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Cells
All living thing are made of cells , and cells are the smallest units that can
be alive.
Prokaryotic cells are smaller and simpler and far older and more diverse
than eukaryotic cells , and do not have a nucleus.
Bacteria cell ( Prokaryotic cell)
Animal cell ( Eukaryoticcell )
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Semi- autonomous organelles :
1 -Organelle which contain their own DNA and ribosome (70S ) .
2-Reproduce independently of the nucleus.
3–Such as Mitochondria
Mitochondria : these are sausage –shaped organelles , where aerobic
respiration takes place in Eukaryotic .
Endosymbiosis : (Probable evolution of Mitochondria)
It is thought that eukaryotic cell organelles like Mitochondria are derived
from prokaryotic cells that became incorporated inside larger prokaryotic
cells.
This idea is called Endosymbiosis.
And is supported by these observations :
1- Organelles contain circular DNA, like bacteria cells.
2- Contain ( 70s) ribosome , like bacteria cells.
3- Organelles have double membranes , as though a single –membrane
cell had been engulfed and surrounded by a larger cell.
4- Organelles reproduce by binary fission , like bacteria cells.
5- Organelles are very like some bacteria that are alive today.
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Cell wall :
The cell wall is the tough, usually flexible but sometimes fairly rigid layer
that surrounds some types of cells. It is located outside the cell membrane.
Cell walls are found in plants , bacteria fungi, algae, and some archaea.
Animals and protozoa do not have cell walls.
The material in the cell wall varies between :
1- species 2- differ depending on cell type 3- developmental stage .
In bacteria, (Prokaryotic cell) the cell wall forms frompeptidoglycan.
Archaean cell walls have various compositions, and may be formed of
glycoproteinS-layers, pseudopeptidoglycan, or polysaccharides.
Fungi possess cell walls made of the polysaccharides ,chitin.
Algae typically possess walls made of glycoproteins and polysaccharides .
Properties:
The cell wall serves a similar purpose in organisms that possess them.
Function of the cell wall is
1- provides these cells structural support and protection .
2- -addition to acting as filtering mechanism through limits the entry of
large molecules that may toxic to the cell
3- act as a pressure vessel preventing over-expansion when water enters
the cell.
4- gives cells rigidity and strength .
5- In multicellular organisms, it permits the organism to build and hold its
shape (morphogenesis) .
6- -It further permits the creation of a stable osmotic environment by
preventing osmotic lysis and helping to retain water .
The composition, properties, and form of the cell wall may change during
the :
1- cell cycle .
2- depend on growth conditions. .
•
Plant cell wall : Layers:
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Up to three strata or layers may be found in plant cell walls :
1-The middle lamella
2-The primary cell wall
3-The secondary cell wall
•
Fungi cell wall:
Most true fungi have a cell wall consisting largely of chitin and other
polysaccharides .
Not all species of fungi have cell walls but in those that do, the plasma
membrane is followed by three layers of cell wall material. From inside out
these are:
1-a layer of chitin .
2-a layer of zymosan.
3- a layer of mannoproteins .
•
Prokaryotic cell walls: Bacterial cell walls
There are broadly speaking two different types of cell wall in bacteria,
called Gram-positive and Gram-negative. The names originate from the
reaction of cells to the Gram stain.
Gram-positive bacteria possess a thick cell wall containing many layers of
peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have
a relatively thin cell wall consisting of a few layers of peptidoglycan
surrounded by a second lipid membrane containing lipopolysaccharides and
lipoproteins. Most bacteria have the Gram-negative cell wall .
Cells & Cell Organelles
Cells have 3 main jobs :
1- make energy (ATP) :
A- Need energy for all activities
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B-Need to clean up waste produced .
2-make proteins:
Proteins do all the work in a cell .
3- make more cells :
A- For growth.
B- To replace damaged or diseased cells .
All these job the Organelles do the work of cells , each structure has a job
to do, keeps the cell alive; keeps you alive.
1- Making energy :
To fuel daily life & growth, the cell must…
1-Take in food & digest it .
2- Take in oxygen (O2) .
3-Make ATP .
4-Remove waste .
Organelles that do this work…
1- Cell membrane , (surrounds the cytoplasm).
2- Lysosomes .
3-Vacuoles & vesicles .
4-Mitochondria .
