III) Cell membrane
(Plasma membrane)
Physiology -I
PHL 215
Dr/ Gamal Gabr
Pharmacy College
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III) Plasma membrane
 Plasma membrane is like a gate
It allow certain substances to enter or leave
the cell.

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• Plasma membrane is a thin, flexible outer
barrier that separates the interior of the cell
from the outside environment.
o Thickness: 8–10 nanometers (nm).
 1 nm = 1/1 000 000 mm
• Plasma membrane is a selectively permeable
membrane = It controls the movement of
substances in and out of cells.
o What chemical materials are plasma
membranes made of?
 It made of proteins floating in a double layer of
phospholipids, which are fluid at body
temperature (like vegetable oil).
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 Composition of plasma membrane
• The plasma membrane is composed
of different types of macromolecules.
•
The most common molecules are:
Lipids
72 %
Proteins Carbohydrates
25%
3 %
Phospholipids:
❶ Lipids
❷ Proteins
❸ Carbohydrates.
55%
Cholesterol:
13%
Glycolipids:
4%
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Fluid-mosaic model
The fluid-mosaic model describes the plasma membrane
of animal cells.
 Fluid:
Because the plasma membrane has the consistency of
vegetable oil at body temperature .
 So, the molecules of lipids and proteins can move
side-to-side within the same layer.
 Mosaic:
Because proteins and other substances such as
cholesterol are distributes in the bilayer, giving the
membrane the look of a mosaic.
o What factors determine fluidity of the cell
membrane?
 Temperature, lipid composition and cholesterol
content
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❶ Membrane lipids
• Lipid molecules = 3 major classes:
• Phospholipids (55%) + Cholesterol (13%) + Glycolipids (4%).

So, the major lipid molecules are Phospholipids.
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 Phospholipids
o Each phospholipid molecule has:
a) Phosphorylated glycerol head that is attracted to water. i.e.
hydrophilic OR water soluble.
b) 2 fatty acid tails that repels water. i.e. hydrophobic OR fat soluble
NOTES:
o Hydrophilic means water loving = Water soluble OR Polar.
o Hydrophobic means water fearing = Fat soluble OR nonpolar.
o




Molecules having both hydrophilic and hydrophobic properties
are called amphipathic. Accordingly, the phospholipid molecules
are considered as amphipathic molecules.
Phosphorylated
glycerol head
Two fatty acid
tails
Which part of a phospholipid molecule is polar and thus
considered hydrophilic?
the phosphate glycerol head
Which part of a phospholipid molecule is nonpolar and thus
is considered hydrophobic?
the 2 fatty acid tails
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 Phospholipids
Lipids bilayer …… Why?
1- Heads of the phospholipid molecules are
hydrophilic, So attracted to water and directed into:
A- ICF (inner surface of the membrane)
B- ECF (outer surface of the membrane).
2- Tails of the phospholipid molecules are
hydrophobic , So repelled by water and arranged tail
to tail forming a hydrophobic region in the center of
the membrane.
• The result = Lipid bilayer
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 Phospholipids
Functions: (The lipid bilayer)
1- Forms a barrier so protects the cell
2- Selectively permeable, so controls the
movement of substances into and out of the
cell:
a) Hydrophobic substances as oxygen, carbon
dioxide, and alcohol can diffuse easily.
b) Hydrophilic substances as
ions, glucose, and
urea can not diffuse easily.
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❷ Membrane Proteins
• The major protein molecules are Glycoproteins.
NOTE:
At physiological temperatures, the lipid bilayer is fluid, so proteins can move
side-to-side within the same layer, but can’t flip from one layer to the other
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Membrane proteins = 2 major types:
A- Integral proteins OR transmembrane proteins
B- Peripheral proteins
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A) Integral proteins:
 They crosses the lipid bilayer of the membrane with one end outside the cell
and the other end inside.
 Integral proteins are not fixed, so they moves through the membrane very
quickly.
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Each integral protein has 3 types of amino acids:
1- Hydrophilic cytosolic amino acids, in contact with
Hydrophilic region
of protein
ICF (Inner surface).
2- Hydrophobic central amino acids in contact with the
fatty acids in the centre of the membrane.
3- Hydrophilic extracellular amino acids in contact
with ECF (Outer surface).
Integral proteins = 3 types:
1- Transport proteins
a) Channel
b) Carrier
Hydrophobic region of protein
proteins
proteins
2- Marker proteins
3- Receptor proteins.
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1- Transport proteins:
• They regulate the movement of substances through the plasma
membrane.
Types:
a- Channel proteins:
 Channel proteins transport water soluble substances, especially the
ions or very small molecules from one side of the membrane to the
other side according to concentration
 this process is known as diffusion
b- Carrier proteins:
 Carrier proteins change their shape to transport certain molecules
from one side of the membrane to the other side against concentration
 this process is known as active transport.
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2- Marker proteins:
 Marker proteins are specific for each organism =
fingerprints.
 They identify cells to the body's immune system.
 They play an important role in organ transplantation.
3- Receptor proteins:
 They have specific binding sites that respond to
hormones, antigens and neurotransmitters.
 They play an important role in intercellular
communication.
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B) Peripheral proteins:
• They does not penetrate the plasma membrane.
• Usually: found on the inside of the membrane.
• They attach the membrane to the cytoskeleton, so keep the shape of the cell.
• They function as enzymes  catalyze some metabolic reactions which occur near
the plasma membrane.
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Functions of membrane proteins:
• Control the movement of substances through cell membranes
 Diffusion & Active transport
• Identification of cells to the body's immune system.
• Act as receptors.
• Act as enzymes.
• Attach the membrane to the cytoskeleton, so keeping the shape of the cell.
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❸ Membrane Carbohydrates:
•
Carbohydrate chains attach to the outer surface of
the plasma membrane of each cell forming a loose
coat called glycocalyx.
•
They attached to :
Proteins
 Glycoproteins
phospholipids  Glycolipids.
OR
•
Glycocalyx of some cells attaches to glycocalyx of
other cells, thus attaching the cells to each other.
•
Many of the carbohydrates act as receptors, as they
bind with some hormones as insulin.
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Function of Carbohydrates:
• Attaches the cells to each others.
• Act as receptors for some hormones.
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Functions of plasma membrane:
1- Protects the cell
2- Controls the movement of substances into and out of the cell (Selectively
permeable)
3- Identification of cells to the body's immune system (marker protein).
4- Communication with other cells and the outside world (receptor protein).
5- keep the shape of the cell.
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