2.9 Membrane Lipids: Cholesterol

• Cholesterol

  • C^27 steroid molecule and precursor for other steroid-based lipids.
  • A specific compound rather than a family of compounds like the phospholipids and sphingoglycolipids.
  • structure differs markedly from that of other membrane lipids in that:(1) there are no fatty acid residues present and (2) neither glycerol or sphingosine is present as the platform molecule.
• Lipid whose structure is based on a fused-ring system of three 6-membered rings and one 5-membered ring.

  A) Steroid

  
• • Cholesterol–protein. A protein carrier system is used for the distribution of cholesterol via the bloodstream. 

  A) Lipoprotein

  
• 2 types of lipoproteins:

  1. Low - density lipoproteins (LDLs)
  2. High - density lipoproteins (HDLs)
• Low - density lipoproteins (LDLs)

  • Carry cholesterol from the liver to various tissues.
  • "Bad cholesterol" because it contributes to increased blood cholesterol levels.
• High - density lipoproteins (HDLs)

  • Carry excess cholesterol from tissues back to the liver.
  • “Good cholesterol” because it contributes to reduced blood cholesterol levels.
• Cholesterol in Food

  • Liver synthesizes almost 1 gram of cholesterol everyday.
  • Cholesterol ingestion decreases biosynthetic cholesterol production.
  • Animal food contains lots of cholesterol.
  • Plant food contains negligible amounts of cholesterol.