Lakhasly

Introduction The classification of the living world, also known as taxonomy, is a science that aims to organize and categorize living organisms into hierarchical groups based on their common characteristics.o Regardless of their form, size, specialization, and lifestyle, all living cells (Eukaryotes or Prokaryotes) exhibit a fundamental structure characterized by the presence of the following three elements: 1- Cell membrane or plasma membrane, without which intracellular biomolecules would be diluted in the surrounding environment.o Ecological Importance: Prokaryotes play a crucial role in terrestrial and marine ecosystems as decomposers, nitrogen fixers, producers of greenhouse gases, and perform various other functions.Here are the main hierarchical levels in biological classification 1-Domain: The three domains of life are as follows: o Archaea (archaea) o Bacteria (bacteria) o Eukaryota (eukaryotes) 2-Kingdom: Each domain is divided into kingdoms.For example, the domain of eukaryotes is subdivided into several kingdoms, including: o Animalia (animals) o Plantae (plants) o Fungi (fungi) o Protista (protists) 3- Phylum: Phyla are even more specific groups within kingdoms.I- Prokaryote In biology and taxonomy, a prokaryotic cell (from the Latin pro, meaning 'before,' and the Greek karyon, meaning 'nucleus') is a unicellular organism (1 to 10um) lacking a nucleus and intracellular organelles (except for ribosomes).Morphology of some bacterial cells (a)Bacillus megaterium; (b)Tetracoccus canadensis Staphylococcus aureus Constant structures: Constant structures are mandatory organelles found in all bacteria.3- Cytoplasm: the region between the nucleus (or DNA) and the cell membrane o The prokaryotes are unicellular organisms belonging to the kingdoms of bacteria and archaea.5-Cytoplasmic Inclusions: This includes reserve granules or gas vacuoles, allowing the vertical movement of the bacterium that contains them.Traditional classification relies on systematic approaches, which encompass nomenclature (naming) and phylogeny (reconstruction of evolutionary relationships).4-Pili (Fimbriae): Similar to flagella but shorter, pili are membranous extensions that facilitate the adhesion of bacteria to substrates.The cell The cell (from the Latin cellula, meaning small chamber or room) is a compartment partitioned (a closed structure) by a biomembrane containing genetic material plus a viscous fluid called cytoplasm.o Reproduction: Prokaryotes primarily reproduce through binary fission, a process where a mother cell divides into two genetically identical daughter cells.The constant structures are: 1- Nucleoid Material: This is the bacterial chromosome, composed of a single double-stranded circular DNA molecule of approximately 1mm.They encode for the synthesis of various enzymes that enable the bacterium to utilize certain substrates or resist antibiotics.o Archaebacteria are typically found in extreme environments (they are also called extremophiles), such as hot springs (~100?C), the deep ocean floor, salt-saturated waters and other environments previously thought to be inhospitable to life.For instance, in the animal kingdom, arthropods (such as insects and arachnids) and chordates (such as vertebrates) are two distinct phyla.o Horizontal Gene Transfer: Prokaryotes can exchange DNA fragments through conjugation, transformation, and transduction, promoting genetic diversity.o Resistance: Some prokaryotes develop resistance to antibiotics and harsh environmental conditions through mechanisms such as mutation and plasmid transfer.The ultrastructure of the cell wall subdivides bacteria into two main groups : o Gram-positive bacteria : their wall is dense and composed of thick peptidoglycan.Their walls consist of a thin layer of peptidoglycan resting on the plasma membrane, surrounded by an outer membrane, making a total of three layers.A pertinent example would be Escherichia coli.Bacteria replicate rapidly through cell division or binary fission.2-Mesosomes: Exclusive to aerobic bacteria, mesosomes are membranous invaginations that penetrate the cytoplasm.Flagella are composed of a contractile protein known as flagellin, and they facilitate the bacterium's locomotion.Taxonomy is an essential tool for biologists and ecologists because it allows them to understand the relationships between organisms and study the evolution of life on Earth.The cell represents the structural, functional, and reproductive unit constituting all (bacteria) or part of a living organism (animal or plant).o Absence of Organelles: Prokaryotes lack membrane-bound organelles such as mitochondria or chloroplasts, which are found in eukaryotes.3-Flagella: These are mobile membranous extensions, ranging from 1 to 8, located in different positions.Figure: Bacterial Cell Structure o The prokaryotes are divided into two major taxonomic groups, EUBACTERIA and ARCHAEBACTERIA.The origins of this group and its relationship with eubacteria and eukaryotes are currently hotly debated among microbiologists.A species is defined as a group of individuals capable of reproducing with each other and producing fertile offspring.2- Region containing genetic material, located inside the cell with molecules capable of reading and copying hereditary instructions.The flagellum and certain pili are involved in locomotion, fimbriae allow the cell to adhere to a substrate, and sex pili are used for DNA exchange.Domain of bacteria A bacterium is a unicellular prokaryotic organism of very small size (1 to 10um).They have various shapes, such as rod-shaped, spherical, cylindrical, etc.4-The cell wall: Thick, ranging from 20 to 80 nm, it externally delimits the bacterium and determines its shape.Gram-positive bacteria retain the dye, appearing violet in color.Their cell walls have a single layer of peptidoglycan resting on the plasma membrane, and both together constitute the cell wall.Gram-negative bacteria are much more permeable to the dye, resulting in a pink coloration.For instance, in the class of mammals, the order of primates includes monkeys, chimpanzees, and humans.7- Genus: 8- Species:

It's important to note that the classification of the living world is constantly evolving as new genetic and anatomical information is discovered.o Lack of Nucleus: Unlike eukaryotes, prokaryotes do not have a nucleus enclosed by a membrane.Bacteria reproduce through binary fission or constriction.The bacterial cell is composed of both constant (essential) and variable structures.It is free in the cytoplasm, not separated by a nuclear envelope to form a true nucleus, hence the name Prokaryote.2-Ribosomes: Grouped in clusters forming polyribosomes.3-Plasma Membrane: Composed of lipids and proteins.o Gram-negative bacteria : their wall is composed of a loose peptidoglycan layer.They are found in certain groups and are absent in others, depending on the species and their environments.They contain enzymes of the respiratory chain, thus serving a role similar to that of mitochondria.o They can metabolize unusual substances like methane, sulfur, and hydrogen gas.The classification of the living world includes several levels, ranging from the most general to the most specific.For example, in the phylum of chordates, the class of mammals is a group that includes animals like humans, dogs, and cats.5- Order: 6- Family: o Families are even more restricted groups, grouping species that are very similar.o Genuses are very close groups, grouping very similar species.They also sometimes contain smaller circular DNA elements called plasmids.The DNA is located in a region known as the nucleoid, which is not surrounded by a membrane.o Diverse Metabolism: Prokaryotes exhibit diverse metabolism.They can be aerobic (using oxygen) or anaerobic (thriving in the absence of oxygen).Some are autotrophic (producing their own food), while others are heterotrophic (obtaining their food from the environment).Bacteria can live individually or grouped in colonies.A living and functional bacterial cell cannot be devoid of any of these organelles.Examples of Gram-positive bacteria include staphylococus.Optional Structures: Optional structures are organelles that may or may not be present in the bacterium.These include: 1-Capsule: Often polysaccharide or polypeptide in nature, the capsule primarily plays a protective role.Its presence is a sign of virulence as it shields the bacterium from phagocytosis.6- Plasmids: Plasmids are double-stranded circular DNA fragments.A plasmid can be present in multiple copies within a single bacterial cell.o As of now, no pathogenic strain of archaebacteria has been identified.Many also have a capsule or slimy envelope made of polysaccharides.o Prokaryotes often have appendages (protrusions) on their surface.o Most prokaryotic cells have a single circular chromosome.It does not contain cholesterol and is low in carbohydrates.It is separated from the plasma membrane by the periplasmic space.They are composed of a protein called pilin.They are extrachromosomal and located in the cytoplasm.However, some of their characteristics place them closer to eukaryotes than eubacteria.4- Class: o Classes group similar animals or plants within a phylum.o This is the lowest level of classification.o Prokaryotic cells are much smaller than eukaryotic cells, o All prokaryotic cells are surrounded by a cell wall.Their synthesis does not require the presence of a nucleolus.It facilitates the transport of nutrients.Eubacteria are the most common forms of bacteria.o Orders are even more specific groups within classes.Its DNA is in the form of a circle or helix.For example, a bacterium.It plays a protective role: a bacterium without a cell wall dies.They can be either pathogenic or non-pathogenic.

Introduction
The classification of the living world, also known as taxonomy, is a
science that aims to organize and categorize living organisms into
hierarchical groups based on their common characteristics.
Traditional classification relies on systematic approaches, which
encompass nomenclature (naming) and phylogeny (reconstruction of
evolutionary relationships).
The classification of the living world includes several levels,
ranging from the most general to the most specific. Here are
the main hierarchical levels in biological classification
1-Domain:
The three domains of life are as follows:
• Archaea (archaea)
• Bacteria (bacteria)
• Eukaryota (eukaryotes)
2-Kingdom:
Each domain is divided into kingdoms. For example, the domain of
eukaryotes is subdivided into several kingdoms, including:
• Animalia (animals)
• Plantae (plants)
• Fungi (fungi)
• Protista (protists)
3- Phylum:
Phyla are even more specific groups within kingdoms. For instance, in
the animal kingdom, arthropods (such as insects and arachnids) and
chordates (such as vertebrates) are two distinct phyla.
4- Class:
• Classes group similar animals or plants within a phylum. For example,
in the phylum of chordates, the class of mammals is a group that
includes animals like humans, dogs, and cats.
• Orders are even more specific groups within classes. For instance, in
the class of mammals, the order of primates includes monkeys,
chimpanzees, and humans.
5- Order:
6- Family:
• Families are even more restricted groups, grouping species that are very
similar.
• Genuses are very close groups, grouping very similar species.
• This is the lowest level of classification. A species is defined as a group of
individuals capable of reproducing with each other and producing fertile
offspring.
7- Genus:
8- Species:

It's important to note that the classification of the living world is
constantly evolving as new genetic and anatomical information is
discovered.
Taxonomy is an essential tool for biologists and ecologists
because it allows them to understand the relationships between
organisms and study the evolution of life on Earth.
The cell
The cell (from the Latin cellula, meaning small chamber or room)
is a compartment partitioned (a closed structure) by a biomembrane
containing genetic material plus a viscous fluid called cytoplasm. The
cell represents the structural, functional, and reproductive unit
constituting all (bacteria) or part of a living organism (animal or plant).

I- Prokaryote
In biology and taxonomy, a prokaryotic cell (from the Latin pro,
meaning 'before,' and the Greek karyon, meaning 'nucleus') is a
unicellular organism (1 to 10μm) lacking a nucleus and intracellular
organelles (except for ribosomes). Its DNA is in the form of a circle or
helix. For example, a bacterium.
• Regardless of their form, size, specialization, and lifestyle, all living cells
(Eukaryotes or Prokaryotes) exhibit a fundamental structure characterized by
the presence of the following three elements:
1- Cell membrane or plasma membrane, without which intracellular biomolecules
would be diluted in the surrounding environment.
2- Region containing genetic material, located inside the cell with molecules
capable of reading and copying hereditary instructions.
3- Cytoplasm: the region between the nucleus (or DNA) and the cell membrane
• The prokaryotes are unicellular organisms belonging to the kingdoms of bacteria and
archaea.
• Prokaryotic cells are much smaller than eukaryotic cells,
• All prokaryotic cells are surrounded by a cell wall. Many also have a capsule or slimy
envelope made of polysaccharides.
• Prokaryotes often have appendages (protrusions) on their surface. The flagellum and
certain pili are involved in locomotion, fimbriae allow the cell to adhere to a substrate,
and sex pili are used for DNA exchange.
• Most prokaryotic cells have a single circular chromosome. They also sometimes contain
smaller circular DNA elements called plasmids.
• Lack of Nucleus: Unlike eukaryotes, prokaryotes do not have a nucleus enclosed by a
membrane. The DNA is located in a region known as the nucleoid, which is not
surrounded by a membrane.
• Absence of Organelles: Prokaryotes lack membrane-bound organelles such as
mitochondria or chloroplasts, which are found in eukaryotes.
• Diverse Metabolism: Prokaryotes exhibit diverse metabolism. They can be aerobic
(using oxygen) or anaerobic (thriving in the absence of oxygen). Some are autotrophic
(producing their own food), while others are heterotrophic (obtaining their food from the
environment).
• Reproduction: Prokaryotes primarily reproduce through binary fission, a process where
a mother cell divides into two genetically identical daughter cells.
• Horizontal Gene Transfer: Prokaryotes can exchange DNA fragments through
conjugation, transformation, and transduction, promoting genetic diversity.
• Resistance: Some prokaryotes develop resistance to antibiotics and harsh environmental
conditions through mechanisms such as mutation and plasmid transfer.
• Ecological Importance: Prokaryotes play a crucial role in terrestrial and marine
ecosystems as decomposers, nitrogen fixers, producers of greenhouse gases, and
perform various other functions.
Domain of bacteria
A bacterium is a unicellular prokaryotic organism of very small size (1
to 10µm). Bacteria can live individually or grouped in colonies. They
have various shapes, such as rod-shaped, spherical, cylindrical, etc.
Bacteria reproduce through binary fission or constriction. The bacterial
cell is composed of both constant (essential) and variable structures.
Morphology of some bacterial cells
(a)Bacillus megaterium; (b)Tetracoccus canadensis Staphylococcus aureus
Constant structures:
Constant structures are mandatory organelles found in all bacteria. A
living and functional bacterial cell cannot be devoid of any of these
organelles. The constant structures are:
1- Nucleoid Material: This is the bacterial chromosome, composed of a
single double-stranded circular DNA molecule of approximately 1mm.
It is free in the cytoplasm, not separated by a nuclear envelope to form a
true nucleus, hence the name Prokaryote.

