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Prokaryotic structure cell
The Structure within Cytoplasm
The Nucleoid
c) Plasmid
d) Ribosomes
e) Endospore
A typical bacterium usually consists of:
a cytoplasmic membrane surrounded by a peptidoglycan
cell wall and maybe an outer membrane;
a fluid cytoplasm containing a nuclear region (nucleoid)
and numerous ribosomes; and
often various external structures such as a glycocalyx,
flagella, and pili.
In bacteria, the cytoplasm refers to everything enclosed
by the cytoplasmic membrane. About 80% of the
cytoplasm of bacteria is composed of water.
Within the cytoplasm can be found nucleic acids (DNA
and RNA), enzymes and amino acids, carbohydrates, lipids,
inorganic ions, and many low molecular weight
The liquid component of the cytoplasm is called
the cytosol.
The Nucleiod
The bacterial genome is
composed of chromosomal
deoxyribonucleic acid or
DNA and represents the
bacterium's nucleoid.
the bacterial nucleoid has no
nuclear membrane or
the bacterial nucleoid does
not divide by mitosis
In general it is thought that
during DNA replication, each
strand of the replicating
bacterial DNA attaches to
proteins at what will become
the cell division plane.
The nucleoid is one long, single molecule of
double stranded, helical, supercoiled DNA
The chromosome is generally around 1000 µm
long and frequently contains as many as 3500
E. coli, a bacterium that is 2-3 µm in length, has a
chromosome approximately 1400 µm long.
Electron Micrograph of Nucleiod DNA
Function of nucleiod?
The nucleoid is the genetic material of the
bacterium. Genes located along the DNA are
transcribed into RNA that, in the case of mRNA, is
then translated into protein at the ribosomes.
In other words, DNA determines what proteins
and enzymes an organism can synthesize and,
therefore, what chemical reactions it is able
to carry out.
Plasmid- Small molecules of autonomously replicating,
circular, extrachromosomal DNA found in many bacteria.
F: They are transferable genetic elements that can be
transferred from one organism to another
- through a process called conjugation, the conjugation pilus
enables the bacterium to transfer a copy of the Rplasmids(g-ve) to other bacteria, making them also
multiple antibiotic resistant and able to produce a
conjugation pilus.
Ribosomes are composed of ribosomal RNA (rRNA)
and protein.
Composed of two subunits with densities of 50S and 30S.
("S" refers to a unit of density called the Svedberg unit.)
The two subunits combine during protein synthesis to
form a complete 70S ribosome about 25nm in diameter.
A typical bacterium may have as many as 15,000
Ribosomes function as
a workbench for protein
synthesis, that is, they receive
and translate genetic
instructions for the formation
of specific proteins. During
protein synthesis, mRNA
attaches to the 30s subunit and
amino acid-carrying transfer
RNAs (tRNA) attach to the 50s
subunit (see Fig. 1). Protein
synthesis is discussed in detail in
Microbial Genes Chapter.
Endospores are dormant
alternate life forms produced by
the genus Bacillus, the
genus Clostridium, and several other
genera of bacteria
including Desulfotomaculum,
Sporosarcina, Sporolactobacillus,
Oscillospira, and Thermoactinomyces.
Bacillus species are obligate
aerobes that live
in soil while Clostridium species
are obligate anaerobes often
found as normal flora of the
gastrointestinal tract in
Under conditions of starvation, especially the lack of
carbon and nitrogen sources, a single endospores form
within some of the bacteria. The process is called
The completed endospore consists of multiple layers of
resistant coats (including a cortex, a spore coat, and
sometimes an exosporium) surrounding a nucleoid,
some ribosomes, RNA molecules, and enzymes.
Endospores are quite resistant to high temperatures
(including boiling), most disinfectants, low energy
radiation, drying, etc.
The endospore can survive possibly thousands of years
until a variety of environmental stimuli
trigger germination, allowing outgrowth of a single
vegetative bacterium
Organelles Used in Bacterial
There are three major groups of photosynthetic bacteria:
cyanobacteria, purple bacteria, and green bacteria.
The cyanobacteria carry out oxygenic
photosynthesis, that is, they use water as an electron
donor and generate oxygen during photosynthesis.The
photosynthetic system is located in an extensive
thylakoid membrane system that is lined with
particles called phycobilisomes.
