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Unit-2: Lipids

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1. Introduction to Lipids

  • Lipids are a diverse group of organic molecules that are insoluble in water but soluble in non-polar solvents (like ether, chloroform).
  • They include:
    • Fats and oils
    • Phospholipids
    • Steroids (including cholesterol and sex hormones)
    • Waxes

Functions:

  • Major source of energy
  • Structural components of cell membranes
  • Serve as signaling molecules in biological systems

2. Fatty Acids

Definition:

  • Organic molecules consisting of a carboxyl group (-COOH) attached to a long hydrocarbon chain.
  • Essential components of lipids involved in energy storage, membrane structure, and cellular signaling.

3. Classification of Fatty Acids

A. Based on Saturation:

  • Saturated Fatty Acids (SFA):
    • No double bonds
    • Solid at room temperature
    • Example: Palmitic acid, Stearic acid
  • Unsaturated Fatty Acids:
    • One or more double bonds
    • Monounsaturated Fatty Acids (MUFA):
      • One double bond
      • Found in olive oil, avocado, nuts
      • Example: Oleic acid
    • Polyunsaturated Fatty Acids (PUFA):
      • Two or more double bonds
      • Found in fish oil, flaxseeds, walnuts
      • Examples: Linoleic acid, Alpha-linolenic acid

B. Based on Essentiality:

  • Essential Fatty Acids (EFA):
    • Cannot be synthesized by the body; must be obtained from the diet
    • Examples:
      • Linoleic acid (Omega-6)
      • Alpha-linolenic acid (Omega-3)
  • Non-Essential Fatty Acids:
    • Can be synthesized by the body

4. Clinical Significance

Essential Fatty Acids:

  • Omega-3 (ALA, EPA, DHA):
    • Anti-inflammatory
    • Supports heart and brain health
  • Omega-6 (Linoleic, Arachidonic acid):
    • Supports skin, immune function
    • Excessive intake may cause inflammation

Deficiency may cause:

  • Dry skin
  • Poor wound healing
  • Cognitive decline

Monounsaturated Fats (MUFA):

  • Increase HDL (good cholesterol), reduce LDL (bad cholesterol)
  • Found in olive oil, avocados, nuts

Polyunsaturated Fats (PUFA):

  • Important for brain function and cardiovascular health
  • Includes Omega-3 and Omega-6

Trans Fatty Acids:

  • Unsaturated fats with at least one trans double bond
  • Found in processed foods, margarine, baked goods
  • Associated with:
    • Increased LDL, decreased HDL
    • Inflammation, diabetes, heart disease

WHO recommends eliminating trans fats from diets for better public health

5. Digestion, Absorption & Metabolism of Lipids

A. Digestion of Lipids

  1. Mouth:
    • Lingual lipase (active mainly in infants)
    • Minimal digestion
  2. Stomach:
    • Gastric lipase
    • Acts mainly on short- and medium-chain triglycerides
  3. Small Intestine:
    • Bile salts (from liver/gallbladder) emulsify fats
    • Pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids
    • Colipase stabilizes pancreatic lipase
    • Phospholipase A2 digests phospholipids
    • Cholesterol esterase hydrolyzes cholesterol esters

B. Absorption of Lipids

  • Lipid digestion products form micelles with bile salts
  • Absorbed by enterocytes in the intestine
  • Inside enterocytes:
    • Lipids are re-esterified
    • Packaged into chylomicrons
    • Chylomicrons enter lymph, then bloodstream
  • Short- and medium-chain fatty acids directly enter portal circulation

6. Lipid Transport

  • Chylomicrons (from intestines): Transport dietary triglycerides to tissues
  • VLDL (Very Low-Density Lipoprotein): Carries triglycerides from liver to tissues
  • LDL (Low-Density Lipoprotein): Delivers cholesterol to cells (high levels = atherosclerosis)
  • HDL (High-Density Lipoprotein): Removes excess cholesterol, returns to liver

7. Lipid Utilization

  1. β-oxidation (Fatty Acid Breakdown):
    • Occurs in mitochondria
    • Produces acetyl-CoA, NADH, and FADH₂ (used for ATP synthesis)
  2. Ketogenesis (in Liver):
    • Happens during fasting or diabetes
    • Produces ketone bodies (acetoacetate, β-hydroxybutyrate) for energy
  3. Lipogenesis (Fat Synthesis):
    • Excess carbohydrates → converted into fats
    • Occurs mainly in liver and adipose tissue