CHAPTER:
2
Prokaryotic
& Eukaryotic Microorganisms
Prokaryotic
Microorganisms:
• The
word prokaryote comes from the Greek word Pro and
karyon
Pro
= before (primitive)
Karyon
= nucleus
• Any
Cellular organism that has no nuclear membrane, no organelles in the cytoplasm
except ribosome and has its genetic materials in the form of single continuous
strands forming coil or loops is known as Prokaryotic Microorganism.
• organisms whose
cells lack a membrane-bound nucleus (karyon)
• Most
prokaryotes are unicellular organisms, although a few such as myxobacteria have multicellular stages
in their life cycles
Examples of Prokaryotic
Microorganisms:
• Streptococcus,
Salmonella, Staphylococcus, Escherischia coli (E. coli) ,Mycobacterium tuberculosis, Clostridium tetani , Actinomyces israelii , Beijerinckia
Klebsiella ,
Cyanobacteria , Azotobacter , Clostridium ,
Desulfovibrio.
• Prokaryotes
are the single-celled organisms and much smaller than eukaryotic cells.
• The
size of most prokaryotes is between 1 µm and 10 µm, but can vary in size from
0.2 µm to 750 µm.
• The
prokaryotes are divided into two domains:
1.
The bacteria, unicellular microorganisms that have wide range of shapes and
ubiquitous in habitat,
2.
The archaea (e.g Halobacteria), single-celled prokaryotic microorganisms
similar to bacteria but possess some genes and several metabolic pathways that
are closely related to those of eukaryotes.
• Exists
in different shapes like, coccus, bacillus, spirillum, coccobacillus, and
spirochete. While some of the prokaryotes are pleomorphic i.e. do not possess
constant shape and some exists as aggregate communities.
Structure and
Reproduction of Prokaryotes:
• Prokaryotic
cell contains Flagellum, cell wall (absent in Mycoplasma), cell membrane,
cytoplasm, ribosome, nucleiod, glycocalyx, and inclusion bodies.
• Membrane
bound organelles like mitochondria, chloroplast, nucleus, etc. are absent in prokaryotes.
• Genetic
material of prokaryotes is not present in nucleus and consists of a large DNA
molecule compacted in an area of cytoplasm called the nucleiod region.
• Cell
wall of the prokaryotes is composed of peptidoglycan, a single large polymer of
amino acids and sugar.
• Some
of the prokaryotes produce endospores during extreme conditions.
• Locomotion
is through flagella.
• Pilus
when present helps in adherance.
• Capsule,
a bacterial glycocalyx protects from host defenses and contributes to the
pathogenicity.
• Prokaryotes
live in nearly all environments even in harsh conditions such as high
temperature and high salinity.
• Some
species contain protein-enclosed microcompartments like carboxysomes or gas
vacuoles that distinct physiological roles.
• Prokaryotes
are asexual and usually reproduction is by binary fission or budding. Genetic
exchange and recombination occur in the form of horizontal gene transfer and is
not a replicative process as in eukaryotes.
• Prokaryotes
have a larger surface-area-to-volume ratio giving them a higher metabolic rate,
a higher growth rate and therefore a shorter generation time compared to
Eukaryotes
Significances of
Prokaryotes:
• Prokaryotes
continue to have an enormous ecological impact.
• Prokaryotes,
along with fungi, are decomposers that recycle chemical elements in ecosystems.
• Some
prokaryotes live with other species in symbiotic relationships.
• Some
parasitic prokaryotes are pathogenic, causing disease in the host.
• Bacteria
have been put to work in laboratories, sewage treatment plants, and the food
and drug industry.
Eukaryotic
Microorganisms:
• The
word Eukaryote comes from the Greek word Eu and karyon
Eu
= Well (true)
Karyon
= nucleus
• A
single-celled or multicellular organism whose cells contain a distinct
membrane-bound nucleus.
• A eukaryote
is any organism whose cells contain a nucleus and
other structures (organelles) enclosed within membranes.
• The
defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells
is the nucleus, or nuclear envelope, within which the genetic
material is carried.
• Most
eukaryotic cells also contain other membrane-bound organelles such
as mitochondria or the Golgi apparatus.
• In addition, plants and algae contain chloroplasts.
• Many unicellular organisms are
eukaryotes, such as protozoa. All multicellular organisms are
eukaryotes,including animals, plants and fungi.
