Download molecular organization of cell membrane

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‫االسراء اية ‪58‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Organization of Human Body
• The human body is made up of different systems
e.g. digestive system, whose functions are
coordinated and integrated together.
• Each system consists of many organs that made up
of many tissues of complementary functions.
• Each tissue consists of millions of similar cells.
• The cell is the basic unit of structure and function
in the body
• Each animal cell is surrounded by a cell membrane
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Cell (Plasma) Membrane
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Cell Membrane
• It is a very thin elastic semi-permeable
membrane (allowing some substances to pass
through it and prevent others) that surrounds
the cell
• It is about 7.5 nm (75 Ao = Angstrom = 10-10 of
meter) in thickness.
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Functions of Cell Membrane
1.
2.
3.
4.
5.
6.
It separates the cytoplasm from ECF.
It maintains cell's internal environment.
It transports of macromolecules into and out of
the cell.
It controls distribution of ions e.g. Na, K
extracellular ICF and ECF.
It generates transmembrane potentials.
It is contains receptors for hormones and
transmitter substances which are important for
signal transduction
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Biophysics , Abdelaziz Hussein
Components of Cell Membrane
• It is formed of 3 components;
1. Lipids 42%.
2. Proteins 55%.
3. Carbohydrates 3%.
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Cell Membrane Lipids
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Cell Membrane Lipids
•Form the basic structure of the membrane.
•Include mainly phospholipids, cholesterol, and glycolipids.
•Phospholipids are arranged into 2 layers (Lipid bilayer)
•Phospholipids molecules resemble the clothes-pin shape
•It consist of;
•Head (PO4 group) (face the ICF and ECF sides) relatively H2O
soluble (polar, hydrophilic).
•Tails (face each other in the interior of the bilayer) → relatively H2O
insoluble (non-polar, hydrophobic).
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Cell Membrane Proteins
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Functions of Cell Membrane Integral
Proteins
1. Integral or intrinsic
proteins→bind
to
hydrophobic center of
the lipid bilayer
2. Peripheral or extrinsic
proteins→ bind to the
hydrophilic polar heads
of the lipids or to the
integral proteins
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Functions of Cell Membrane Integral
Proteins
i)Transmembrane proteins →
span the entire bilayer which act
as:
a. Channels → for diffusion of
small, water–soluble substances.
b. Carriers actively transport
materials across the bilayer.
c. Pumps actively transport ions
across the bilayer.
d.Receptors initiate intracellular
reactions when activated.
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Functions of Cell Membrane Integral
Proteins
ii)Present only on one side of the membrane
They act as enzymes that activate or inactive metabolic
processes.
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Functions of Cell Membrane Peripheral
Proteins
The peripheral protein may be:
i) Peripheral proteins: bind to
the intracellular surface of the
membrane and contribute to the
cytoskeleton.
ii) Peripheral proteins: bind to
the extracellular surface of the
membrane and contribute to the
glycocalyx.
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Glycocalyx
•It is composed of glycolipids and
glycoproteins and covers the cell
membrane.
•Functions
1. It binds to the extracellular Ca to
stabilize membrane structures.
2. It acts as an attachment matrix for
other cells (cell adhesion
molecules).
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Transport through the Cell
Membrane
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Transport through the Cell Membrane
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Transport through the Cell Membrane
There are 3 mechanisms for transport of substances through cell
membranes;
a) Passive transport (Diffusion):
It is movement of substances across the cell membrane down
its electrochemical gradient.
b) Active transport:
It is movement of substances across the cell membrane against
its electrochemical gradient.
c) Vesicular transport:
It is the process by which large sized substances are engulfed by
the cell membrane to be either pushed inside the cell
(endocytosis) or pushed outside the cell (exocytosis)
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Diffusion
It is divided into 3 subtypes:
1.Simple diffusion.
2.Facilitated diffusion.
3.Osmosis (water only)
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Simple Diffusion
It is movement of substances across the cell membrane down its
electrochemical gradient by simple movement without the
necessity of binding with carrier proteins
Mechanisms:
It
occurs
either
through;
•The interstices of the
lipid bilayer.
•The watery proteins
channels.
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Simple Diffusion
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Simple Diffusion
Characters:
It is characterized by;
a) It occurs down an electrochemical gradient.
b) It is passive i.e. no
external energy is required.
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Simple Diffusion
Characters:
It is characterized by;
c) It is not rate-limiting i.e.
linear with concentration
gradients.
d) The diffusion process is not
saturable.
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Osmosis or Diffusion of Water
Water diffuses through lipid
bilayer of the membrane at
high rate like bullets due
to;
Its small size.
Its very high kinetic energy
Water moves from high to
low concentrations of water
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Facilitated Diffusion
It is the diffusion of substances across the cell membrane (down
electrochemical gradient) which needs the presence of carrier
proteins→ carrier mediated diffusion
Characters
i) It occurs down an electrochemical gradient
ii) It is passive i.e. no energy is required.
iii) It requires the presence of a transport carrier
protein.
iv) It is rate-limiting and saturable because it depends
on the availability of a definite number of carrier or
channel proteins
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II) Active Transport
Def.,
It is the movement of substances across the cell membranes
against an electrochemical gradient.
Types:
There are 2 subtypes of active transport:
i) Primary active transport → obtain its energy directly from the
hydrolysis of ATP e.g. Na-K Pump, Ca ATPase, H-K ATPase
ii) Secondary active transport → use the energy stored in
the Na concentration gradient e.g. Na-glucose co-transport
and Na-Ca exchange
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II) Active Transport
Characters
1. It occurs against the electrochemical gradient
2. It is active i.e. energy is required
3. It requires the presence of a transport carrier protein
4. It exhibits stereospecificity, saturation, and competition
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1ry Active Transport
Site
(Na-K ATPase)
Ca-ATPase
K-H-ATPase
Na-K pump
(Ca-pump)
(proton pump)
It presents in the
It
is
present
cell membranes.
sarcoplasmic
in It
presents
stomach
reticulum and cell cells
membranes.
Importance maintains
low It
intracellular
and
maintains
Na intracellular
in
parietal-
and
renal
distal tubular cells.
the It transports H ions
Ca into
the
lumen
high ions concentration against
intracellular K.
low.
its
electrochemical
gradient.
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2ry Active Transport
Na-glucose co-
Na-Ca Exchange
transport
Site
It presents in the cell It is present in ventricular
membranes.
ms cells
Importance Transport of Na & It transports Ca uphill from
glucose into the cells low ICF Ca to high ECF
Ca
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Differences ( ) simple diffusion, facilitated diffusion and
active transport
Simple diffusion
Facilitated diffusion
Active transport
Downhill
Downhill
Uphill
Energy
Doenot need energy
Doenot need energy
Needs energy
Rate
Not limited
Limited
Limited
Saturation
Not Saturable
Saturable
Saturable
Carrier
Doenot need carrier
Needs carrier
Needs carrier
Competition
Not show competition
Shows competition
Shows competition
Electrochemical
gradient
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III) Vesicular Transport
It is the mechanism by which the large sized substances can
cross the cell membranes
a) Endocytosis
The extracellular material is trapped within vesicles that are
formed by invagination of the cell membrane
It is active process which uses energy
It is 3 types
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III) Vesicular Transport
a) Exocytosis
The intracellular material is trapped within vesicles, then the
vesicles fuse with the cell membrane and release their
contents to the ECF
It is active process which uses energy
e.g. release of hormones, digestive enzymes,
and synaptic transmitters
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THANKS