Cholesterol Structure and Function in Cell Membranes

Overview

• Cholesterol is a critical component in animal cell membranes.
• Absent in plant cell membranes due to the presence of a cellulose cell wall.
• Recognizable by its unique four-ring structure.

Structure of Cholesterol

• Composition:
  • Four linked hydrocarbon rings forming the steroid portion.
  • Hydrocarbon chain extends from one end.
  • Hydroxyl group extends from the other end.
• Properties:
  • Amphipathic molecule:
    • Hydrophobic parts: Steroid portion and hydrocarbon tail.
    • Hydrophilic part: Hydroxyl group.

Position in the Cell Membrane

• Cholesterol is embedded in the phospholipid bilayer:
  • Hydroxyl group aligns with the hydrophilic phospholipid heads.
  • Hydrophobic parts (steroid rings and hydrocarbon chain) are embedded in the hydrophobic membrane portion with phospholipid fatty acid tails.

Functions of Cholesterol in the Cell Membrane

• Regulation of Fluidity:

  • Stabilizes the membrane, providing support in the absence of a cell wall.
  • Maintains membrane fluidity, but reduces overall fluidity to a balanced level.
  • Prevents phospholipids from stiffening or crystallizing.

• Regulation of Permeability:

  • Balances membrane permeability to prevent excessive molecule passage.
  • Ensures appropriate fluidity for controlled molecule passage into the cell.

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