Cell membrane :
The cell membrane (also known as the plasma membrane or cytoplasmic
membrane) is a biological membrane surrounds the cytoplasm of cells that
separates the interior of all cells from the outside environment(physically
separating the intracellular components from the extracellular environment).
The cell membrane is selectively permeable to ions and organic molecules
and controls the movement of substances in and out of cells. The movement
of substances across the membrane can be either "passive", occurring
without the input of cellular energy, or active, requiring the cell to expend
energy in transporting it.
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Structure :
Lipids:
Consist of two type :
1-Polar portion (hydrophilic)
2- Non-polar portion (hydrophobic)
Phospholipids:
Lipid vesicles or liposomes are circular pockets that are enclosed by a lipid
bilayer. These structures are used in laboratories to study the effects of
chemicals in cells by delivering these chemicals directly to the cell, as well
.as getting more insight into cell membrane permeability
Carbohydrates :
Plasma membranes also contain carbohydrates, predominantly
glycoproteins, but with some glycolipids (cerebrosides and gangliosides).
The glycocalyx is an important feature in all cells, especially epithelia with
microvilli. Recent data suggest the glycocalyx participates in cell adhesion.
Proteins :
The cell membrane has large content of proteins, typically around 50% of
membrane volume These proteins are important for cell because they are
responsible for various biological activities .
1- Trans-membrane proteins .
It's have many function such as composed of mechanical structure of
membrane , molecule carrier , transport channel.
2-Integral protein .
3-Peripherial protein.
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Illustration of a Eukaryotic cell membrane
Function :
The basic function of the cell membrane is to:
1- protect the cell from its surroundings ( It is consists of the lipid
bilayer with embedded proteins).
2- It is selectively permeable and able to regulate what enters and exits
from the cell (thus works as a selective filter that allows only certain
things to come inside or go outside the cell) .
3- It is facilitating the transport of materials needed for survival , and
also maintains the cell potential .
4- It is involved in a variety of cellular processes such as :
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A -cell
adhesion .
B- ion
conductivity .
C- cell
signaling .
Genetics
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D-
serve as the attachment surface for several extracellular structures:
including the cell wall , and intracellular cytoskeleton .
5- It is also plays a role in anchoring the cytoskeleton to provide shape to
the cell, and in attaching to the extracellular matrix and other cells to
help group cells together to form tissues.
Fluid mosaic model :
According to the fluid mosaic model of S.J. Singer and G.L. Nicolson
(1972), biological membranes can be considered as a two-dimensional
liquid in which lipid and protein molecules diffuse more or less easily.
Although the lipid bilayers that form the basis of the membranes do indeed
form two-dimensional liquids by themselves, the plasma membrane also
contains a large quantity of proteins, which provide more structure.
Lipid bilayer :
Diagram of the arrangement of amphipathic lipid molecules to form a lipid bilayer. The yellow polar
head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular
environments.
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Lipid bilayers form through the process of self-assembly:
1-
The cell membrane consists primarily of a thin layer of
amphipathicphospholipids which spontaneously arrange so that the
hydrophobic "tail" regions are isolated from the surrounding polar
fluid .
2-
causing the more hydrophilic "head" regions to associate with the
intracellular (cytosolic) and extracellular faces of the resulting bilayer
34-
This forms a continuous, spherical lipid bilayer.
The forces that contribute to the formation ofthe lipid bilayer such as:
A- van der Waals .
B- electrostatic .
C- hydrogen bonds .
D-non covalent interactions .
F- Overall, hydrophobic interactions are the major driving force in the
formation of lipid bilayers .
Substances movement across Cell Membranes :
Cell membranes are a barrier to most substances, and this property allows
materials to be concentrated inside cells, excluded from cells, or simply
separated from the outside environment . And there are four main methods
by which substances can move across a cell membrane:
1-Passive Transport .
A- Lipid Diffusion(or Simple Diffusion).
B- Facilitated diffusion .
2- Osmosis.
3- Active Transport (or Pumping) .
4- Vesicles .
A- . Endocytosis.
B- Exocytosis .
1- Passive transport
ASimple Diffusion
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Doesn’t require energy
Moves high to low concentrationgradient .
Lipid diffusion cannot be controlled by the cell .
Example: very small molecules, such as H2O, O2 and CO2. For these
molecules the membrane is no barrier at all.
B-
Facilitated diffusion :
Doesn’t require energy
Uses transport proteins to move substance from high to low
concentration .
