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1. Structure of prokaryotic cell
Prepared by: Dhiman Dutta

2. Prokaryotic cell
 Prokaryotes are unicellular organisms
that lack organelles or other internal
membrane-bound structures.
 Therefore, they do not have a nucleus,
but, instead, generally have a single
chromosome: a piece of circular,
double-stranded DNA located in an
area of the cell called the nucleoid.
 Most prokaryotes have a cell wall
outside the plasma membrane.

3. Structure of a prokaryotic cell

4. A typical prokaryotic cell

5. Features of prokaryotic cell
 Cell wall is present in most of the
prokaryotic cells.The composition of the
cell wall differs significantly between the
domains Bacteria and Archaea, the two
domains of life into which prokaryotes
are divided.
 Some bacteria have a capsule outside
the cell wall.
 Some species also have flagella used for
locomotion and pili used for attachment
to surfaces.
 Plasmids, which consist of extra-
chromosomal DNA, are also present in
many species of bacteria and archaea.

6. Capsule
 Many prokaryotes have a sticky
outermost layer called the capsule,
which is usually made of
polysaccharides (sugar polymers).
 The capsule helps prokaryotes cling to
each other and to various surfaces in
their environment, and also helps
prevent the cell from drying out.
 In the case of disease-causing
prokaryotes that have colonized the body
of a host organism, the capsule or slime
layer may also protect against the host’s

7. Cell wall in prokaryotes
 Most of the prokaryotic cells have a stiff cell
wall, located underneath the capsule (if there
is one).
 For most prokaryotic cells, the cell wall is
critical to cell survival, yet there are some that
do not have cell walls. Eg. Mycoplasma
 Cell wall maintains the cell’s shape, protects
the cell interior, and prevents the cell from
bursting when it takes up water.
 The chemical composition of the cell walls
varies between archaea and bacteria.
 It also varies between bacterial species.

8. Prokaryotic cell wall
 The cell wall of most bacteria
contains peptidoglycan, a polymer of linked
sugars and polypeptides.
 Peptidoglycan is unusual in that it contains not
only L-amino acids, the type normally used to
make proteins, but also D-amino acids ("mirror
images" of the L-amino acids).
 Archaeal cell walls don't contain peptidoglycan,
but some include a similar molecule called
pseudopeptidoglycan, while others are
composed of proteins or other types of polymers.
 Based on Gram staining technique bacteria are
grouped into 2 categories: Gram positive and
Gram negative

9. Gram positive bacteria cell
wall
 The primary component of bacterial cell
walls is peptidoglycan.
 Peptidoglycan is a macromolecule
composed of sugars and amino
acids that are assembled structurally like
woven material.
 The amino sugar component consists of
alternating molecules of N-
acetylglucosamine (NAG) and N-
acetylmuramic acid (NAM).
 These molecules are crosslinked
together by short peptides which help
give peptidoglycan strength and
structure.

10. Gram negative bacteria cell
wall
 Like Gram positive bacteria, the Gram
negative bacterial cell wall is composed of
peptidoglycan. However, the peptidoglycan is a
single thin layer compared to the thick layers in
Gram positive cells.
 The cell wall structure of Gram negative bacteria
is more complex than that of Gram positive
bacteria.
 Located between the plasma membrane and the
thin peptidoglycan layer is a gel-like matrix called
periplasmic space.
 Unlike in Gram positive bacteria, Gram negative
bacteria have an outer membrane layer that is
external to the peptidoglycan cell wall.
Membrane proteins, murein lipoproteins, attach
the outer membrane to the cell wall.

12. Cell membrane in prokaryotes
 The plasma membrane, also called the cytoplasmic membrane, is the
most dynamic structure of a prokaryotic cell. Its main function is as
a selective permeability barrier that regulates the passage of substances
into and out of the cell.
 Bacterial membranes are composed of 40 percent phospholipid and 60
percent protein. The phospholipids are amphiphilic molecules with a polar
hydrophilic glycerol "head" attached via an ester bond to two nonpolar
hydrophobic fatty acid tails, which naturally form a bilayer in aqueous
environments.
 Dispersed within the bilayer are various structural and enzymatic proteins
which carry out most membrane functions.
 Some membrane proteins are located and function on one side or another of
the membrane, most proteins are partly inserted into the membrane, or
possibly even traverse the membrane as channels from the outside to the
inside. It is possible that proteins can move laterally along a surface of the
membrane, but it is thermodynamically unlikely that proteins can be rotated
within a membrane.
 This arrangement of proteins and lipids to form a membrane is called
the fluid mosaic model.
 The membranes of bacteria are structurally similar to the cell
membranes of eukaryotes, except that bacterial membranes consist of

13. Cell membrane in prokaryotes

14. Mesosomes
 Mesosomes are the invaginated structures formed by
the localized infoldings of the plasma membrane. The
invaginated structures comprise of vesicles, tubules of
lamellar whorls.
 Generally mesosomes are found in association with
nuclear area or near the site of cell division. They are
absent in eukaryotes.
 Mesosomes are supposed to take part in respiration but
they are not analogous to mitochondria because they
lack outer membrane. Respiratory enzymes have been
found to be present in cell membrane.
 Mesosomes might play a role in reproduction also.
During binary fission a cross wall is formed resulting in
formation of two cells. Mesosomes begin the formation
of septum and attach bacterial DNA to the cell
membrane. It separates the bacterial DNA into each
daughter cell.
 In addition, the infoldings of mesosomes increase the
surface area of plasma membrane that in turn increases

15. Mesosome diagram

16. Cytoplasm in prokaryotic
cell
 Cytoplasm is the internal "soup" of the
prokaryotes cell. It is bounded on the outside
by the cell membrane. The cytoplasm is
mostly water, but within it are some of the
bacterial organelles and inclusions -
nucleoid, plasmids, ribosomes and storage
granules - as well as the components
necessary for bacterial metabolism.
 Cytoplasm may also contain
chromatophores. Chromatophores are
internal membrane systems present in
photosynthetic prokaryotes. These develop
as membrane lined sacs or thylakoids from
plasma membrane. Thylakoid membranes
contain photosynthetic pigments in
cyanobacteria and purple bacteria.

