Prokaryotic cell structure

1. Prokaryotes
• Prokaryotes are unicellular organisms that lack organelles or other internal
membrane-bound structures
• They are found in both domains – Bacteria and Archaea
• Key points :-
• Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed the
nucleoid, in which a single chromosomal, circular, double-stranded DNA molecule is
located.
• Archaeal membranes have replaced the fatty acids of bacterial membranes with
isoprene; some archaeal membranes are monolayer rather than bilayer.

2. 
• Key terms :-
• nucleoid: the irregularly-shaped region within a prokaryote cell
where the genetic material is localized
• plasmid: a circle of double-stranded DNA that is separate from
the chromosomes, which is found in bacteria and protozoa
• osmotic pressure: the hydrostatic pressure exerted by a solution
across a semipermeable membrane from a pure solvent
• The Plasma Membrane :-
• In archaeal cell membranes, isoprene (phytanyl) chains linked to
glycerol replace the fatty acids linked to glycerol in bacterial
membranes.
• Some archaeal membranes are lipid monolayers instead of
bilayers.

3. 

4. 
• Archaeal phospholipids differ from those found in Bacteria and
Eukarya in two ways.
• First, they have branched phytanyl(isoperene) sidechains
instead of linear ones.
• Second, an ether bond instead of an ester bond connects the
lipid to the glycerol.
• The compound ester possesses a carbon-carbonyl-oxygen bond,
while the ether compound has a carbon-oxygen-carbon bond.

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6. 
• The P.M. also provide specific site at which the single DNA
remain attached. It is the point from where DNA replication
start.

7. P.M. Intrusion
• Infolding of the p.m. of all gram + and gram – bacteria give rise
two main types of structures :
• Mesosomes ( chondriods) :- an organelle of bacteria that
appears as an invagination of the plasma membrane and
functions either in DNA replication and cell division or excretion
of exoenzymes
• They are seen in Nitrosomonas(chemoautotrophic) and
Rhodopseudomonas (photosynthetic pigment) bacteria.

8. 
• Mesosome are also involved in cross-wall (septum)
formation during the division of cell.
• Chromatophores :-
• these are photosynthetic pigment-bearing membranous
structure of photosynthetic bacteria.
• they vary in form as vesicles, tubes, bundled tubes, stacks
or thylakoid ( as in cynobacteria)
• Rhodospirillum rubrum

9. 
• Cell wall :-
• The cytoplasm of prokaryotic cells has a high concentration of
dissolved solutes. Therefore, the osmotic pressure within the
cell is relatively high. The cell wall is a protective layer that
surrounds some cells and gives them shape and rigidity.
• The cell wall is absent in Mycoplasma
• Bacterial cell walls contain peptidoglycan composed of
polysaccharide chains that are cross-linked by unusual peptides
containing both L- and D-amino acids, including D-glutamic acid
and D-alanine.
• Proteins normally have only L-amino acids; as a consequence,
many of our antibiotics work by mimicking D-amino acids and,
therefore, have specific effects on bacterial cell wall
development.

10. 
• Peptidoglycan layer contain carbohydrates and proteins
• Bacteria are divided into two major groups: gram-positive and
gram-negative, based on their reaction to gram staining.
• Note that all gram-positive bacteria belong to one phylum
• bacteria in the other phyla (Proteobacteria, Chlamydias,
Spirochetes, Cyanobacteria, and others) are gram-negative.
• Gram-positive bacteria can be divided into two major
subdivisions:
• the phylum Actinobacteria, also described as the high-G+C
gram-positives.
• the phylum Firmicutes, also known as the low-G+C gram-
positives, a group that includes such well-known genera
as Bacillus and Clostridium.

11. 
• Gram-positive bacteria typically have a cell wall consisting of a
single thick layer of peptidoglycan.
• the most thoroughly studied gram-positive bacteria include
human pathogens (e.g., Mycobacterium tuberculosis, Bacillus
anthracis)
• Gram postive cell wall consist of : peptidoglycan and teichoic acid
• Peptidoglycan is complex of oligosaccharide and short peptide
• Oligosaccharide are N-acetylglucosamine & N-acetylmuramic acid
• Teichoic acid are coploymer of glycerol phosphate or ribitol
phosphate and carbohydrate linked via phosphodiester bond
• They are covalently linked to N-acetylmuramic acid or a terminal
D-alanine in the tetrapeptide
• The main function of teichoic acids is to provide rigidity to the cell-
wall by attracting cations such as magnesium and sodium.

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13. 
• Teichoic acids may be covalently linked to lipids in the plasma
membrane to form lipoteichoic acids.
• Lipoteichoic acids anchor the cell wall to the cell membrane.