Examples of Eukaryotic
Microorganisms:
• Rhizopus
, R. arrhizus , Aspergillus , Penicillium , Giardia
lamblia , Entamoeba histolytica , Plasmodium, Saccharomyces
cerevisiae , Candida albicans, Amanita,
Acaulospora, Entrophospora, Gigaspora, Glomus,
Sclerocystis , Scutellospora Lactarius
Structure of
Prokaryotic Microorganisms:
Prokaryotic cells are
not as complex as eukaryotic cells. They have no true nucleus as the DNA
is not contained within a membrane or separated from the rest of the cell, but
is coiled up in a region of the cytoplasm called the nucleoid. Using bacteria
as our sample prokaryote, the following structures can be found in bacterial
cells:
Internal Structures of
Prokaryotic Cells:
- Plasma Membrane:
The plasma
membrane is a double-layer of phospholipids with associated proteins and
other molecules. The lipids occur in two layers (a bilayer).
Proteins embedded in the bilayer appear to float within the lipid, so the
membrane is constantly in flux. The membrane is therefore referred to as
a fluid mosaic structure. Within the fluid mosaic
structure, proteins carry out most of the membrane's functions. It is
essentially the “bag” that holds all of the intracellular material and regulates
the movement of materials into and out of the cell.
- Cytoplasm:
The cytoplasm in
prokaryotic cells is a gel-like, yet fluid, substance in which all of
the other cellular components are suspended.
It is very similar to the eukaryotic cytoplasm, except that
it does not contain organelles. Recently, biologists have
discovered that prokaryotic cells have a complex and functional cytoskeleton similar
to that seen in eukaryotic cells.
3. Cytoskeleton:
It's only recently been
discovered that rod-shaped bacteria and Archaea possess cytoskeletal proteins
that function in a similar way to the cytoskeleton of eukaryotic cells.
cytoskeleton is an interconnected system of fibers, threads, and interwoven
molecules that give structure to the cell. The main components of the
cytoskeleton are microtubules, microfilaments, and intermediate filaments. All
are assembled from subunits of protein. This scaffolding provides structural
support to the cell and plays a role in cell-division.
4. Ribosomes:
Ribosomes in prokaryotes are smaller than eukaryotes. It consists 70s
type ribosome. These ribosomes possess
two subunits : one smaller which is 30s in nature and next larger one which is
50s in nature. Ribosomes are the protein-making machinery of the cell.
- Genetic Material of Prokaryotes:
All prokaryotic cells contain
large quantities of genetic material in the form of DNA and RNA.
Because prokaryotic cells, by definition, do not have a
nucleus, the single large circular strand of DNA containing most of the genes
needed for cell growth, survival, and reproduction is found in the cytoplasm.
The DNA tends to look like a mess of string in the middle of the cell.
a. Nucleoid:
The nucleoid is the region of the
prokaryotic cytoplasm that contains the genome—the main genetic material (DNA)
of the cell. Bacteria and Archaeans typically have a single, circular
chromosome.
b. Plasmids:
In addition to the bacterial chromosome,
bacteria may also contain one or more plasmids. A plasmid is a non-essential
piece of DNA that confers an advantage to the bacteria, such as antibiotic
resistance, virulence (the ability to cause disease) and conjugation (a
bacterium’s ability to share its plasmids with other bacteria). Plasmids are
also found in some eukaryotic microbes, such as yeasts.
External Structures of
Prokaryotic Cells:
- Cell wall:
All prokaryotes have a
protective cell wall that surrounds some cells and gives them shape and
rigidity. It is located outside the cell membrane and prevents osmotic lysis (bursting
due to increasing volume).The composition of cell walls vary depending on the
type of organisms, but most cell walls contain a combination of the major
organic molecules—proteins, carbohydrates and lipids. Bacteria have a unique
molecule called peptidoglycan in their cell wall. Archaean cell walls do not
contain this molecule. Cell wall composition of bacteria allow scientists to
classify them as either Gram-positive or Gram-negative.
Bacteria
are divided into two major groups: gram-positive and gram-negative, based on
their reaction to gram staining:
Gram –Positive and Gram
Negative Bacteria:
Both groups have a cell wall composed of peptidoglycan:
in gram-positive bacteria, the wall is thick, whereas in gram-negative
bacteria, the wall is thin. In gram-negative bacteria, the cell wall is
surrounded by an outer membrane that contains lipopolysaccharides and
lipoproteins. Porins, proteins in this cell membrane, allow substances
to pass through the outer membrane of gram-negative bacteria. In gram-positive
bacteria, lipoteichoic acid anchors the cell wall to the cell membrane.
- Glycocalyx:
The glycocalyx is a
layer present in some bacteria, and located outside of the cell wall. There
are two types of glycocalyces: slime layers and capsules. Slime layers help
bacteria stick to things and protect them from drying out, particularly in
hypertonic environments. Capsules also allow bacteria to stick to things, but
have the added benefit of helping encapsulated bacteria hide from the hosts
immune system.
- Pili:
Hair-like structures on
the surface of the cell that attach to other bacterial cells. Shorter pili
called fimbriae help bacteria attach to surfaces.
- Flagella:
Long whip-like
extensions that help bacteria move about the environment.
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