17. Refer to cytoplasm in bacteria
cell

18. Nucleoid
 DNA in the bacterial cell is generally confined to the
central region known as nucleoid. Though it isn't
bounded by a membrane, it is visibly distinct (by
transmission microscopy) from the rest of the cell
interior.
 The bacterial genome is present in the cell within the
nucleoid.
 The nucleoid contains the double stranded genomic
DNA, and molecules of RNA and proteins. The main
proteins of the nucleoid are: RNA polymerase,
topoisomerases and the histone-like proteins: HU, H-NS
(H1), H, HLP1, IHF and FIS. The DNA molecule in the
nucleoid is under helical tension or supercoiling.
 DNA supercoiling is generated by the activity of the
topoisomerases and by DNA-protein interactions.

19. Nucleoid

20. Nucleoid/Bacterial
chromosome

21. Plasmid
 A plasmid is a small, circular, double-stranded DNA
molecule that is distinct from a cell's chromosomal
DNA.
 Plasmids naturally exist in bacterial cells, and they also
occur in some eukaryotes.
 Often, the genes carried in plasmids provide bacteria
with genetic advantages, such as antibiotic resistance.
 Plasmids have a wide range of lengths, from roughly
one thousand DNA base pairs to hundreds of
thousands of base pairs.
 When a bacterium divides, all of the plasmids contained
within the cell are copied such that each daughter cell
receives a copy of each plasmid.
 Bacteria can also transfer plasmids to one another
through a process called conjugation.
 Episomes are plasmids that can integrate into the
genome.

23. Ribosomes
 Ribosomes give the cytoplasm of
bacteria a granular appearance in
electron micrographs.
 Though smaller than the ribosomes in
eukaryotic cells, these organelles
have a similar function in translating
the genetic message in messenger
RNA into the production of peptide
sequences (proteins).

24. Prokaryotic ribosome

25. Three dimensional structure of
ribosome

26. Flagella
 Bacterial flagella are long, thin (about 20 nm),
whip-like appendages that move the bacteria
towards nutrients and other attractants.
 Flagella are free at one end and attached to
the cell at the other end.
 The long filament of flagella is composed of
many subunits of a single protein, flagellin,
arranged in several intertwined chains. The
energy for movement, the proton motive
force, is provided by ATP.
 Flagella are usually found in gram-negative
bacilli. Gram-positive rods
(e.g., Listeria species) and
cocci(some Enterococcus species, Vagococc
us species) also have flagella.

27. Structure of Bacterial flagella
 Flagella are helical shaped structure
composed of subunits of a protein
called flagellin. The wider region at the base
of the flagellum is called a hook. It is different
in structure than that of the filament. Hook
connects filament to the motor portion of the
flagellum called a basal body.
 The basal body is anchored in the
cytoplasmic membrane and cell wall. There
are presence of rings that are surrounded by
a pair of proteins called Mot.
 These proteins actually drive the flagellar
motor causing rotation of the
filament. Another set of proteins called Fli
proteins function as the motor switch,
reversing the rotation of the flagella in
response to intracellular signals.

28. Diagram of Bacterial flagella

29. Diagram of Bacterial flagella

30. Pili and Fimbriae
 The pilus is a hair-like structure associated
with bacterial adhesion and related to bacterial
colonization and infection.
 Pili are primarily composed of oligomeric pilin proteins,
which arrange helically to form a cylinder. New pilin
protein molecules insert into the base of the pilus.
 Pili are not locomotive structures.
 They are classified into ordinary pilus or sex pilus
according to their morphology, distribution, and function.
 Pili function as adhesive factors for mucosal
surface attachment.
 Fimbriae are bristle like short fibres occurs on the
surface of bacteria. They are composed of fimbrillin
proteins. Pili are long hair like tubular microfibres like
structures present on the surface of
bacteria. Fimbriae are present on both Gram positive
and Gram negative bacteria. Pili are present only on
some Gram negative bacteria.

31. Diagram of pili and fimbriae

32. Difference between Fimbriae and Pili
 The fimbriae differ from the pili in the following ways:
 Pili are fine hair like microfibers having pilin – a thick tubular
structure while the fimbriae are tiny bristle-like fibers emerging
from the surface of the bacterial cells made up of fimbrillin
protein.
 Pili are longer than fimbriae
 Occurrence of the fimbriae in each cell is about 200-400 while
the occurrence of pili are lesser than one to ten every cell
 Fimbriae are found in the both the gram negative and positive
bacteria both, pili are present in the gram negative bacteria
only
 The fimbriae are composed of fimbrillin protein while the pilin
protein makes up the pili
 The fimbriae are less rigid compared to the pili
 The formation of fimbriae is administered by the bacterial
genes in the nucleoid area while the pili is administered by the
plasmid genes
 Fimbriae plays a role in attaching cells to the surface while pili
are critical in bacterial conjugation