The transport proteins tend to be specific for one molecule (a bit like
enzymes) , So substances can only cross a membrane if it contains the
appropriate protein .
Examples: Ions ,Glucose or amino acids moving from blood into a cell.
There are two kinds of transport protein:
1- Channel Proteins : form a water-filled pore or channel in the membrane.
This allows charged substances (usually ions) to diffuse across
membranes. Most channels can be gated (opened or closed), allowing the
cell to control the entry and exit of ions .
2- Carrier Proteins :have a binding site for a specific solute and constantly
flip between two states so that the site is alternately open to opposite sides
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of the membrane. The substance will bind on the side where it at a high
concentration and be released where it is at a low concentration .
Channel Proteins
Carrier Proteins
2-Osmosis
Osmosis is the diffusion of water across a membrane. It is in fact just
normal lipid diffusion, but since water is so important and so abundant in
cells , the diffusion of water has its own name - osmosis. The contents of
cells are essentially solutions of numerous different solutes, and the more
concentrated the solution, the more solute molecules there are in a given
volume, so the fewer water molecules there are. Water molecules can
diffuse freely across a membrane, but always down their concentration
gradient, so water therefore diffuses from a dilute to a concentrated
solution.
Osmotic Pressure (OP). This is an older term used to describe osmosis. The
more concentrated a solution, the higher the osmotic pressure. It therefore
means the opposite to water potential, and so water move from a low to a
high OP.
Cells and Osmosis. The concentration (or OP) of the solution that surrounds
a cell will affect the state of the cell, due to osmosis. There are three
possible concentrations of solution to consider :
•Isotonic solution
a cell
a solution of equal OP (or concentration) to
•Hypertonic solution
than a cell
a solution of higher OP (or concentration)
•Hypotonic solution
than a cell.
a solution of lower OP (or concentration)
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* Move the water across cell membrane through especial protein called
(Aquaporin) .
The effects of these solutions on cells are shown in this diagram:
3- Active Transport (or Pumping) :
Active transport is the pumping of substances across a membrane by a
trans-membrane protein pump molecule. The protein binds a molecule of
the substance to be transported on one side of the membrane, changes
shape, and releases it on the other side. The proteins are highly specific, so
there is a different protein pump for each molecule to be transported. The
protein pumps are also ATPase enzymes, since they catalyse the splitting of
ATP g ADP + phosphate (Pi), and use the energy released to change shape
and pump the molecule. Pumping is therefore an active process, and is the
only transport mechanism that can transport substances up their
concentration gradient .Examples: Pumping Na+ (sodium ions) out and K+
(potassium ions) in against strong concentration gradients. (Called Na+-K+
Pump ) .
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4-Vesicles :
The processes described so far only apply to small molecules. Large
molecules (such as proteins, polysaccharides and nucleotides) and even
whole cells are moved in and out of cells by using membrane vesicles.
Endocytosis :
Endocytosis is the process in which cells absorb molecules by engulfing
them. The plasma membrane creates a small deformation inward, called an
invagination, in which the substance to be transported is captured. The
deformation then pinches off from the membrane on the inside of the cell,
creating a vesicle containing the captured substance. Strictly speaking the
material has not yet crossed the membrane, so it is usually digested and the
small product molecules are absorbed by the methods above. When the
materials and the vesicles are small (such as a protein molecule) the process
is known as pinocytosis (cell drinking), and if the materials are large (such
as a white blood cell ingesting a bacterial cell) the process is known as
phagocytosis (cell eating). Endocytosis requires energy and is thus a form
of active transport.
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Endocytosis – Phagocytosis
•Exocytosis :
Just as material can be brought into the cell by invagination and formation
of a vesicle, the membrane of a vesicle can be fused with the plasma
membrane, extruding its contents to the surrounding medium. This is the
process of exocytosis. Exocytosis occurs in various cells to remove
undigested residues of substances brought in by endocytosis, to secrete
substances such as hormones and enzymes, and to transport a substance
completely across a cellular barrier. In the process of exocytosis, the
undigested waste-containing food vacuole or the secretory vesicle budded
from Golgi apparatus, is first moved by cytoskeleton from the interior of the
cell to the surface. The vesicle membrane comes in contact with the plasma
membrane. The lipid molecules of the two bilayers rearrange themselves
and the two membranes are, thus, fused. A passage is formed in the fused
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membrane and the vesicles discharges its contents outside the cell. is the
transport of materials out of a cell. It is the exact reverse of endocytosis .
