Red Cell Membrane Structure and Function

Red Cell Membrane

• The red blood cell is a specialized eukaryotic cell, unique for lacking cytoplasmic organelles and nuclei.
• It's essentially a hemoglobin-filled sac.
• The cell's small size (8 microns) and need to navigate narrow capillaries (3 microns) necessitates its deformability, which prevents fragmentation.

Red Cell Membrane Structure

• Lipid Bilayer (40%): Composed of 20% phospholipids and 20% cholesterol.
• Membrane Proteins (52%): Include both peripheral and integral proteins.
• Carbohydrates (8%): Located only on the outer surface.

Membrane Lipids

• The membrane's lipids consist of phospholipids, cholesterol, and glycolipids.
• Phospholipids form a bilayer, asymmetrically arranged.
• Outer layer lipids: phosphatidylcholine and sphingomyelin (uncharged).
• Inner layer lipids: phosphatidylethanolamine, phosphatidylinositol, and phosphatidylserine.

Phospholipid Transport

• The asymmetric phospholipid distribution is crucial for cell function and maintained by proteins:
  • Flippases: move phospholipids from the outer to the inner monolayer.
  • Floppases: move phospholipids from the inner to the outer monolayer.

Importance of Asymmetric Distribution

• Macrophages recognize and remove red cells with exposed phosphatidyserine (PS).
• Premature destruction of red cells (e.g., sickle or thalassemia) can stem from disrupted lipid asymmetry, resulting in PS exposure.
• Exposure of PS can cause the red blood cells to adhere to endothelial cells, impairing their transport through the microvasculature

Cholesterol

• Unesterified cholesterol resides between the phospholipid layers.
• Mature red blood cells cannot synthesize their own lipids, so they rely on exchange between plasma and membrane lipids.

Membrane Proteins

• Integral Proteins are embedded within the lipid bilayer. Examples include:
  • Band 3
  • Glycophorin
  • Aquaporin
• Peripheral Proteins are located on the cytoplasmic surface, forming the membrane skeleton. This skeleton provides structural integrity. Examples include:
  • Spectrin
  • Actin
  • Protein 4.1
  • Pallidin (band 4.2)
  • Ankyrin
  • Adducin
  • Tropomyosin
  • Tropomodulin

Membrane Protein Function

• Membrane skeleton proteins give red blood cells their flexibility and deformability. This enables them to squeeze through capillaries.
• They recover their normal discoid shape easily.
• Over 50 proteins exist in the membrane with about 25 carrying blood group antigens (e.g., A, B, Rh).

Protein Connections

• Vertical Connections: Beta chain of spectrin, ankyrin, band 3, protein 4.2.
• Horizontal Connections: Alpha and beta spectrin chains, protein 4.1, actin, tropomyosin.

Analyzing Membrane Proteins

• Membrane proteins can be separated by size using SDS-PAGE.
• Visualization of the separated bands is possible using stains like Coomassie blue.
• On electron microscopy, these proteins form a hexagonal lattice.

Red Cell Membrane Functions

• Mechanical Strength and Flexibility: The membrane withstanding shearing forces in the circulatory system.
• Volume Regulation: Maintained through cation pumps (Na+/K+ ATPase), regulating sodium and potassium levels.
• Calcium Control: Maintained at a very low level via membrane ATPase.
• Anion Transport: Band 3 protein facilitating bicarbonate (HCO3-) exchange with chloride (Cl-) ions.

Membrane Interactions and Other Functions

• Adhesion and Interactions: Interact with other cells, specifically endothelial cells.
• Signaling Receptors: Participate in cell signaling
• Specific Transporters: Include Diego blood group antigen, Aquaporin 1 (water transport), Glut 1 (glucose and other molecules), and Kidd (urea) and Rh (gas transporter) antigens.

Biochemistry of Red Blood Cells

• Red blood cells need energy to maintain their structure, active transport (cations), and functions (e.g., nucleotide salvage and glutathione synthesis).

Explore the fascinating structure and functionality of red blood cell membranes in this quiz. Learn about the unique composition, including lipids and proteins, and discover how these elements contribute to the cell's deformability and role in circulation.