Ultra-structure of Prokaryotic Cells
Gr., pro-primitive or naked; karyon-nucleus) As the name indicates these cells are simple in
structural organization and most primitive types. They include Bacteria, Pleuropneumonia-
Like Organisms (PPLO), Blue green algae etc.
Prokaryotic cells are the simplest of most primitive cells. The records of microfossils suggest
that they have evolved 2.5 billion years ago and existed as the only organisms on earth for
the next one billion years until eukaryotes evolved about 1.5 billion years ago. Earlier claims
that oldest prokaryotic microfossils, the stromatolites (i.e., giant colonies of ancient
cyanobacteria) of 3.5 billion years ago are actually lifeless mineral artifacts.
The Prokaryotic ceil Is the structural unit of two microbial groups: the archaebacteria and
the eubacteria. Each prokaryotic cell is essentially a one envelop system that consists of
protoplasm encased within cell envelope. The ultrastructure of a prokaryotic cell,
particularly a typical bacterial cell consists of cell envelope, cytoplasm, nucleoid, plasmids
and surface appendage.
1. Cell envelope:
It is the protective covering of bacterial cell that has three basic layers: the outermost
glycocalyx, middle cell wall and innermost cell membrane (plasma membrane),
, i) Glycocalyx:
It is the outermost layer of cell envelope which chemically composed of polysaccharides
with or without proteins. When glycocalyx is thick and tough, it is called capsule, and when
it forms a loose sheath it is called slime layer.
Though not essential for bacterial survival, glycocalyx has many functions:
(a) Protects cell from desiccation, toxins and phagocytes.
(b) Helps in adhesion, immunogenicity and virulence.
ii) Cell wall:
It is the rigid middle layer of cell envelope that provides shape and prevents a bacterium
from osmotic bursting in a hypotonic solution. It is composed of peptidoglycan (murein or
mucopeptide), which consists of a three dimensional network of glyean strands cross linked
by peptide chains. Each glycan strand is 20-130 units long consists of two alternating amino
sugars, N-acetylglucosamine (NAG) and N-acetylmuramicacid (NAM).
In Gram-positive bacteria, cell wall is single layered and almost uniform in thickness (10 to
80nm). The walls of Gram- positive bacteria also contain teichoic acids. Teichoic acids act as
surface antigen.
In Gram-negative bacteria cell wall is two- layered and only 7.5-12 nm thick. In some Gram-
negative bacteria (Mycobacterium, Noccardia) the wall contains long chain fatty acids called
mycolic acids.
(iii) Cell membrane:
It is the innermost layer of cell envelope. It is a semi-permeable, quasi-fluid, dynamic
membrane similar to that of eukaryotic membrane. But the only difference is that, in
bacteria they lack sterols instead hopanoids present. The hopanoids are pentacyclic sterol-
like molecules that stabilize the bacterial cell membrane.
In a bacterial cell, plasma membrane performs many functions:
(a) It retains the cytoplasm
(b) Prevent loss of essential components through leakage
(c) Aids in the movement of nutrients, wastes and secretions across the membrane
(d) Holds receptor molecules that detect and respond chemicals in their surroundings.
Such as respiration, photosynthesis, synthesis of lipids and cell wall constituents
(e) It invaginates to form mesosome and thylakoids of cyanobacteria.
2. Cytoplasm:
Gr., pro-primitive or naked; karyon-nucleus) As the name indicates these cells are simple in
structural organization and most primitive types. They include Bacteria, Pleuropneumonia-
Like Organisms (PPLO), Blue green algae etc.
Prokaryotic cells are the simplest of most primitive cells. The records of microfossils suggest
that they have evolved 2.5 billion years ago and existed as the only organisms on earth for
the next one billion years until eukaryotes evolved about 1.5 billion years ago. Earlier claims
that oldest prokaryotic microfossils, the stromatolites (i.e., giant colonies of ancient
cyanobacteria) of 3.5 billion years ago are actually lifeless mineral artifacts.
The Prokaryotic ceil Is the structural unit of two microbial groups: the archaebacteria and
the eubacteria. Each prokaryotic cell is essentially a one envelop system that consists of
protoplasm encased within cell envelope. The ultrastructure of a prokaryotic cell,
particularly a typical bacterial cell consists of cell envelope, cytoplasm, nucleoid, plasmids
and surface appendage.
1. Cell envelope:
It is the protective covering of bacterial cell that has three basic layers: the outermost
glycocalyx, middle cell wall and innermost cell membrane (plasma membrane),
, i) Glycocalyx:
It is the outermost layer of cell envelope which chemically composed of polysaccharides
with or without proteins. When glycocalyx is thick and tough, it is called capsule, and when
it forms a loose sheath it is called slime layer.
Though not essential for bacterial survival, glycocalyx has many functions:
(a) Protects cell from desiccation, toxins and phagocytes.
(b) Helps in adhesion, immunogenicity and virulence.
ii) Cell wall:
It is the rigid middle layer of cell envelope that provides shape and prevents a bacterium
from osmotic bursting in a hypotonic solution. It is composed of peptidoglycan (murein or
mucopeptide), which consists of a three dimensional network of glyean strands cross linked
by peptide chains. Each glycan strand is 20-130 units long consists of two alternating amino
sugars, N-acetylglucosamine (NAG) and N-acetylmuramicacid (NAM).
In Gram-positive bacteria, cell wall is single layered and almost uniform in thickness (10 to
80nm). The walls of Gram- positive bacteria also contain teichoic acids. Teichoic acids act as
surface antigen.
In Gram-negative bacteria cell wall is two- layered and only 7.5-12 nm thick. In some Gram-
negative bacteria (Mycobacterium, Noccardia) the wall contains long chain fatty acids called
mycolic acids.
(iii) Cell membrane:
It is the innermost layer of cell envelope. It is a semi-permeable, quasi-fluid, dynamic
membrane similar to that of eukaryotic membrane. But the only difference is that, in
bacteria they lack sterols instead hopanoids present. The hopanoids are pentacyclic sterol-
like molecules that stabilize the bacterial cell membrane.
In a bacterial cell, plasma membrane performs many functions:
(a) It retains the cytoplasm
(b) Prevent loss of essential components through leakage
(c) Aids in the movement of nutrients, wastes and secretions across the membrane
(d) Holds receptor molecules that detect and respond chemicals in their surroundings.
Such as respiration, photosynthesis, synthesis of lipids and cell wall constituents
(e) It invaginates to form mesosome and thylakoids of cyanobacteria.
2. Cytoplasm:
