Download Structure of prokaryotic cells

Structure of prokaryotic cell
Structure of prokaryotic cell
Structure of prokaryotic cells
There are four main structures
shared by all prokaryotic cells
1. The plasma membrane
2. Cytoplasm
3. Ribosomes
4. Genetic material
1. Plasma membrane
• Prokaryotic cells can have multiple plasma membranes.
• The plasma membrane in prokaryotic cells is
responsible for controlling what gets into and out of
the cell. A series of proteins stuck in the membrane
also aid prokaryotic cells in communicating with the
surrounding environment.
• Among other things, this communication can include
sending and receiving chemical signals from other
bacteria and interacting with the cells of eukaryotic
organisms during the process of infection.
2. 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.
• The cytoskeleton helps prokaryotic cells divide
and helps the cell maintain its plump, round
shape.
3. Ribosomes
Prokaryotic ribosomes are smaller and have a
slightly different shape and composition than
those found in eukaryotic cells.
Bacterial ribosomes have about 1/2 of the
amount of ribosomal RNA (rRNA) and 1/3
fewer ribosomal proteins (53 vs. ~83) than
eukaryotic ribosomes have.
Just like in eukaryotic cells, prokaryotic
ribosomes build proteins by translating
messages sent from DNA.
Genetic material
• 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
Pili, flagella
• Some prokaryotic cells also have other
structures like the cell wall, pili (singular
pillus), and flagella (singular flagellum).
• Each of these structures and cellular
components plays a critical role in the growth,
survival, and reproduction of prokaryotic cells.
Summary of differences!
Prokaryotic Cells
Eukaryotic cells
small cel
larger cells (> 10 mm)
ls (< 5 mm)
always unicellular
often multicellular
no nucleus or any membrane-bound
organelles
always have nucleus and other
membrane-bound organelles
DNA is circular, without proteins
DNA is linear and associated with
proteins to form chromatin
ribosomes are small (70S)
ribosomes are large (80S)
no cytoskeleton
always has a cytoskeleton
cell division is by binary fission
cell division is by mitosis or meiosis
reproduction is always asexual
reproduction is asexual or sexual
Eukaryotic cell
As in eukaryotic cells, the prokaryotic chromosome is
intimately associated with special proteins involved in
maintaining the chromosomal structure and regulating
gene expression.
In addition to a single large piece of chromosomal
DNA, many prokaryotic cells also contain small pieces of
DNA called plasmids.
These circular rings of DNA are replicated
independently of the chromosome and can be transferred
from one prokaryotic cell to another through pili, which
are small projections of the cell membrane that can form
physical channels with the pili of cells.
Eukaryotic cell
A cell is defined as eukaryotic if it has
a membrane-bound nucleus.
What do eukaryotic cells have
All eukaryotic cells have
A nucleus
Genetic material
A plasma membrane
Ribosomes
Cytoplasm, including the cytoskeleton
What others?
Most eukaryotic cells also have other membrane-bound
internal structures called organelles.
Organelles include
Mitochondria
Golgi bodies
Lysosomes
Endoplasmic reticulum
Vesicles
NUCLEUS
The nucleus in the cell is analogous to the
brain in the body.
It is a control center for a cell.
The nucleus stores all the information the
cell needs to grow, reproduce, and function.
This information is contained in long but thin
molecules of deoxyribonucleic acid, or DNA.
One of the functions of the nucleus is to
protect the cell’s DNA from damage.
The nucleus also contains a small round body
called a nucleolus that holds nucleic acids and
proteins.
The nuclear membrane has pores through
which the contents of the nucleus communicate
with the rest of the cell.
The nuclear membrane tightly controls what
gets into the nucleus and what gets out.
Chromosomes are also located in the nucleus
and are basically organized structures of DNA and
proteins.
In eukaryotes chromosomal DNA is packaged
and organized into a condensed structure
called chromatin.
Chromosomes are single pieces of DNA along
with genes, proteins, and nucleotides, and
chromatin is a condensed package of
chromosomes that basically allows all the
necessary DNA to fit inside the nucleus.
In eukaryotic organisms, the DNA inside
the nucleus is also closely associated with
large protein complexes called histones.
Along with the nuclear membrane,
histones help control which messages get
sent from the DNA to the rest of the cell.
The information stored in DNA gets
transferred to the rest of the cell.
The central dogma of
biology is:
DNA → RNA → Protein
and it all starts in the
nucleus
Eukaryotic plasma membrane
The plasma membrane in eukaryotic cells is
responsible for controlling what gets into and
out of the cell.
A series of proteins stuck in the membrane
help the cell communicate with the
surrounding environment. Communication can
include Sending and receiving chemical
signals from other eukaryotic cells
Interacting with the cells of prokaryotic
organisms during the process of infection.
Eukaryotic ribosomes
•Eukaryotic ribosomes are larger and have a
slightly different shape and composition than
those found in prokaryotic cells.
•Eukaryotic ribosomes have twice the amount
of ribosomal RNA (rRNA) and 1/3rd more
ribosomal
proteins
than
prokaryotic
ribosomes.
•Despite these differences, the function of the
eukaryotic ribosome is virtually identical to the
prokaryotic version.
Eukaryotic Cytoplasm and Cytoskeleton
The cytoplasm in eukaryotic cells is a gel-like,
yet fluid, substance in which all of the other
cellular components are suspended, including all
of the organelles. The underlying structure and
function of the cytoplasm, and of the cell itself,
is largely determined by the cytoskeleton, a
protein framework along which particles in the
cell, including proteins, ribosomes, and
organelles,
move
around.
Mitochondria
They serve as power house of cell, and are surrounded by two
membranes.
They have their own genome.
They also divide independently of the cell in which they
reside, meaning mitochondrial replication is not coupled to
cell division.
Mitochondria
The inner membrane of mitochondria has
restricted permeability like the plasma
membrane of a cell.
The inner membrane is loaded with proteins
involved in electron transport and ATP
synthesis.
This membrane surrounds the mitochondrial
matrix, where the citric acid cycle produces
the electrons that travel from one protein
complex to the next in the inner membrane.
Mitochondrial genomes show a great
deal of variation as a result of divergent
evolution.
Mitochondrial genes conserved across
evolution include rRNA genes, tRNA
genes, and small number of genes that
encode proteins involved in electron
transport
and
ATP
synthesis.
The mitochondrial genome retains
similarity to its prokaryotic ancestor.
Mitochondrial rRNAs more closely
resemble bacterial rRNAs than the
eukaryotic rRNAs found in cell cytoplasm.
In addition, some of the codons that
mitochondria use to specify amino acids
differ from the standard eukaryotic
codons.
Cells have extensive sets of
intracellular
membranes,
which
together
compose
the
endomembrane
system.
The
endomembrane system was first
discovered in the late 1800s when
scientist Camillo Golgi noticed that a
certain stain selectively marked only
some internal cellular membranes.
Golgi thought that these intracellular
membranes were interconnected, but
advances in microscopy and biochemical
studies of the various membraneencased organelles later made it clear
the organelles in the endomembrane
system are separate compartments with
specific functions. These structures do
exchange membrane material, however,
via a special type of transport.
The
endomembrane
system
includes the
Endoplasmic reticulum(ER),
Golgi apparatus, and
Lysosomes,
Vesicles also allow the exchange
of membrane components with a
cell's plasma membrane.
The ER, Golgi apparatus, and
lysosomes are all members of a
network of membranes, but they are
not continuous with one another.
Therefore, the membrane lipids and
proteins that are synthesized in the ER
must be transported through the
network to their final destination in
membrane-bound vesicles.
Golgi Apparatus
The Golgi apparatus functions as a molecular
assembly line in which membrane proteins
undergo
extensive
post-translational
modification. Many Golgi reactions involve
the addition of sugar residues to membrane
proteins and secreted proteins. The
carbohydrates that the Golgi attaches to
membrane proteins are often quite complex,
and their synthesis requires multiple steps.
Golgi apparatus
In electron micrographs, the Golgi
apparatus looks like a set of flattened
sacs.
Vesicles that bud off from the ER fuse
with the closest Golgi membranes,
called the cis-Golgi.
Molecules then travel through the
Golgi apparatus via vesicle transport
until they reach the end of the
assembly line at the farthest sacs from
the ER — called the trans-Golgi.
At each workstation along the
assembly line, Golgi enzymes catalyze
distinct reactions. Later, as vesicles of
membrane lipids and proteins bud
off from the trans-Golgi, they are
directed to their appropriate
destinations — either lysosomes,
storage vesicles, or the plasma
membrane.
Cytoskeletal proteins
Cytoskeletal proteins are proteins that make up
the cytoskeleton, flagella or cilia of cells.
Generally, cytoskeletal proteins are polymers,
and include tubulin (the protein component of
microtubules), actin (the component of
microfilaments) and lamin (the component of
intermediate filaments).
Contractile proteins
Contractile proteins are proteins that
mediate sliding of contractile fibres
(contraction) of a cell’s cytoskeleton, and
of cardiac and skeletal muscle. Heart and
muscle contractile fibres consist of
bundles of actin polymers that slide
alongside each other by the activity of
the motor protein myosin and associated
contractile proteins such as troponin and
titin.
Actin
Actin is the most abundant protein in most
eukaryotic cells.
It is highly conserved and participates in more
protein-protein interactions than any known
protein.
These properties, along with its ability to
transition between monomeric (G-actin) and
filamentous (F-actin) states under the control of
nucleotide hydrolysis, ions, and a large number
of actin-binding proteins, make actin a critical
player in many cellular functions, ranging from
cell motility and the maintenance of cell shape
and polarity to the regulation of transcription.
Moreover, the interaction of filamentous actin
with myosin forms the basis of muscle
contraction. Owing to its central role in the cell,
the actin cytoskeleton is also disrupted or taken
over by numerous pathogens.
Structure of
Membrane
the
Plasma
Like all other cellular membranes, the plasma
membrane consists of both lipids and proteins.
The fundamental structure of the membrane is
the phospholipid bilayer which forms a stable
barrier between two aqueous compartments.
In the case of the plasma membrane, these
compartments are the inside and the outside of
the cell. Proteins embedded within the
phospholipid bilayer carry out the specific
functions of the plasma membrane, including
selective transport of molecules and cell-cell
recognition.
The plasma membranes of animal cells contain
four
major phospholipids(phosphatidylcholine, phosph
atidylethanolamine,
phosphatidylserine,
and sphingomyelin), which together account for
more than half of the lipid in most membranes.
These
phospholipids
are
asymmetrically
distributed between the two halves of the
membrane bilayer.
The outer leaflet of the plasma membrane consists
mainly of phosphatidylcholine and sphingomyelin,
whereas
phosphatidylethanolamine
and
phosphatidylserine
are
the
predominant
phospholipids of the inner leaflet.
A fifth phospholipid, phosphatidylinositol, is also
localized to the inner half of the plasma
membrane. Although phosphatidylinositol is a
quantitatively minor membrane component, it
plays an important role in cell signaling. The head
groups of both phosphatidylserine and
phosphatidylinositol are negatively charged, so
their predominance in the inner leaflet results in
a net negative charge on the cytosolic face of the
plasma membrane.
Phospholipid molecule
Head-Glycerol and phosphates,
Hydrophilic
Tail-Fatty acid chains, Hydrophhobic
In 1925, two Dutch scientists
(E. Gorter and R. Grendel) were
instrumental in finding that membranes
consist of double layers
Active transport
Active transport describes when a cell
uses energy to transport something. It deals with
the movement of individual molecules across
the cell membrane. The liquids inside and
outside of cells have different substances.
Sometimes a cell has to work and use some
energy to maintain a proper balance of ions and
molecules.
Active transport usually happens across
the cell membrane.
There are thousands of proteins
embedded in the cell's lipid bilayer. Those
proteins do much of the work in active
transport. They are positioned to cross the
membrane so one part is on the inside of the
cell and one part is on the outside.
 Only when they cross the bilayer are they
able to move molecules and ions in and out
of the cell.
The membrane proteins are very
specific. One protein that moves glucose
will not move calcium (Ca) ions. There
are hundreds of types of these
membrane proteins in the many cells of
our body.
Active transport requires energy and
work, passive transport does not.
The process of moving sodium and
potassium ions across the cell
membrance is an active transport
process involving the hydrolysis of ATP
to provide the necessary energy.
Exocytosis and endocytosis
The movement of macromolecules such
as proteins or polysaccharides into or out
of the cell is called bulk transport. There
are
two
types
of
bulk
transport: exocytosis and endocytosis,
and both require the expenditure of
energy (ATP).
Exocytosis
In exocytosis, materials are exported out of the cell
via secretory vesicles. In this process, the Golgi
complex packages macromolecules into transport
vesicles that travel to and fuse with the plasma
membrane. This fusion causes the vesicle to spill its
contents out of the cell.
Exocytosis is important in expulsion of waste
materials out of the cell and in the secretion of
cellular products such as digestive enzymes or
hormones.
Exocytosis
Exocytosis is a process in which an intracellular
vesicle (membrane bounded sphere) moves to the
plasma membrane and subsequent fusion of the
vesicular membrane and plasma membrane ensues.
Many cellular processes involve exocytosis. Examples of
few of the processes that use exocytosis are:
secretion of proteins like enzymes, peptide hormones
and antibodies from cells.
release of neurotransmitter from presynaptic neurons
placement of integral membrane proteins
acrosome reaction during fertilization
antigen presentation during the immune response
recycling of plasma membrane bound receptors
Endocytosis
Endocytosis, on the other hand, is the process by which
materials move into the cell. There are three types of
endocytosis:
1. Phagocytosis,
2. Pinocytosis, and
3. Receptor-mediated endocytosis.
In phagocytosis or “cellular eating,” the cell’s plasma
membrane surrounds a macromolecule or even an
entire cell from the extracellular environment and buds off
to form a food vacuole or phagosome. The newly-formed
phagosome then fuses with a lysosome whose hydrolytic
enzymes digest the “food” inside.
In pinocytosis or “cellular drinking,” the cell
engulfs drops of fluid by pinching in and
forming vesicles that are smaller than the
phagosomes formed in phagocytosis. Like
phagocytosis, pinocytosis is a non-specific
process in which the cell takes in whatever
solutes that are dissolved in the liquid it
envelops.
Unlike
phagocytosis
and
pinocytosis,
receptor-mediated
endocytosis is an extremely selective
process of importing materials into the cell.
This specificity is mediated by receptor
proteins located on depressed areas of the
cell membrane called coated pits.
The cytosolic surface of coated pits is
covered by coat proteins. In receptormediated endocytosis, the cell will only
take in an extracellular molecule if it binds to
its specific receptor protein on the cell’s
surface.
Similar to the digestive process in non-specific
phagocytosis, this coated vesicle then fuses with
a lysosome to digest the engulfed material and
release it into the cytosol.
Mammalian cells use receptor-mediated
endocytosis to take cholesterol into cells.
Cholesterol in the blood is usually found in lipidprotein
complexes
called
low-density
lipoproteins (LDLs).
LDLs bind to specific receptor proteins on the
cell surface, thereby triggering their uptake by
receptor-mediated endocytosis.
Lysosomes
Lysosomes are central, acidic and membrane
bound organelles that contain hydrolase
enzyme for the breakdown of all types of
biological polymers - proteins, nucleic acids,
carbohydrates and lipids. They are mostly found
in animal cells, while in yeast and plants, it acts
as lytic vacuoles. It is enclosed by membrane
known as lysosomal membrane that maintains
the digestive enzyme at pH 4.5.
Functions of lysosomes:
• Maintains pH by pumping protons from
cytosol across the membrane via proton
pumps and chloride ion channels.
• Protects the cytosol and rest of the cells
from degradative enzymes within the
lysosome.
• Acts as digestive system of the cell, serving
both to degrade material taken up from the
outside of the cell and to digest obsolete
components of cell itself.
• Sequestration of lysosomal enzymes.
• Mediation of fusion events between
lysosomes and other organelles.
• Transport of degradation products to the
cytoplasm
Lysosomal membrane
To perform its function with efficacy the lysosomal
membrane needs some additional features
in its membrane. It is slightly thicker than that of the
plasma membrane.
It contains substantial amounts of carbohydrate componen
, particularly sialic acid. In fact, most lysosomal membrane
proteins are highly glycosylated, which may help protect
them from the lysosomal proteases in the lumen.
The lysosomal membrane has another unique property of
fusing with other membranes of the cell.
The entire process of digestion is carried out within
the lysosome.
Most lysosomal enzymes act in an acid medium.
Acidification of lysosomal contents depends on an
ATP-dependent proton pump which is present in the
membrane of the lysosome and accumulates H+ inside
the organelle.
Lysosomal membrane also contains transport proteins
that allow the final products of digestion of
macromolecules to escape so that they
can be either excreted or reutilized by the cell.
Lysosomal
membrane
composition:
The V-class H+ ATPase pump is generally present in
lysosomal membrane. This class of ATPase pump only
transports H+ ions. Its main function is to acidify the
lumen of the organelles. The proton gradient between
the lysosomal lumen (pH ≈4.5–5.0) and the cytosol
(pH ≈7.0) depends on ATP production by the cell.
These V-class proton pumps contain two domains: a
cytosolic
hydrophilic
domain
(V1)
and
a
transmembrane domain (V0) with multiple subunits in
each domain. Binding and hydrolysis of ATP by the B
subunits in V1 provides the energy for pumping of
H+ ions through the proton-conducting channel
formed by the c and a subunits in V0. These V-class
proton pumps are not phosphorylated and
dephosphorylated during proton transport. Figure 2
CELL CYCLE
The cell cycle or cell-division cycle is the series of events that
take place in a cell leading to its division and duplication
(replication) that produces two daughter cells.
In prokaryotes which lack a cell nucleus, the cell cycle occurs
via a process termed binary fission.
In cells with a nucleus, as in eukaryotes, the cell cycle can be
divided into three phases: interphase, the mitotic (M) phase,
and cytokinesis.
During interphase, the cell grows, accumulating nutrients
needed for mitosis, preparing it for cell division
and duplicating its DNA. During the mitotic phase, the cell
splits itself into two distinct daughter cells. During the final
stage, cytokinesis, the new cell is completely divided. To
ensure the proper division of the cell, there are control
mechanisms known as cell cycle checkpoints.
Various phases
G0-A resting phase where the cell has left the cycle
and has stopped dividing
G1-Cells increase in size
S (Synthesis)–DNA replication takes place
G2-During the gap between DNA synthesis and mitosis
cells continue to grow. The G2 check point ensures
that it is ready to enter M-Phase
M-Cell growth stops and cellular energy is focused on
Orderly division into 2 daughter cells
Cancer
The fundamental abnormality resulting in the
development of cancer is the continual unregulated
proliferation of cancer cells.
 Rather than responding appropriately to the signals
that control normal cell behavior, cancer cells grow and
divide in an uncontrolled manner, invading normal
tissues and organs and eventually spreading throughout
the body.
The generalized loss of growth control exhibited by
cancer cells is the net result of accumulated
abnormalities in multiple cell regulatory systems and is
reflected in several aspects of cell behavior that
distinguish cancer cells from their normal counterparts.
Cancer
Cancer can result from abnormal proliferation
of any of the different kinds of cells in the body,
so there are more than a hundred distinct types
of cancer, which can vary substantially in their
behavior and response to treatment.
The most important issue in cancer pathology
is the distinction between benign and malignant
tumors .
A tumor is any abnormal proliferation of cells,
which may be either benign or malignant.
A benign tumor, such as a common skin wart, remains
confined to its original location, neither invading
surrounding normal tissue nor spreading to distant
body sites.
A malignant tumor however, is capable of both
invading surrounding normal tissue and spreading
throughout the body via the circulatory or lymphatic
systems (metastasis).
Only malignant tumors are properly referred to as
cancers, and it is their ability to invade and metastasize
that makes cancer so dangerous. Whereas benign
tumors can usually be removed surgically, the spread
of malignant tumors to distant body sites frequently
makes them resistant to such localized treatment.
Benign and malignant tumors
Both benign and malignant tumors are classified
according to the type of cell from which they
arise. Most cancers fall into one of three main
groups:
1. carcinomas –Malignancies of epithelial cells
(90%)
2. Sarcomas –solid tumors of connective tissues
3. leukemias or lymphomas- arise from the
blood-forming cells and from cells of the
immune system
Carcinomas, which include approximately 90% of
human cancers, are malignancies of epithelial
cells. Sarcomas, which are rare in humans, are
solid tumors of connective tissues, such as muscle,
bone,
cartilage,
and
fibrous
tissue. Leukemias and lymphomas, which account
for approximately 8% of human malignancies, arise
from the blood-forming cells and from cells of the
immune system, respectively. Tumors are further
classified according to tissue of origin (e.g., lung or
breast carcinomas) and the type of cell involved.
For example, fibrosarcomas arise from fibroblasts,
and erythroid leukemias from precursors
of erythrocytes (red blood cells).
Benign tumor
Benign tumors can be serious if they press on vital structures
such as blood vessels or nerves. Therefore, sometimes they
require treatment and other times they do not.
Causes of Benign Tumors
Often the cause is unknown. But the growth of a benign tumor
might be linked to:
Environmental toxins, such as exposure to radiation
Genetics
Diet
Stress
Local trauma or injury
Inflammation or infection
Source: National Cancer Institute, USA
Malignant tumors
•Malignant tumours vary in size and
shape. They grow in an uncontrolled,
abnormal way and can grow into (invade)
nearby tissues, blood vessels or lymphatic
vessels.
•They can interfere with body functions
and become life-threatening.
Source: Mayo clinic, USA
Mutagenesis
Mutagenesis is a process by which the
genetic information of an organism is
changed in a stable manner, resulting in
a mutation. It may occur spontaneously in
nature, or as a result of exposure
to mutagens. It can also be achieved
experimentally using laboratory procedures.
In nature, mutagenesis can lead to cancer
and various heritable diseases, but it is also
a driving force of evolution.
Tumor suppressor genes
The activation of cellular oncogenes represents only
one of two distinct types of genetic alterations involved
in tumor development; the other is inactivation of
tumor suppressor genes.
Oncogenes drive abnormal cell proliferation as a
consequence of genetic alterations that either
increase gene expression or lead to uncontrolled
activity of the oncogene encoded proteins. Tumor
suppressor genes represent the opposite side of cell
growth control, normally acting to inhibit cell
proliferation and tumor development. In many tumors,
these genes are lost or inactivated, thereby removing
negative regulators of cell proliferation and
contributing to the abnormal proliferation of tumor
cells.
An oncogene is a gene that has the
potential to cause cancer. In tumor cells, they
are often mutated or expressed at high levels.
Most normal cells will undergo a
programmed form of rapid cell death
(apoptosis) when critical functions are
altered. Activated oncogenes can cause those
cells designated for apoptosis to survive and
proliferate instead. Most oncogenes require
an additional step, such as mutations in
another gene, or environmental factors, such
as viral infection, to cause cancer.
Proto-oncogene
A proto-oncogene is a normal gene that can become
an oncogene due to mutations or increased expression.
The resultant protein encoded by an oncogene is termed
oncoprotein.
Proto-oncogenes code for proteins that help to
regulate cell growth and differentiation. Protooncogenes are often involved in signal transduction and
execution of mitogenic signals, usually through
their protein products.
Upon activation, a proto-oncogene (or its product)
becomes
a
tumor-inducing
agent,
an
oncogene.
Examples
of
proto-oncogenes
include RAS, WNT, MYC, ERK, and TRK.