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Cytoplasm
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:
The cytoplasm is the gel-like substance residing within the cell membrane
holding all the cell's internal sub-structures (called organelles), outside the
nucleus.
All the contents of the cells of prokaryote organisms (such as bacteria,
which lack a cell nucleus) are contained within the cytoplasm.
Within the cells of eukaryote organisms the contents of the cell nucleus
are separated from the cytoplasm, and are then called the nucleoplasm.
The cytoplasm is about 70% to 90% water and usually colourless.
It is within the cytoplasm that most cellular activities occur, such as many
metabolic pathways including :glycolysis, and processes such as cell
division.
The inner, granular mass is called the endoplasmand the outer, clear and
glassy layer is called the cell cortex orthe ectoplasm.
Constituents:
The cytoplasm has three major elements :
A-The cytosol .
B-Inclusions.
C- Organelles
A-The cytosol :
The part of the cytoplasm that is not held within organelles or It is the
portion of the cytoplasm not contained within membrane-bound
organelles, It is makes up about 70% of the cell volume .
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The cytosol is a complex mixture of :
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Cytoskeleton filaments .
Dissolved molecules .
Salts .
Organic molecules .
Small structures such as ribosomes .
Water that fills much of the volume of a cell .
Due to this network of fibers and high concentrations of dissolved
macromolecules, such as proteins, an effect called macromolecular
crowding occurs and the cytosol does not act as an ideal solution.
This crowding effect alters how the components of the cytosol interact
with each other .
B-The inclusions :
small particles of insoluble substances suspended in the cytosol. A huge
range of inclusions exist in different cell types, and range from crystals of
calcium oxalate or silicon dioxide in plants, to granules of energy-storage
materials such as starch, glycogen .
A particularly widespread example are lipid droplets, which are spherical
droplets composed of lipids and proteins that are used in both prokaryotes
and eukaryotes as a way of storing lipids such as fatty acids andsterols.
C- Organelles :
Organelles are a specialized subunit within a cell that has a specific
function, and is usually separately enclosed within its own lipid bilayer.
Some major organelles that are suspended in the cytosol are the
mitochondria, the endoplasmic reticulum, the Golgi apparatus, vacuoles,
lysosomes and nucleus.
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2-Lysosomes:
1- Lysosomes are involved in cellular digestion. Food can be taken from
outside the cell into food vacuoles by a process called endocytosis. These
food vacuoles fuse with lysosomes which break down the components so
that they can be used in the cell. This form of cellular eating is called
phagocytosis.
2- Lysosomes are also used to destroy defective or damaged organelles in a
process called endophagocytosis. They fuse with the membrane of the
damaged organelle digesting it.
3-
Vacuoles and Vesicles :
A- Vacuoles :
A vacuole is a membrane-bound organelle which is present in all plant
and fungal cells and some, animal and bacterial cells.
Vacuoles are essentially enclosed compartments which are filled with
water containing inorganic and organic molecules including enzymes in
solution called (cell sap ) , though in certain cases they may contain solids
which have been engulfed.
The organelle has no basic shape or size; its structure varies according to
the needs of the cell.
The function and importance of vacuoles varies greatly according to the
type of cell in which they are present .
B-
Vesicles:
In cell biology, a vesicle is a small bubble within a cell, and thus a type of
organelle. Enclosed by lipid bilayer, vesicles can form naturally, for
example, during endocytosis (protein absorption).
There are two kinds ofvesicles :
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A-Transport vesicles:
Transport vesicles can move molecules between locations inside the cell,
e.g., proteins from the rough endoplasmic reticulum to the Golgi
apparatus.
B- Secretory vesicles:
Secretory vesicles contain materials that are to be excreted from the cell.
Cells have many reasons to excrete materials.
1- To dispose of wastes.
2- Tied to the function of the cell.
3- Some cells are specialized to produce certain chemicals(Within a
larger organism.) . These chemicals are stored in secretory vesicles
and released when needed.
4- Mitochondrion:
In cell biology, a mitochondrion (plural mitochondria) is a membraneenclosed organelle found in most eukaryotic cells. Mitochondria are
sometimes described as "cellular power plants" because they-Generate
most of the cell's supply of adenosine triphosphate (ATP), used as a source
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of chemical energy. energy from sugar + O2
cellular respiration ).
