Question

The erythrocyte membrane consists of different phospholipids that are asymmetrically distributed across the membrane. Explain the different lipid composition of the inner and outer leaflets of the erythrocyte membrane. Which leaflet would you expect to be the most permeable to Na+ or Cl– ions? Explain your answer.

Answer 1

Answer:

• Head group superlattices play a central role in the maintenance of the phospholipid composition of the inner and outer leaflets of the erythrocyte membrane.

Head group SL are classified on two assumptions:

1) phospholipid head groups represent the basic lattice elements.

2) phospholipid species with an identical or similar polar head group are considered equivalent.

• Based on that, the choline phospholipids (CPs) - phosphatidylcholine (PC) and sphingomyelin (SM) form one class.
• The acidic phospholipids (APs) - phosphatidylserine (PS), phosphatidylinositol (PI), and phosphatidic acid (PA) form another and phosphatidylethanolamine (PE) alone forms the third class.
• The head group composition of the outer leaflet of the erythrocyte membrane contains two different head group classes while the inner leaflet contains three head group classes.
• The 2 head group classes of outer leaflet are choline phospholipids (CPs) and acidic phospholipids(APs) while 3 head group composition of inner leaflet contain choline phospholipids (CPs), acidic phospholipids(APs) and phosphatidylethanolamine (PE).
• K+ and organic anions are present at higher concentrations inside the cell than outside.
• In contrast, N+ and Cl− are usually present at higher concentrations outside the cell.
• Therefore outer leaflet expect to be the most permeable to Na+ or Cl- ions.
• As they are charged ions they can't pass directly through the hydrophobic lipid regions of the membrane.
• So they have to use specialized channel proteins that provide a hydrophilic tunnel across the membrane.

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