2-Ribosomes: Grouped in clusters forming polyribosomes. Their
synthesis does not require the presence of a nucleolus.
3-Plasma Membrane: Composed of lipids and proteins. It does not
contain cholesterol and is low in carbohydrates. It facilitates the
transport of nutrients.
4-The cell wall:
Thick, ranging from 20 to 80 nm, it externally delimits the bacterium
and determines its shape. It plays a protective role: a bacterium without
a cell wall dies. It is separated from the plasma membrane by the
periplasmic space.
The ultrastructure of the cell wall subdivides bacteria into two main
groups :
• Gram-positive bacteria : their wall is dense and composed of thick
peptidoglycan. Gram-positive bacteria retain the dye, appearing violet in color.
Their cell walls have a single layer of peptidoglycan resting on the plasma
membrane, and both together constitute the cell wall. Examples of Gram-positive
bacteria include staphylococus.
• Gram-negative bacteria : their wall is composed of a loose peptidoglycan layer.
Gram-negative bacteria are much more permeable to the dye, resulting in a pink
coloration. Their walls consist of a thin layer of peptidoglycan resting on the
plasma membrane, surrounded by an outer membrane, making a total of three
layers. A pertinent example would be Escherichia coli.Bacteria replicate rapidly through cell division or binary fission. They
can be either pathogenic or non-pathogenic.
Optional Structures:
Optional structures are organelles that may or may not be present
in the bacterium. They are found in certain groups and are absent in
others, depending on the species and their environments. These include:
1-Capsule: Often polysaccharide or polypeptide in nature, the capsule
primarily plays a protective role. Its presence is a sign of virulence as it
shields the bacterium from phagocytosis.
2-Mesosomes: Exclusive to aerobic bacteria, mesosomes are
membranous invaginations that penetrate the cytoplasm. They contain
enzymes of the respiratory chain, thus serving a role similar to that of
mitochondria.
3-Flagella: These are mobile membranous extensions, ranging from 1 to 8,
located in different positions. Flagella are composed of a contractile protein
known as flagellin, and they facilitate the bacterium's locomotion.
4-Pili (Fimbriae): Similar to flagella but shorter, pili are membranous
extensions that facilitate the adhesion of bacteria to substrates. They are
composed of a protein called pilin.
5-Cytoplasmic Inclusions: This includes reserve granules or gas vacuoles,
allowing the vertical movement of the bacterium that contains them.
6- Plasmids: Plasmids are double-stranded circular DNA fragments.
They are extrachromosomal and located in the cytoplasm. A plasmid can
be present in multiple copies within a single bacterial cell. They encode
for the synthesis of various enzymes that enable the bacterium to utilize
certain substrates or resist antibiotics.
Figure: Bacterial Cell Structure
• The prokaryotes are divided into two major taxonomic groups, EUBACTERIA and
ARCHAEBACTERIA. Eubacteria are the most common forms of bacteria.
• As of now, no pathogenic strain of archaebacteria has been identified.
• Archaebacteria are typically found in extreme environments (they are also called
extremophiles), such as hot springs (~100°C), the deep ocean floor, salt-saturated waters
and other environments previously thought to be inhospitable to life.
• They can metabolize unusual substances like methane, sulfur, and hydrogen gas. The
origins of this group and its relationship with eubacteria and eukaryotes are currently
hotly debated among microbiologists. However, some of their characteristics place them
closer to eukaryotes than eubacteria.
Archaea domain

