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Red cell membrane
Dr. Suhair Abbas Ahmed
objectives
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After studying this lecture you should be able to:
1-List the main functions of the red cell membrane.
2-Outline the composition of the red cell
membrane.
3-Differentiate between the integral and peripheral
proteins in the red cell membrane.
4-Describe the structure and composition of the
red cell cytoskeleton.
5-Outline the ABO and Rh systems.
Red cell membrane
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The primary function of the red cell
is the transport of respiratory gases
to and from the tissues.
To achieve this task the red cell
should be capable of traversing the
microvascular system without
mechanical damage, and that the
cell should retain a shape which
facilitates gaseous exchange.
Red cell membrane
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In order to meet with the demands of
function, the red cell membrane should be
extremely tough yet highly flexible.
This strength and flexibility of the red cell
membrane is due to the design of its
protein cytoskeleton and the way the
cytoskeleton interacts with the membrane
lipid bilayer.
Functions of red cell membrane
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To separate the contents of the cell
from the plasma.
To maintain the characteristic shape
of the red cell.
To regulate intracellular cation
conc.
To act as the interface between the
cell and its environment via
membrane surface receptors.
Red cell membrane
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The red cell membrane consists of:
Proteins~50%
Lipids~40%
Carbohydrates~10%
Composition of the red cell membrane
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CARBOHYDRATES
They occur only on the external
surface of the red cell.
They are composed of glycoproteins
and glycolipids.
Composition of the red cell
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PROTEINS
These are either:
-Peripheral or
-Integral, penetrating the lipid
bilayer.
Composition of the RBC memb. (cont.)
LIPIDS
Lipid components of the red cell
membrane are:
30% free unesterified cholesterol.
10% glycerides and free fatty
acids.
60% phospholipids
Red cell membrane lipids
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All of the lipids associated with red
cells is present in the cell
membrane.
The mature red cell has no capacity
to synthesize lipids, alteration in
membrane lipid content can only
occur by exchange with plasma
lipids.
Lipids (cont)
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The red cell membrane phospholipids, are:
Phosphatidyl choline, Phosphatidyl ethanolamine,
Sphingomyelin and Phosphatidyl serine.
Phospholipid molecules are characterized by a polar
head group attached to a non-polar fatty acid tail.
The polar head group is hydrophilic ( water loving),
while the fatty acid tail is hydrophobic (water fearing).
Thus the phospholipids in the cell membrane tend to
arrange themselves in a bilayer with their hydrophilic
heads pointing towards the inner and outer aqueous
phases ( the cytoplasm and plasma), while the
hydrophobic tails point towards each other.
Lipids (cont)
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The membrane cholesterol is unesterified
and lies between the two layers of the
lipid bilayer.
The concentration of cholesterol in the
membrane is an important determinant of
membrane surface area and fluidity: an
increase in membrane cholesterol leads to
an increased surface area and decreased
deformability.
Red cell membrane proteins
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PERIPHERAL PROTEINS
The red cell peripheral proteins interact to form a
cytoskeleton.
The cytoskeleton acts as a tough supporting
framework for the lipid bilayer.
Four peripheral proteins play a key role in the
structure of the red cell cytoskeleton, these are :
- Spectrin, which is the most abundant and
consists of two chains a and b spectrin.
- Ankyrin
- Protein 4.1 and
- Actin
Red cell membrane proteins
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SPECTRIN
is the most abundant, consists of two
chains, a and b, wound around each other
to form heterodimers which then selfassociate head to head to form tetramers.
These tetramersare linked at the tail end
to actin and are attached to protein band
4.1.
At the head end, the b-spectrin chains
attach to ankyrin which connects to band
3(anion channel). Protein 4.2 enhances
this interaction.
Red cell membrane proteins
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INTEGRAL PROTEINS
These penetrate the lipid bilayer and are
firmly anchored within it via interactions
with the core.
-Band 3 (acts as anion transport
channel).
-Glycophorins A, B, and C.
-Na+/K+ ATPase.
-glucose transport protein.
-surface receptors. (the most
important is the transferrin receptors).
Red cell membrane proteins
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BAND 3
One of the integral proteins
Has two major functions within the red
cell membrane:
1- Its primary function is to facilitate
anion transport via the red cell
membrane.
2- It is an important binding site for
cytoskeletal and other red cell proteins.
Red cell membrane proteins
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Na+/K+ ATPase
This enzyme catalyses the
hydrolysis of ATP to ADP, liberating
energy in this process.
Each ATP molecule hydrolysed via
this system results in the ejection of
three Na+ ions from the cell and the
transport of two K+ ions into the
cell.
Blood group antigens
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Blood group antigens: are antigens found on the
red cell membrane, they are responsible for the
determination of the blood group of the individual.
Blood group antigens are found on both the
protein and the carbohydrate components of the
membrane glycoproteins and the glycolipids.
The Rh and Fya antigens are considered as minor
membrane proteins.
The MN and Ss are found on larger membrane
proteins.
Some blood group antigens are absorbed
passively from the plasma, e.g. Lewis, Chido and
Rogers.
Defects of the red cell membrane
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Defects of the proteins may explain some
of the abnormalities of the shape of the
red cell membrane, e.g. hereditary
spherocytosis and elliptocytosis.
While alterations in lipid composition
because of congenital or acquired
abnormalities in plasma cholesterol or
phospholipids may be associated with
other membrane abnormalities, e.g.
target cells and acanthocytes.