
be destroyed). This particular antigen (the LPS unit) is referred to as an O antigen.
However, it is a weak antigen, and does not stimulate much of an immune response.
Within the LPS membrane is a toxin called Lipid A. It is toxic when it is released
from the LPS unit, but not while it is attached. This is important to keep in mind
when designing antibiotics that attack the plasma membrane of bacteria, possibly
releasing this toxin. The natural immune response can also release it. A cell wall
separates the inner plasma membrane from the outer plasma membrane in a Gram
negative bacterium.
2. CELL WALL: Surrounds the plasma membrane, gives it structure and shape. It
is more complex in Prokaryotes (bacteria) than in Eukaryotes (humans). It keeps the
organism from exploding from osmotic shock (too much water entering into the cell).
The cell wall is composed of peptidoglycan, which is a combination of peptide
(protein) and glycan (sugar). Peptidoglycan is only found in bacteria, not in any other
organism. Therefore, this structure is important to study because we can create
antibiotics that attack peptidoglycan and it will not harm the cells of the patient.
Peptidoglycan consists of a chain of two types of sugars (NAM and NAG) linked by
proteins. The sugars are arranged in this order: NAG-NAM-NAG. This creates
rigidity to help prevent osmotic lysis (rupture), and helps maintain the shape of the
cell. Mycobacterium and Mycoplasma are the only bacteria without a normal cell
wall.
Mycobacterium
 This causes TB or leprosy, depending on the species.
 The cell wall of Mycobacterium is 60% waxy.
 It is neither Gram-positive nor Gram-negative.
 It is called “Acid-fast” because it takes an acidic stain to color it.
Mycoplasma
 Mycoplasma has no cell wall.
 This makes Mycoplasma strains resistant to many kinds of common antibiotics
like penicillin, that act on bacteria cell walls.
 One species causes pneumonia.
The cell wall of a Gram positive bacterium is different than a Gram negative
bacterium, and the antibiotics to attack each type of cell wall are different. Gram
positive organisms have much more peptidoglycan than Gram negatives. The
peptidoglycan is what takes up the Crystal Violet stain. The mechanism of the Gram
stain is based on differences in the structure of the cell walls of gram-positive and
gram-negative bacteria. Crystal Violet, the primary stain, stains both gram-positive
and gram-negative cells purple because the dye enters the cytoplasm of both types of
cells. When the iodine is applied, it forms large crystals with the dye that are too
large to escape through the cell wall. The application of alcohol dehydrates the
peptidoglycan of gram-positive cells to make it more impermeable to the Crystal
Violet-iodine. The effect on gram-negative cells is quite different; alcohol dissolves
the outer membrane of the gram-negative cells and even leaves small holes in the thin
peptidoglycan layer through which the Crystal Violet-iodine complex diffuses.
Because gram-negative bacteria are colorless after the alcohol wash, the addition of
safranin turns these cells pink. Although gram-positive and gram-negative cells both
absorb safranin, the pink color of safranin is masked by the darker purple dye
previously absorbed by gram-positive cells.
Not only do Gram negative bacteria have less peptidoglycan than Gram positives,
they also have an inner and outer plasma membrane. The outer plasma membrane is
external to the cell wall, and the inner plasma membrane is internal to the cell wall.
GRAM POSITIVE CELL WALL GRAM NEGATIVE CELL WALL
No outer plasma membrane
Inner and outer plasma membrane
Thick peptidoglycan
Thin peptidoglycan
3. GLYCOCALYX: many prokaryotes secrete on-air surface a substance called the
glycocalyx (sugar coat). It is made inside the cell and secreted to the cell surface. If the
substances organized and firmly attached to the cell wall, the glycocalyx is described as a
capsule. If the substance is unorganized and loosely attached to the cell wall, the
glycocalyx is described as a slime layer.
a. CAPSULE: non-slimy protein (made of polypeptides) or sugars (polysaccharides)
covering the bacterium. It is neatly organized. Not every bacterium has a capsule.