ATP in process called,(
Involved in other tasks such :
1-as signaling
1- Cellular differentiation.
2- Cell death .
3- Control of the cell cycle .
4- Cell growth .
Although most of a cell's DNA is contained in the cell nucleus, the
mitochondrion has its own independent genome. Further, its DNA shows
substantial similarity to bacterial genomes .
Structure:
A mitochondrion contains outer and inner membranes composed of
phospholipid bilayers and proteins. The two membranes have different
properties. Because of this double-membraned organization, there are
five distinct parts to a mitochondrion. They are:
1-The outer mitochondrial membrane.
2-The intermembrane space (the space between the outer and inner
membranes).
3-The inner mitochondrial membrane.
4-The cristae space (formed by infoldings of the inner membrane).
5-The matrix (space within the inner membrane).
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Cell nucleus :
The nucleus is a membrane-enclosed organelle found in eukaryotic cells
and it is the largest cellular organelle in animals, In mammalian cells, the
average diameter of the nucleus is approximately 6micrometers (μm),
which occupies about 10% of the total cell volume. The viscous liquid within
it is called nucleoplasm is :
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Similar in composition to the cytosol found outside the nucleus.
It appears as a dense, roughly spherical organelle.
It contains most of the cell's genetic material,
Organized as multiple long linear DNA molecules in complex with a
large variety of proteins, such as histones, to form chromosomes.
The genes within these chromosomes are the cell's nuclear genome. The
main structures making up the nucleus are the nuclear envelope: a double
membrane that encloses the entire organelle and unifies its contents from
the cellular cytoplasm, and the nucleoskeleton : a mesh work within the
nucleus that adds mechanical support, much like the cytoskeleton, which
supports the cell as a whole .
Because the nuclear membrane is impermeable to large molecules,
nuclear pores are required to allow movement of molecules across the
envelope.
These pores cross both of the membranes, providing a channel that
allows free movement of small molecules and ions.
The movement of larger molecules such as proteins is carefullycontrolled,
and requires active transport regulated by carrier proteins. Nuclear
transport is crucial to cell function, as movement through the pores is
required for both :
1- gene expression 2- chromosomal maintenance.
The interior of the nucleus does not contain any membrane-boundsub
compartments, its contents are made up of :
1- unique proteins, 2- RNA molecules, 3- particular parts of the
chromosomes.
The best-known of these is the nucleolus .
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Function:
The main function of the cell nucleus is to :
1- Control gene expression .
2- Mediate the replication of DNA during the cell cycle.
3 - The nucleus provides a site for genetic transcription that is
segregated from the location of translation in the cytoplasm, allowing
levels of gene regulation that are not available to prokaryotes.
The nucleus is, therefore, the control center of the cell.
Nucleolus:
The nucleolus is a non-membrane bound structure composed of proteins
and nucleic acids (RAN) found within the nucleus of cells.Some types of
cells contain more than one nucleolus .
Its function : is to
1- Transcriberibosomal RNA (rRNA)
2- Assemble it within the cell.
3- which is mainly involved in the produced and assembly of
ribosomesare exported to the cytoplasm where they translate mRNA .
Structure :
Three major components of the nucleolus are recognized:
1- The fibrillar centers (FC).
2- The dense fibrillar components (DFC).
3- Granular components (GC).
However, it has been proposed that this
particular organization is only observed in
higher eukaryotes.
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Chromosomes :
The cell nucleus contains the majority of the cell's genetic material in the
form of multiple linear DNA molecules organized into structures called
chromosomes.
Each human cell contains 2m of DNA . During most of the cell cycle these
are organized in a DNA-protein complex known as chromatin, and during
cell division the chromatin can be seen to form the well-defined
chromosomes familiar from a karyotype.
A small fraction of the cell's genes are located instead in the
mitochondria.
There are two types of chromatin :
1- Euchromatin is the less compact DNA form, and contains genes that
are frequently expressed by the cell.
2- Heterochromatin, is the more compact form, and contains DNA that
are infrequently transcribed.
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Ribosome :
The ribosome is a large and complex molecular machine, found within all
livingcells, that serves as the primary site of biological protein synthesis.
