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Red cell membrane – Normal composition and functions

Red cell membrane – Normal composition and functions

Red cell membrane – Normal composition and functions 

- 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 that facilitates gaseous exchange. 

- 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 are due to the design of its protein cytoskeleton and the way the cytoskeleton interacts with the membrane lipid bilayer. 

Functions of the red cell membrane 

- To separate the contents of the cell from the plasma. 

- To maintain the characteristic shape of the red cell. 

- To regulate intracellular cation concentration. 

- To act as the interface between the cell and its environment via membrane surface receptors. 

Red cell membrane 

- The red cell membrane consists of: 




Composition of the red cell membrane 


- They occur only on the external surface of the red cell. 

- They are composed of glycoproteins and glycolipids.  


- These are either: 

-Peripheral or 

-Integral, penetrating the lipid bilayer. 


Lipid components of the red cell membrane are: 

30% free unesterified cholesterol. 

10% glycerides and free fatty acids. 

60% phospholipids 

- All the lipids associated with red cells are present in the cell membrane. 

- The mature red cell cannot synthesize lipids, alteration in membrane lipid content can only occur by exchange with plasma lipids. 

- The red cell membrane phospholipids are Phosphatidylcholine, Phosphatidylethanolamine, Sphingomyelin, and Phosphatidylserine. 

- 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. 

- 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 

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 


- is the most abundant, consists of two chains, a and b, wound around each other to form heterodimers which then self-associate head to head to form tetramers. 

- These tetramers are 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. 

Integral Proteins 

- These penetrate the lipid bilayer and are firmly anchored within it via interactions with the core. 

-Band 3 (acts as an anion transport channel). 

-Glycophorins A, B, and C. 

-Na+/K+ ATPase. 

-glucose transport protein. 

-surface receptors. (the most important is the transferrin receptors). 

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 a principal binding site for cytoskeletal and other red cell proteins. 

Na+/K+ ATPase 

- This enzyme catalyzes the hydrolysis of ATP to ADP, liberating energy in this process. 

- Each ATP molecule hydrolyzed 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 

Blood group antigens are antigens located 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 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 

- 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.