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‫الزي هذاوا لهزا وما كىا لىهخذي لىال أن هذاوا اهلل‬
‫والصالة والسالم على أششف األوبٍاء‬
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‫‪222Cell Biology‬‬
Lecture 6
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Osmosis
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Osmosis : is the diffusion of water across a membrane
It is crucial for cells that water moves across their membrane.
Water moves across membranes in response to solute
concentration inside and outside of the cell by a process
called osmosis
Osmosis will move water across a membrane down its
concentration gradient until the concentration of solute is
equal on both sides of the membrane
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Osmosis, the diffusion of water across
a membrane
Lower concentration of solute, Higher concentration of
solute, Equal concentration of solute
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Types of biological solution
Tonicity is a term that describes the ability of a solution to cause
a cell to gain or lose water
 Tonicity is dependent on the concentration of a non penetrating
solute on both sides of a membrane
1- Isotonic solution: indicates that the concentration of a solute
of a cell and its isotonic environment are equal and the cell
gains and loses water at the same rate.
2- Hypertonic solution indicates that the concentration of
solute is higher than that inside a cell.
3- Hypotonic solution is a solution with a solute
concentration lower than that of a cell.
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How animal and plant cells behave in different
solutions
Isotonic solution Hypotonic solution Hypertonic solution
Animal
cell
A) Normal ً‫طبٍع‬
(B) Lysed ‫ححللج‬
(C) Shriveled ‫ربلج‬
Plant
cell
(D) Flaccid ‫حشهلج‬
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F) Shriveled ‫ربلج‬
(E) Turgid ‫ إوخفخج‬plasmolyzed ‫مبلزمت‬
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Cell junctions
A) Tight junctions or occludens :
1) No spaces between membrane of adjacent cells
2) Tightly pressed against each other to prevent leakage of fluid.
e.g. cells of digestive tract ( intestinal cells)
B) Anchoring junctions or desmosomes
Each desmosome is made of regions of dense material on the
cytoplasmic sites of the 2 plasma membranes, plus protein filaments in
the narrow intercellular space between them. Desmosomes are
anchored to intermediate filaments inside the cells. e.g. cells of
epidermis of skin
C) Gap junctions
They are communicating junctions. They allow small molecules (ions) to
flow through protein pores between cells
e.g. heart muscle cells . In plant cells are called
‘’Plasmodesmata’’
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Three types of cell junctions in animal
tissues
Tight junctions
‫االحصاالث المحكمت‬
Anchoring junction
‫االحصاالث المثبخت‬
Gap junctions
‫االحصاالث الثغشٌت‬
Plasma membranes
of adjacent cells
Extracellular matrix
‫المادة الخاسج خلىٌت‬
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Modification of cell membranes
1) Modification of cell membrane for protection and insulation
as myelin sheath which surrounds axon and is a modification of Schwann
cells
2) The cell membrane of RBCs is highly selective to allow exchange of
gases through it and is plastic to allow RBCs to be squeezed inside
narrow capillaries.
3) Photoreceptors of retina which are sensory receptors detect and
transmit light to the brain .
4) Microvilli : Projections of cell membranes, they increase surface areas
of cells e.g. in cells of intestine.
5) Presence of basal and lateral infolding of cell membranes as in cells of
kidney tubules.
6) Formation of pseudopodia as in amoeba and WBCs.
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Na+/K+ pump
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Membranes proteins called sodium-potassium
pumps actively transport Na+ out of the cell and K+
inside, helping keep the concentration of Na+ low in
the cell and K+ high.
The membrane potential can change from its resting
value if the membrane’s permeability to a
particular ions changes.
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Na+/K+ pump
Neuron
Axon
Plasma
membrane
Outside of cell
K+
Na+
Na+
channel
K+ channel
Na+-K+
pump
Protein
Inside of cell
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Causes of resting membrane potential
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Na-K pump gates are working pumping 3Na+ ions
out & allow 2K+ ions to pass inside
K ions diffuses out through passive K gates creating
an electrochemical gradient (+ve out & -ve in).
A nerve signal begins as a change in the membrane
potential :
A stimulus is any factor that causes a nerve signal
to be generated. e.g. light, sound. If a stimulus is
strong enough, a neuron fires a nerve impulse or
action potential
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Mitochondria
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Mitochondria
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Mitochondria are the energy factories of the cells.