You can do a capsule stain to see if it’s there. Its purpose is to store nutrients and also
to protect it from phagocytosis (ingestion) by our protective white blood cells which
are trying to eat it and kill it. Phagocytosis is inhibited by capsules. There are several
types of white blood cells. One type is called a monocyte. Its job is to circulate in the
blood stream, looking for bacteria and other debris to eat. Once the bacteria is
phagocytized (ingested), the WBC releases a sac of enzymes to dissolve it. A
monocyte has special receptor cells called chemoreceptors, which sense chemicals
emitted by bacteria. If a monocyte senses a chemical made by bacteria in the tissues,
it squeezes out of the blood vessel and travels in the tissues to the bacteria and
phagocytizes it there. When a monocyte leaves the vessel to enter the tissues, it is
called a macrophage. Sometimes, a macrophage is able to phagocytize a bacterium
with a capsule (for example a tuberculosis organism in the lungs). The bacterium can
then live inside the macrophage because the capsule also protects it from the
destructive enzymes. The WBC now has become an infected host to the bacteria. The
body will try to surround the infected WBC with calcium deposits to kill the WBC,
but although the WBC dies, the bacteria go on living in this calcified nodule. An xray of a TB patient will show these nodules in the lungs. Another example is
Streptococcus pneumoniae: some strains have a capsule, and so a virulent (cause
disease), and other strains do not have a capsule, so they are avirulent (do not cause
disease). A strain is a subtype or a variation of the typical organism. There can be
thousands of strains of one bacterium. Some may cause disease and some do not. The
capsule itself is an antigen, called the K antigen. It stimulates an immune response.
b. SLIME LAYER: slimy protein covering the entire bacterium. Not neatly
organized. Not every bacterium has a slime layer. The function of the slime layer is to
attach to some structure in the host. An example is the bacteria in the mouth. Oral
bacteria frequently have a slime layer. That’s why your teeth feel slimy. They break
down sugars that we eat which are left behind on our teeth. For instance, they break
down sucrose into its components: glucose and fructose. This process is called
fermentation. The products of fermentation are acidic, so they break down the
protective enamel on the tooth and cause a cavity.
4. FLAGELLUM: whip-like tail used for motility. This structure is difficult to see
under the microscope in live cells, but you can see the bacteria swimming around. To
see flagella, you need a special stain, and that kills the bacterium. It is made of a
protein called flagellin. The entire structure consists of a filament, hook, and turning
disk within a basil body. It uses ATP for energy to turn the disk, which turns the
flagella. The bacterium “decides” which way to move depending on the chemicals it
senses in the environment. This is called chemotaxis (chemo = chemical; taxis =
movement). It can also move in response to a physical stimulus. Bacteria flagella
contain a protein called an H antigen (Flagellar antigen), which our white blood cells
recognize as a bad foreign element. When a WBC comes across this antigen it
stimulates an immune response to produce antibodies against the bacteria. There is
one strain of E. coli called 0157.H7 (weird name!). The letter “O” followed by a
number indicates the type of cell wall lipopolysaccharide (LPS) and the H7 indicates
the type of flagellar antigen. This strain of E. coli is the main pathogen that you hear
about on the news. It produces intestinal bleeding, especially in babies. It is found in
cattle feces. If there is an outbreak of bleeding diarrhea that is traced to having eaten
spinach at one restaurant, the Center for Disease Control (CDC) has to find out where
that spinach came from and pull that product from the grocery store shelves because it
probably did not have the fertilizer cleaned off it properly.
Flagella come in various arrangements:
A. Peritricous: Many flagella all around the perimeter of the cell.
B. Lophotrichous: A group of flagella gathered at one end of the cell.
C. Amphitrichous: One flagellum coming out of each end of the cell.
D. Monotrichous: Only one flagellum, comes out of one end of the cell.
Flagella cause various types of motility:
A. Run: move in a straight line from point A to point B.
B. Tumble: roll around themselves like a rock tumbling down a slope.
C. Run and Tumble: Doing both movements alternately.
5. AXIAL FILAMENTS: These are special flagella found only in a type of bacteria
called a spirochete (spiral shaped). The axial filament attaches from the “head” to
the “tail end” of the bacterium. When it contracts, it allows the spirochete to move
in a motion like a corkscrew. This allows it to penetrate tissue. An example of a
spirochete is the bacterium that causes syphilis.
