Prokaryotes are unicellular organisms that lack membrane-bound structures, the most noteworthy of which is the nucleus. ... While prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions. In prokaryotic cells, DNA bundles together in a region called the nucleoid.

1. PROKARYOTIC CELL

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3. PROKARYOTES
• Prokaryotes are the most numerous and
widespread organisms on earth and are so
classified because they have no defined
membrane-bound nucleus.
• Prokaryotes range in size from 0.1 to 10 m and
have one of three basic shapes:
1. spherical (cocci)
2. rodlike (bacilli)
3. helically coiled (spirilla)

4. • They can be divided into two separate groups: the
eubacteria and the archaebacteria.
EUBACTERIA:
They are the commonly encountered bacteria in soil,
water and living in or on larger organisms, and include
the Gram-positive and Gram-negative bacteria, and
cyanobacteria (photosynthetic blue-green algae).
ARCHAEBACTERIA:
They grow in unusual environments such as salt brines,
hot acid springs and in the ocean depths, and include the
sulfur bacteria and the methanogens.

5. CELL STRUCTURE
• Prokaryotic cell is bounded by a plasma membrane that completely encloses the cytosol and separates the
cell from the external environment.
• The plasma membrane, which is about 8 nm thick, consists of a lipid bilayer containing proteins.
• Although prokaryotes lack the membranous subcellular organelles characteristic of eukaryotes their plasma
membrane may be infolded to form mesosomes.
• mesosomes may be the sites of deoxyribonucleic acid (DNA) replication and other specialized enzymatic
reactions.
• In photosynthetic bacteria, the mesosomes contain the proteins and pigments that trap light and generate
adenosine triphosphate (ATP).
• The aqueous cytosol contains the macromolecules [enzymes, messenger ribonucleic acid (mRNA), transfer
RNA (tRNA) and ribosomes], organic compounds and ions needed for cellular metabolism.
• Also within the cytosol is the prokaryotic ‘chromosome’ consisting of a single circular molecule of DNA
which is condensed to form a body known as the nucleoid.
• Many bacterial cells have one or more tail-like appendages known as flagella which are used to move the
cell through its environment

7. BACTERIAL CELL WALL
• To protect the cell from mechanical injury and osmotic pressure, most
prokaryotes are surrounded by a rigid 3–25 nm thick cell wall.
• The cell wall is composed of peptidoglycan, a complex of oligosaccharides
and proteins.
• The oligosaccharide component consists of linear chains of alternating N-
acetylglucosamine (GlcNAc) and N-acetylmuramic acid (NAM)).
• Chains are attached via an amide bond to the lactic acid group on NAM is
a D-amino acid-containing tetrapeptide.
• Adjacent parallel peptidoglycan chains are covalently cross-linked through
the tetrapeptide side-chains by other short peptides.

10. • The extensive cross-linking in the peptidoglycan cell wall gives it its strength and rigidity.
• The presence of D-amino acids in the peptidoglycan renders the cell wall resistant to the action of
proteases which act on the more commonly occurring L-amino acids but provides a unique target for the
action of certain antibiotics such as penicillin.
• Penicillin acts by inhibiting the enzyme that forms the covalent cross-links in the peptidoglycan, thereby
weakening the cell wall.
• The glycosidic linkage between NAM and NAG is susceptible to hydrolysis by the enzyme lysozyme which
is present in tears, mucus and other body secretions.
• Bacteria can be classified as either Gram-positive or Gram-negative depending on whether or not they
take up the Gram stain.
• Gram-positive bacteria have a thick (25 nm) cell wall surrounding their plasma membrane.
• Gram-negative bacteria (e.g. Escherichia coli) have a thinner (3 nm) cell wall and a second outer
membrane.
• In contrast with the plasma membrane , this outer membrane is very permeable to the passage of
relatively large molecules (molecular weight > 1000 Da) due to porin proteins which form pores in the
lipid bilayer.
• Between the outer membrane and the cell wall is the periplasm, a space occupied by proteins secreted
from the cell.