14. 
• Gram negative :- Escherichia coli
• Cell wall consist of three layers Peptidoglycan, Periplasmic space,
Outer membrane
• Peptidoglycan layer is thin lacks teichoic acid and lies next to
P.M.
• The periplasmic space lies outside the peptidoglycan layer and
contains proteins secreted by cell
• The outer layer consists of lipid bilayer and contains
lipopolysaccharide
• It is permeable and also contain proteins called porin which lines
channels large enough to let many chemicals pass

15. 
• Peptidoglycan are chemically similar in all prokaryotes and
differ only in the types of AA present in peptide portion of
molecule

16. 
characteristics Gram positive Gram negative Blue green algae
Layers 1 3 3
peptidoglycan + (thick) + (thin) + (thin)
Teichoic acid + - -
lipopolysaccharide - + +
Gram stain + - +

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18. 
• Capsule :- in some bacteria, the cell wall is surrounded by an
additional slime or gel layer called capsule.
• It is thick, gummy, mucilaginous and is secreted by the P.M.
• The capsule serves mainly as a protective layer against attack by
phagocytes and by viruses
• It also help regulating the concentration and uptake of essential
ions and water.
• Capsule (also known as K antigen) is a major virulence factor of
bacteria, e.g. all of the principal pathogens which cause
pneumonia and meningitis, including Streptococcus
pneumoniae, Haemophilus influenzae, Neisseria meningitidis,
Klebsiella pneumoniae, Escherichia coli, and group B
streptococci have polysaccharide capsules on their surface

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20. Forms of Bacteria
• Cocci:- these bacteria are spherical or round in shape
• E.g. Diplococcus pneumonia(diplococci), streptococus
pyrogenes(bead like chain, cause sore throat)

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• Bacilli :- rod like bacteria
• Bacillus cause certain disease of man like
• Tuberculosis – mycobacterium or bacillus tuberculosis
• Tetanus – clostridium tetani, Typhoid – salmonella typhous
• Diptheria – corynebacterium diptheriae,
• Leprosy – mycobacterium leprae
• Dysentery or food poisoning – closteridium botylinum
• Anthrax – bacillus anthracis

22. 
• Spirilla :- these are also called spirochetes
• these are spiral shaped and motile bacteria
• Spirilla cause human disease such as syphilis(treponema
pallidium)
• Vibrio :- these are comma-shaped or bent-rod like bacteria
• Cause cholera – vibrio cholerae

23. Flagella
• Flagella for cellular locomotion
• Consist of single type of protein - flagellin

24. 
• The flagellum is attached at its base, by a short flexible hook
that is rotated like a propeller of ship, By the flagellar rotatory
motor
• The flagellar motor comprises four distinct parts :-
• The Rotor(M ring), Stator, Bearing ( S ring ) and Rod
• The Rotor is a protein disc integrated into the P.M.
• Rotor is driven by energy stored in the trans-membrane proton
H+ gradient & rotate rapidly in the lipid bilayer against another
protein disc called Stator
• A rod links the rotor to a hook & flagellum, thereby causing
them to rotate
• The protein Bearing serves as a seal the outer membrane of the
cell wall as the rotating rod passes through it
• The stator & bearing remains stationary

25. 
• The shaft bears a pair of ring in Gram + and two pairs of ring in
Gram negative
• The bacterial flagellum usually does not have any bounding
membranes and it grows at the tip
• Eukaryotic flagellum grows at the base adjacent to the basal
body

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27. 
• Fimbriae or pili :- are non-motile but adhesive structure
• they enable the bacteria to stick firmly to other cells, to a
surface
• Pili help in conjugation ( sex pili)
• E.g. Gonorrhea- causing coccus, Neiseria gonorrhoeae

28. Transfer of Genetic Material in Bacterial Cells
• It take place in three ways :-
1) Transformation:-Bacterial transformation is a process of
horizontal gene transfer by which some bacteria take up
foreign genetic material (naked DNA) from the environment.
* It was first reported in Streptococcus pneumoniae by Griffith
in 1928.
* DNA as the transforming principle was demonstrated by Avery
et al in 1944.

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30. 
• Transduction :-a process of genetic recombination in bacteria
in which genes from a host cell (a bacteria) are incorporated into
the genome of a bacterial viruse (bacteriophage) and then carried
to another host cell when the bacteriophage initiates another
cycle of infection.

31. 
• Conjugation :- is the process by which one bacterium transfers genetic
material to another through direct contact.
• During conjugation, one bacterium serves as the donor of the genetic material,
and the other serves as the recipient.
• The donor bacterium carries a DNA sequence called the fertility factor, or F-
factor.
• The F-factor allows the donor to produce a thin, tubelike structure called a
pilus, which the donor uses to contact the recipient.
• The pilus then draws the two bacteria together, at which time the donor
bacterium transfers genetic material to the recipient bacterium.

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Thanks
Sukhmeet Kang