Which of the following membrane components is found in the lowest abundance on the outer leaflet of the eukaryotic plasma membrane (especially in human red blood cells)? A. phosphatidylcholine B. glycolipids C. phosphatidylserine D. cholesterol E. sphingomyelin
The correct answer and explanation is:
Correct Answer: C. Phosphatidylserine
Explanation (300 words):
The eukaryotic plasma membrane, including that of human red blood cells, is asymmetrical in the distribution of its lipid components across the inner (cytoplasmic) and outer (extracellular) leaflets. Each side has a unique lipid composition that reflects its specific functions.
Let’s examine the key lipid types:
- Phosphatidylcholine (PC): This is one of the most abundant phospholipids in the plasma membrane and is found primarily on the outer leaflet, facing the extracellular space.
- Glycolipids: These lipids, which have sugar groups attached, are also mostly found on the outer leaflet. They play critical roles in cell-cell recognition, signaling, and interactions with the environment.
- Sphingomyelin: Like PC, sphingomyelin is mainly located in the outer leaflet and is involved in forming lipid rafts, which are platforms for signaling molecules.
- Cholesterol: Cholesterol is distributed evenly between both leaflets and plays a structural role by modulating membrane fluidity and permeability.
- Phosphatidylserine (PS): This phospholipid is almost exclusively found on the inner leaflet of the plasma membrane under normal conditions. Its presence on the outer leaflet is minimal in healthy cells. However, when cells undergo apoptosis (programmed cell death), PS becomes exposed on the outer leaflet as a signal for phagocytosis.
Because phosphatidylserine (PS) is actively kept on the inner leaflet by enzymes such as flippases, its abundance on the outer leaflet is extremely low compared to other membrane components.
Therefore, among the listed options, phosphatidylserine (C) is the component found in the lowest abundance on the outer leaflet of the eukaryotic plasma membrane, particularly in red blood cells.
Conclusion:
✅ Correct answer: C. Phosphatidylserine
This reflects the asymmetric lipid organization essential to normal cell function and signaling.