Photograph of the cyanobacteria Anabaena.
Photograph of the cyanobacteria Oscillatoria.
Photograph of the cyanobacteria Anabaena.
Photograph of the
cyanobacteria Oscillatoria.
Cyanobacteria, as well as algae and green plants, use hydrogen
atoms from water to reduce carbon dioxide to form carbohydrates,
and during this process oxygen gas is given off (an oxygenic
process). Cyanobacteria were the first organisms on earth to carry
out oxygenic photosynthesis.
The green bacteria carry out anoxygenic
photosynthesis. They use reduced molecules such as H2,
H2S, S, and organic molecules as an electron source and
generate NADH and NADPH. The photosynthetic system
is located in ellipoidal vesicles called chlorosomes that
are independent of the cytoplasmic membrane.
The purple bacteria carry out anoxygenic
photosynthesis. They use reduced molecules such as H2,
H2S, S, and organic molecules as an electron source and
generate NADH and NADPH. The photosynthetic system
is located in spherical or lamellar membrane
systems that are continuous with the cytoplasmic
Structure Outside The Cell Wall
b) Flagella
c) Pili
a) TheGlycocalyx (Capsules and
Slime Layers
All bacteria secrete some sort of glycocalyx (an
outer viscous covering of fibers extending from the
The possession of a glycocalyx on bacteria is
associated with the ability of the bacteria to
establish an infection.
Can assume several forms.
If in a condensed form that is relatively tightly associated
with the underlying cell wall, the glycocalyx is referred to
as a capsule.
A more loosely attached glycocalyx that can be removed
from the cell more easily is referred to as a slime layer.
Capsule stain of Streptococcus
2 important functions of Glycocalyx
The glycocalyx enables certain bacteria to resist
phagocytic engulfment by white blood cells in the
body or protozoans in soil and water.
The glycocalyx also enables some bacteria to adhere to
environmental surfaces (rocks, root hairs, teeth,
etc.), colonize, and resist flushing.
2) Flagella
Outside cell wall
Made of chains of flagellin
Attached to the protein hook
Anchored to the wall and membrane by the basal body
•The filament of the bacterial flagellum is
connected to a hook which, in turn, is
attached to a rod.
•The basal body of the flagellum consists
of a rod and a series of rings that anchor
the flagellum to the cell wall and the
cytoplasmic membrane.
•In gram-negative bacteria, the L ring
lipopolysaccharide layer of the outer
•while the P ring anchors the flagellum to
the peptidoglycan portion of the cell wall.
•The MS ring is located in the cytoplasmic
membrane and the C ring in the
cytoplasm. The Mot proteins surround the
MS and C rings of the motor and function
to generate torque for rotation of the
Arrangement of Bacterial Flagella
1. monotrichous: a single flagellum, usually at one pole
2. amphitrichous: a single flagellum at both ends of the
3. lophotrichous: two or more flagella at one or both poles
4. peritrichous: flagella over the entire surface
Flagella are the organelles of locomotion for most of
the bacteria that are capable of motility.
The bacterial flagellum can rotate both counterclockwise
and clockwise. A protein switch in the molecular motor
of the basal body controls rotation.
Clockwise rotation results in a tumbling motion
and changes the direction of bacterial movement.
On the other hand, counterclockwise rotation leads
to long, straight or curved runs without a change
in direction
3) Fimbriae and Pili
Fimbriae allow attachment
They are found in virtually all gram-negative bacteria but
not in many gram-positive bacteria.
There are two basic types of pili:
1) short attachment pili, also known as fimbriae, that
are usually quite numerous and (fig.a).
2) long conjugation pili, also called "F" or sex pili that
are very few in number (fig.b).
Function of pili
The short attachment pili or fimbriae are organelles
of adhesion allowing bacteria to colonize
environmental surfaces or cells and resist flushing.
. Because both the bacteria and the host cells have a
negative charge, pili may enable the bacteria to bind to
host cells without initially having to get close enough to
be pushed away by electrostatic repulsion. Once attached
to the host cell, the pili can depolymerize and enable
adhesions in the bacterial cell wall to make more intimate