Figure: archaea colony

Viruses
The term "virus" is derived from the Latin word for poison. Viruses are
the causative agents of numerous viral diseases, some of which remain
fatal and incurable to this day.
Viruses are not considered living beings because they lack a cellular
structure, have no independent metabolism, and can only be
reproduced by infecting living cells to ensure their multiplication
cycle (absolute parasitism)
1-Definition and Characteristics of Viruses:
• Viruses are obligatory parasites of specific hosts (either eukaryotic or
prokaryotic). They consist of genetic material (either DNA or RNA, but
not both), a capsid with variable symmetry, and potentially an envelope.
• Devoid of cytoplasm and a nucleus, viruses lack their own metabolism
and replication capabilities. Their size ranges from 15 to 300 nm.
• Being devoid of cellular organelles, viruses do not constitute a true cell;
they are merely internal particles incapable of autonomous evolution. They
parasitize living cells, introducing their genetic material and utilizing the
host's organelles for their own benefit.
• Viruses are very simple, with a relatively short piece of genetic code
inside a packaging of protein called a capsid.
• They do not have a transcription or translation system.
• When the virus infects a cell, it inserts its genetic code into the cell and
takes over the machinery of the host cell to make lots of copies of its
genetic code and proteins for its capsid.
• The new viruses are then assembled inside the host cell and eventually
burst the cell and kill it. Viral destruction of cells causes disease.
• Viruses parasitize both multicellular organisms (animals and plants)
and unicellular organisms (bacteria and protists).
Glycoproteins
: transmembrane
proteins that serve as ligands (key)
for cellular receptors (lock) and as a recognition system for antibodies.
Envelope
: specific to certain
viruses
, its origin is cellular, and it
is vulnerable to disinfectants
.
Capsid
: provides protection for the
nucleic acid
, composed of proteins
assembled in
a geometric fashion,
some of which have antigenic
properties
.
Nucleic acid
(genome
)
: DNA or
RNA
.
virus structure
2-Classification of Viruses:
Viruses can be classified based on various criteria, as follows:
1-Nature of Nucleic Acid: Adenovirus (DNA) or Retrovirus (RNA).
2-Capsid Symmetry: Helical or Cubic.
3-Presence or Absence of Envelope: Enveloped or Non-enveloped viruses.

The difference between bacteria and viruses
Cell membrane Presence Absence
Protein synthesis Presence Absence
DNA or RNA Presence One of the two
Multiplication Autonomous/stand-alone Host-dependent
Respiration Aerobic/anaerobic Absence
Eukaryotic Cell
A eukaryotic cell (10 à 100μm), is a type of cell characterized by
the presence of a membrane-bound nucleus, which contains the
genetic material (DNA).
Unlike prokaryotic cells, which do not have a distinct nucleus,
eukaryotic cells have various specialized internal organelles, also
surrounded by membranes.
Characteristics:
• Nucleus: Contains DNA organized into chromosomes and is
surrounded by a nuclear membrane.
• Plasma membrane: Surrounds the cell and regulates exchanges
between the interior of the cell and the external environment.
• Cytoplasm: The medium where organelles are located.
• Cytoskeleton: A network of fibers that gives the cell its shape and
facilitates the movement of organelles.
• Organelles:
➢ Mitochondria: Responsible for energy production in the form of
ATP.
➢Endoplasmic reticulum: Involved in protein and lipid synthesis.
➢Golgi apparatus: Modifies, sorts, and packages proteins for
transport.Lysosomes: Contain enzymes to digest substances.
➢Chloroplasts (only in plants and certain algae): Involved in
photosynthesis.
Eukaryotic cells are found in multicellular organisms, such as plants,
animals, fungi, and protists.

Animal Cell and Plant Cell
• The cells of plants and animals are eukaryotic and very similar in their structure.
• The differences are mainly related to the presence or absence of a wall
surrounding the plasma membrane and the presence or absence of chloroplasts.
• Plant cells are surrounded by a rigid cellulose wall and contain all the organelles
common to other animal cells.
• The cytoplasm of plant cells is occupied by a large vacuole.
• Animal cells lack a cell wall, chloroplasts, and large vacuoles.
• When vacuoles are present (in animal cells), they are small and
dispersed throughout the cytoplasm