12. CYTOPLASMIC MATRIX
• The cytoplasmic matrix is the substance lying between the plasma
membrane and the nucleoid.
• The matrix is largely water (about 70% of bacterial mass is water).
• It is featureless in electron micrographs but often is packed with
ribosomes.
• Although bacteria may lack a true cytoskeleton, they do have a
cytoskeleton like system of proteins in their cytoplasmic matrix.
• The plasma membrane and everything within is called the
protoplast; thus the cytoplasmic matrix is a major part of the
protoplast.

13. Inclusion Bodies
• A variety of inclusion bodies, granules of organic or inorganic
material that often are clearly visible in a light microscope, is
present in the cytoplasmic matrix.
• These bodies usually are used for storage e.g., carbon compounds,
inorganic substances, and Energy.
• Some inclusion bodies are not bounded by a membrane and lie free
in the cytoplasm.—for example glycogen granules.
• Other inclusion bodies are enclosed by a membrane about 2.0 to
4.0 nm thick, which is single-layered and not a typical bilayer
membrane.
• Because inclusion bodies are used for storage, their quantity will
vary with the nutritional status of the cell.

14. RIBOSOMES
• the cytoplasmic matrix often is packed with ribosomes; they also may be loosely
attached to the plasma membrane IN PROKARYOTIC CELL.
• Ribosomes look like small, featureless particles but actually very complex objects made
of both protein and ribonucleic acid (RNA).
• They are the site of protein synthesis, synthesize proteins destined to remain within the
cell, whereas the plasma membrane ribosomes make proteins for transport to the
outside.
• prokaryotic ribosomes are smaller than eukaryotic ribosomes.
• They commonly are called 70S ribosomes, have dimensions of about 14 to 15 nm by 20
nm
• molecular weight of approximately 2.7 million, and are constructed of a 50S and a 30S
subunit.
• The S in 70S and similar values stands for Svedberg unit.

15. THE NEUCLEOID
• Probably the most striking difference between procaryotes and eucaryotes is the way in
which their genetic material is packaged.
• The procaryotic chromosome is located in an irregularly shaped region called the nucleoid
(other names are also used: the nuclear body, chromatin body, nuclear region).
• Usually procaryotes contain a single circle of double-stranded deoxyribonucleic acid (DNA),
but some have a linear DNA chromosome.
• Many bacteria possess plasmids in addition to their chromosome.
• Plasmids are double-stranded DNA molecules, usually circular, that can exist and replicate
independently of the chromosome or may be integrated with it, in either case they normally
are inherited or passed on to the progeny.
• Plasmids are not required for host growth and reproduction, although they may carry genes
that give their bacterial host a selective advantage.
• Plasmid genes can render bacteria drug-resistant, give them new metabolic abilities, make
them pathogenic, or endow them with a number of other properties.

16. Capsules, Slime Layers, and S-Layers
• Some bacteria have a layer of material lying outside the cell wall.
• When the layer is well organized and not easily washed off, it is called a
capsule.
• A slime layer is a zone of diffuse, unorganized material that is removed
easily.
• Slayers also are very common among Archaea, where they may be
• the only wall structure outside the plasma membrane.
• Capsules and slime layers usually are composed of polysaccharides, but
they may be constructed of other materials.
• Capsules are clearly visible in the light microscope when special capsule
stains are employed

17. FLAGELLA
• Most motile bacteria move by use of flagella, threadlike locomotors appendages extending
outward from the plasma membrane and cell wall.
• They are slender, rigid structures, about 20 nm across and up to 15 or 20 m long.
• Flagella are so thin they cannot be observed directly with a bright-field microscope, but must
be stained with special techniques designed to increase their thickness.
• Bacterial species often differ distinctively in their patterns of flagella distribution.
1. Monotrichous bacteria (trichous means hair) have one flagellum; if it is located at an end, it
is said to be a polar flagellum
2. Amphitrichous bacteria (amphi means “on both sides”) have a single flagellum at each
pole.
3. lophotrichous bacteria (lopho means tuft) have a cluster of flagella at one or both ends.
4. Flagella are spread fairly evenly over the whole surface of peritrichous (peri means
“around”) bacteria.
• Flagellation patterns are very useful in identifying bacteria.