1 lipids

• Lipids are indispensable for cell structure and function.
• Unique nature of lipids

  • Hydrophobic
  • Nonpolar
• Triacylglycerols are the most abundant lipids comprising 85-90% of body lipids.
• Most of the triacylglycerols are stored in the adipose tissue and serve as energy reserve of the body.
• Fats are the fuel reserve of the body.
• Main reasons for fat being the fuel reserve

  • Highly concentrated form of energy
  • Non-polar and hydrophobic nature
• Triacylglycerols yield 9 Cal/g, in contrast to carbohydrates and proteins that produce only 4 Cal/g.
• One gram of glycogen combines with 2 g of water for storage.
• Long chain fatty acids are the ideal storage fuel reserves of the body.
• Fats can support the body's energy needs for long periods of food deprivation.
• In extreme cases, humans can fast and survive for 60-90 days, and obese persons can survive even longer (6 months to one year) without food.
• Other important body lipids

  • Phospholipids
  • Glycolipids
  • Cholesterol
• Insoluble lipids are solubilized in association with proteins to form lipoproteins for transport in the bloodstream.
• Free lipids are undetectable in blood.
• The plasma levels of lipids are often useful for assessing the health of individuals.
• Body lipids are continuously being degraded and resynthesized.
• Triacylglycerols transported from intestine and liver are stored in the adipose tissue.
• Triacylglycerol (TG) is the stored fat in the adipose tissue.
• Mobilization of fat from adipose tissue

  1. Hormone sensitive lipase removes fatty acid

  2. Forms diacylglycerol

  3. Cleavage by additional lipases

  4. Complete degradation to glycerol and free acids
• Lipolysis


  Complete degradation of triacylglycerol to glycerol and free acids
• Regulation of hormone-sensitive TG-lipase


  1. Inactive form 'b' activated by cAMP dependent protein kinase

  2. Hormones enhance lipase activity

  3. Insulin decreases cAMP levels and inactivates lipase

  4. Caffeine promotes lipolysis
• Increased levels of cAMP promote lipolysis.
• Lipolysis and lipogenesis are not simultaneously operative.
• The adipose tissue lacks the enzyme glycerol kinase.
• Glycerol produced in lipolysis cannot be phosphorylated in adipose tissue.
• Glycerol is transported to the liver where it is activated to glycerol-3-phosphate.
• Glycerol-3-phosphate may be used for the synthesis of triacylglycerols and phospholipids.