6. SEX PILUS: longer than flagella. Helps cells connect to each other during
conjugation.
7. FIMBRAE: hair-like structures also made of protein. In Eukaryotes, they are
called cilia. In bacteria, fimbrae allow them to attach to the host like roots of a
plant in the soil. An example is Neisseria gonorrhoeae (causes gonorrhea). This
bacterium has fimbrae that allow them to get into the urinary tract and anchor
there. This creates pus and painful urination in the patient.
INTERNAL COMPOSITION OF PROKARYOTE CELLS
1. CYTOPLASM: in prokaryotes, the cytoplasm refers to the watery substance inside of
the plasma membrane. It is made up of 80% water and contains proteins (enzymes),
carbohydrates, and lipids. It also contains the following:
A. NUCEOID: a nuclear area (prokaryotes have no nucleus). There is only one
chromosome, and the DNA is circular instead of linear. The chromosome contains the
cell’s genetic information, which carries all of the information required for the cell’s
structure and function. Since there is not much DNA, prokaryotes have no histones,
which are structures eukaryotes use to organize their DNA by wrapping around it.
B. PLASMIDS: some bacteria contain these small pieces of DNA fragments
which are separate from the chromosome. These little plasmids may carry genes for
antibiotic resistance, production of toxins, etc. Plasmids can be transferred from one
bacterium to another. In fact, plasmid DNA is used for gene manipulation and
biotechnology.
C. RIBOSOMES: these are like little factories that make proteins. All eukaryotic
and prokaryotic cells contain ribosomes. Cells that have high rates of protein synthesis
have more ribosomes than others. The cytoplasm can contain tens of thousands of these
ribosomes, which give the cytoplasm a granular appearance. Several antibiotics work by
inhibiting the protein synthesis of ribosomes, such as streptomycin, gentamicin,
erythromycin, and chloramphenicol.
D. INCLUSIONS are reserve deposits of nutrients within the cytoplasm. These
nutrients can be in the form of phosphate, glycogen, starch, and lipids.
2. ENDOSPORES: when essential nutrients are depleted, certain gram-positive bacteria
form specialized resting cells called endospores.
An example is Clostridium, which causes diseases such as gangrene, tetanus, botulism,
and food poisoning. Another example is Bacillus, some species of which cause anthrax
and food poisoning.
Only bacteria make endospores. They are highly durable, dehydrated cells with thick
walls. They are formed inside the cell membrane; when released into the environment,
they can survive extreme heat, lack of water, and exposure to toxic chemicals and
radiation.
One endospore that was estimated to be 7500 years old germinated when it was placed in
a nutrient medium. Other endospores which were found fossilized in tree resin have
germinated even after 40 million years!
The process of endospore formation within a vegetative (parent) cell is known as
sporulation. When a key nutrient becomes unavailable, the cytoplasm of the vegetative
cell dries up, the cell wall ruptures, and the endospore is released into the environment.
An endospore that is located at one end of the cell is called a terminal endospore. If it is
near the end of the cell it is called a sub terminal endospore, and if it is in the center it is
called a central endospore. These distinctions make it possible to identify the species of
bacteria being viewed. Endospores require a special stain to be visualized.
An endospore returns to its vegetative state by a process called germination. It is
triggered by a change in the environment. Water enters into the endospore, and
metabolism resumes. Only one cell comes from one endospore, therefore sporulation is
not reproduction. Endospores are important from a clinical viewpoint in the food
industry because they are resistant to processes that normally kill vegetative cells. Such
processes include heating, freezing, desiccation (drying), use of chemicals, and radiation.
Endospores can survive in boiling water for several hours or more. Endospore-forming
bacteria are a problem in the food industry because they aren't likely to survive underprocessing, and if conditions for growth occur, some species produce toxins and disease.