Location:
Ribosomes are classified as being either "free" or "membrane-bound". Free
and membrane-bound ribosomes differ only in their spatial distribution; they
are identical in structure. Whether the ribosome exists in a free or membranebound state depends on the presence of an ER-targeting signal sequence
on the protein being synthesized.
so an individual ribosome might be membrane-bound when it is making one
protein, but free in the cytosol when it makes another protein.
Structure :
The ribosomal subunits of prokaryotes and eukaryotes are quite similar.
The unit of measurement is the Svedberg unit, a measure of the rate
ofsedimentationin centrifugation rather than size, and this accounts for why
fragment names do not add up (70S is made of 50S and 30S) .
Prokaryotes have 70S ribosomes, each consisting of a small (30S) and a large
(50S) subunit,
Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large
(60S) subunit.
The ribosomes found in chloroplasts and mitochondria of eukaryotes.
Ribosomes from bacteria, archaea and eukaryotes (the three domains of life
on Earth) differ in their : 1- size
2- sequence
3- structure
4- the ratio of
protein to RNA.
The differences in structure allow some antibiotics to kill bacteria by
inhibiting their ribosomes, while leaving human ribosomes unaffected.
Function :
Ribosomes are the workhorses of protein biosynthesis, the process of
translating mRNA into protein.
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Protein synthesis:
1- Process starts from DNA through “transcription”. by mRNA .
2- The mRNA comprises a series of codonsthat dictate to the ribosome
the sequence of the amino acids needed to make the protein.
3- Using the mRNA as a template.
4- The ribosome traverses each codon (3 nucleotides) of the mRNA,
pairing it with the appropriate amino acid provided by an aminoacyl-tRNA.
aminoacyl-tRNA contains a complementary anticodon on one end and
the appropriate amino acid on the other. (each tRNA has own
nucleotide triplet which binds to matching triplet on mRNA, ex., tRNA with
code AAA (triple adenine) would match up with mRNA that has code
UUU (triple uracil).
-The ribosome then contains three RNA binding sites :
1- The A site binds an aminoacyl -tRNA;
2- The P site binds a peptidyl-tRNA (a tRNA bound to the peptide
beingsynthesized) , mRNA binds to the P site of the ribosome first.
3- The E site binds a free tRNA before it exits the ribosome.
Ribosomes consist of two major subunits--the small ribosomal subunit
readsthe mRNA while, the large subunit joins amino acids to form a
polypeptide chain.
Each subunit is composed of one or more ribosomal RNA (rRNA)
molecules and a variety of proteins (ribonucleoprotein).
Protein synthesis begins at a start codon AUG (methionine ) near 5 end of the
mRNA , the first phase of translation called Initiation .
Stop
or nonsense codon ( UAA , UAG , UGA ),
this is
third phase of
translation called End of translation .
Ribosomes link amino acids together in the order specified by messenger
RNA(mRNA) molecules.
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Amino acids are selected, collected and carried to the ribosome by transfer
RNA (tRNA molecules), which enter one part of the ribosome and bind to the
messenger RNA chain.
The attached amino acids are then linked together by another , this is
second phase of translation called Elongation.
{The ribosome assembles polymericproteinmolecules whose sequence is controlled by the sequence of
messenger RNA molecules}
In bacteria and archaea, more than one ribosome may move along a single
mRNA chain at one time, each "reading" its sequence and producing a
corresponding protein molecule.
The ribosomes in the mitochondria of eukaryotic cells functionally resemble in
many features those in bacteria, reflecting the likely evolutionary origin of
mitochondria .
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Endoplasmic reticulum:
Endoplasmic
reticulum
a
type
of organelle in
the cells of eukaryoticorganisms that forms an interconnected network of
flattened , membrane -enclosed sacs or tube like structures known
as cisternae. These sac-like structures are held together by the
cytoskeleton.
ER are continuous with the outer membrane of the nuclear envelope.
Endoplasmic reticulum occurs in most types of eukaryotic cells, but is
absent from red blood cells and spermatozoa.
There are two types of endoplasmic reticulum, rough and smooth. The
outer (cytosolic) face of the rough endoplasmic reticulum is studded
with ribosomes that are the sites of protein synthesis. The rough
endoplasmic reticulum is Increase the proportion of its presence in the
stomach lining cells and endocrine cells, as these cells are responsible for
the secretion of enzymes and hormones
Sooth endoplasmic reticulum especially prominent in cells such
as hepatocytes. It happen conversion of glucose into glycogen stores in
the liver. Shift some toxic compounds to less toxic compounds.