The energy currency for the work that animals must do is the
energy-rich molecule adenosine triphosphate (ATP)
The ATP is produced in the mitochondria using energy stored
in food
Just as the chloroplasts in plants act as sugar factories for the
supply of ordered molecules to the plant
The mitochondria in animals and plants act to produce the
ordered ATP molecules as the energy supply for the processes
of life.
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Mitochondria
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A typical animal cell will have on the order of 1000 to 2000
mitochondria
So the cell will have a lot of structures that are capable of
producing a high amount of available energy
This ATP production by the mitochondria is done by the
process of respiration, which in essence is the use of oxygen
in a process which generates energy.
This is a very efficient process for using food energy to make
ATP.
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Mitochondria
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All living cells have mitochondria
Hair cells and outer skin cells are dead cells and no longer
actively producing ATP, but all cells have the same structure.
Some cells have more mitochondria than others.
Your fat cells have many mitochondria because they store a
lot of energy
Muscle cells have many mitochondria, which allows them to
respond quickly to the need for doing work
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Structure
A mitochondria contains outer and inner membranes
composed of phospholipid bilayers and proteins
 The two membranes, however, have different properties
Because of this double-membraned organization, there are
five distinct compartments within the mitochondrion:
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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Outer membrane
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The outer mitochondrial membrane, which encloses the entire
organelle, has a protein-to-phospholipid ratio similar to that of the
eukaryotic plasma membrane.
It contains large numbers of integral proteins called porins.
These porins form channels that allow molecules 5000 Daltons or
less in molecular weight to freely diffuse from one side of the
membrane to the other.
Larger proteins can enter the mitochondrion if binds to a large
multisubunit protein called translocase of the outer membrane,
which then actively moves them across the membrane.
The mitochondrial outer membrane can associate with the ER
This is important in ER-mitochondria calcium signaling and involved
in the transfer of lipids between the ER and mitochondria.
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Intermembrane space
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The intermembrane space is the space between the outer
membrane and the inner membrane.
Because the outer membrane is freely permeable to small
molecules, the concentrations of small molecules such as ions and
sugars in the intermembrane space is the same as the cytosol.
However, as large proteins must have a specific signaling
sequence to be transported across the outer membrane
The protein composition of this space is different than the protein
composition of the cytosol.
One protein that is localized to the intermembrane space in this way
is cytochrome c
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Inner membrane
The inner mitochondrial membrane contains proteins with five types of
functions:
1-Those that perform the redox reactions of oxidative phosphorylation
2- ATP synthase, which generates ATP in the matrix
3-Specific transport proteins that regulate metabolite passage into and out of
the matrix
4-Protein import machinery.
5-Mitochondria fusion and fission protein
 The inner membrane is home to around 1/5 of the total protein in a
mitochondrion.
 The inner membrane is rich in an unusual phospholipid, cardiolipin
 Cardiolipin contains four fatty acids rather than two and may help to
make the inner membrane impermeable
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Cristae
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The inner mitochondrial membrane is compartmentalized into numerous
cristae , which expand the surface area of the inner mitochondrial
membrane, enhancing its ability to produce ATP.
These are not simple random folds, which can affect overall chemiosmotic
function
In typical liver mitochondria, for example, the surface area, including
cristae, is about five times that of the outer membrane.
Mitochondria of cells that have greater demand for ATP, such as muscle
cells, contain more cristae than typical liver mitochondria.
These folds are studded with small round bodies known as F1 particles or
oxysomes
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Cross-sectional image of cristae in rat liver mitochondrion to
demonstrate the likely 3D structure and relationship to the
inner membrane
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Matrix
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The matrix is the space enclosed by the inner membrane
It contains about 2/3 of the total protein in a mitochondrion.
The matrix is important in the production of ATP with the aid of the
ATP synthase contained in the inner membrane.
The matrix contains a highly-concentrated mixture of hundreds of
enzymes, special mitochondrial ribosomes , tRNA and several copies
of the mitochondrial DNA genome
Of the enzymes, the major functions include oxidation of pyruvate
and fatty acids, and the citric acid cycle
Mitochondria have their own genetic material, and the machinery to
manufacture their own RNAs and proteins
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Growth and Development
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Mitochondria arise by division and growth of preexisting
mitochondria. Because they synthesize only a few proteins
and RNA molecules
They must import many proteins and other materials from
the cytoplasm.
A mitochondrion contains at least 100 proteins that are
encoded by nuclear genes
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