The functions : of the endoplasmic reticulum can be summarized : (- as
the synthesis and export of proteins and membrane lipids ), but varies
between ER and cell type and cell function.
The quantity of both rough and smooth endoplasmic reticulum in a cell
can slowly interchange from one type to the other, depending on the
changing metabolic activities ofthe cell .
Rough endoplasmic reticulum :
The surface of the rough endoplasmic reticulum (RER) is studded with
protein-manufacturing ribosomes giving it a "rough" appearance (hence its
name).
The binding site of the Ribosome on RER is the translocon. However, the
ribosomes bound to the RER at any one time are not a stable part of this
organelle's structure as ribosomes are constantly being bound and
released from the membrane
The membrane of the RER forms large double membrane sheets that are
located near, and continuous with the outer layer of the nuclear
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Pathological Analysis Dep
Genetics
AFAQTALIB
envelope. Although there is no continuous membrane between the
RERand the Golgi apparatus, membrane-bound vesicles shuttle proteins
between these two compartments.
rough endoplasmic reticulum is involved in the :
1-Synthesis of proteinssuch as : lysosomal enzymes and also add
Integral membrane proteins
2- A membrane factory for the cell .
Smooth endoplasmic reticulum :
The smooth endoplasmic reticulumconsists of tubules that are located
near the cell periphery. These tubes sometimes branch forming a network
that is reticular in appearance.
functions in several metabolic processes. It : synthesizes lipids,
phospholipids, and steroids. Cells which secrete these products, such as
those in the testes, ovaries, and sebaceous glands have an abundance of
smooth endoplasmic reticulum. It also carries out the metabolism
ofcarbohydrates, drug detoxification, attachment of receptorson
cellmembrane proteins, and steroid metabolism.
In muscle cells, it regulates calcium ion concentration.
Smooth endoplasmic reticulum is found in a variety of cell types (both
animal and plant).
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College of Science
Pathological Analysis Dep
Genetics
AFAQTALIB
Illustration of a endoplasmic reticulum (Rough and Smooth ).
Golgi apparatus:
The Golgi apparatus, also known as the Golgi complex, Golgi body, or
simply the Golgi, is an organelle found in most eukaryoticcells. Golgi
apparatus complex is usually located near the cell nucleus, close to the
centrosome ,Part of the cellular endomembrane system.Found within the
cytoplasm of both plant and animal cells,
Structure :
the Golgi is composed of stacks of membrane-bound structures known as
cisternae (singular: cisterna).
Function :
Cells synthesize a large number of different macromolecules.
The Golgi apparatus is : packages proteins inside the cell before they are
sent to their destination; it is particularly important in the processing of
proteins for secretion.
1- integral in modifying 2- sorting 3- packaging these macromolecules
for cell secretion (exocytosis) oruse within the cell.
1-It primarily modifies proteins delivered from the rough endoplasmic
reticulum.
2- Involved in the transport of lipids around the cell.
3- The creation of lysosomes .
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College of Science
Pathological Analysis Dep
Genetics
AFAQTALIB
The endomembrane system :
The endomembrane system is composed of the different membranes
that are suspended in the cytoplasm within a eukaryotic cell.
These membranes divide the cell into functional and structural
compartments, or organelles.
In eukaryotes the organelles of the endomembrane system include:
1 The-nuclear membrane 2- The endoplasmic reticulum 3- The Golgi
apparatus 4- Lysosomes, 5- Vesicles 6- Vacuoles 7- The cell membrane.
The system is defined more accurately as the: set of membranes that
form a single functional and developmental unit, either being connected
directly, or exchanging material through vesicle transport .
Importantly, the endomembrane system does not include the membranes
of mitochondria or chloroplasts .
The organelles of the endomembrane system are related through direct
contact or by the transfer of membrane segments as vesicles.
Despite these relationships, the various membranes are not identical in
structure and function. 1- The thickness, 2- molecular composition, 3metabolic behavior of a membrane are not fixed, they may be modified
several times during the membrane's life.
One unifying characteristic the membranes share is a lipid bilayer, with
proteins attached to either side or traversing them .
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College of Science
Pathological Analysis Dep
Genetics
AFAQTALIB
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