Lipid Classification and Chemistry

Lipid Classification - Greasy Groups

• Simple Lipids: Esters of fatty acids (FAs) with alcohols.
  • Fats & Oils (TAGs): FAs + glycerol. Primary energy storage.
  • Waxes: FAs + long-chain monohydric alcohols. Protective.
• Complex Lipids: FAs + alcohol + additional groups (e.g., phosphate, carbohydrate).
  • Phospholipids: + Phosphate. Cell membranes (e.g., Lecithin, Sphingomyelin).
  • Glycolipids: + Carbohydrate + sphingosine. Cell recognition (e.g., Cerebrosides).
  • Lipoproteins: Lipids + apolipoproteins (e.g., LDL, HDL). Blood lipid transport.
• Derived Lipids: Hydrolysis products or precursors.
  • FAs, glycerol, steroids (cholesterol, hormones), ketone bodies, fat-soluble vitamins (A,D,E,K).

⭐ Dipalmitoylphosphatidylcholine (DPPC), a lecithin, is the main lipid in lung surfactant, preventing alveolar collapse (atelectasis).

Fatty Acids - Chain Champions

• Carboxylic acids with hydrocarbon chains ($R-COOH$). Amphipathic.
• Chain Length: Short (<6C), Medium (6-12C), Long (14-20C), Very Long (>20C).
• Saturation:
  • Saturated (SFA): No C=C. E.g., Palmitic (16C). Solid.
  • Unsaturated (UFA): $\ge$1 C=C. Liquid.
    • MUFA: 1 C=C. E.g., Oleic (18:1 $\Delta$9).
    • PUFA: $\ge$2 C=C. E.g., Linoleic (18:2 $\Delta$9,12), Linolenic (18:3 $\Delta$9,12,15).
• Nomenclature: $\Delta$ from $COOH$; $\omega$ from $CH_3$ end.
• Essential (EFAs): Linoleic ($\omega$-6), $\alpha$-Linolenic ($\omega$-3). 📌 LA ($\omega$-6), ALA ($\omega$-3).
• Properties: MP: SFA > UFA. MP ↑ chain length, ↓ C=C. Natural UFAs: cis.

⭐ Arachidonic acid (20:4 $\omega$-6), derived from Linoleic acid, is a key precursor for prostaglandins, thromboxanes, and leukotrienes.

Glycerides & Complex Lipids - Structure Stars

• Glycerides (Acylglycerols): Esters: glycerol & fatty acids.
  • Triglycerides (TAGs): Glycerol + 3 FAs. Energy store; neutral.
  • Saponification: TAG + Alkali $\rightarrow$ Glycerol + Soap.
• Complex Lipids: Contain additional groups (e.g., $PO_4^{3-}$, carbohydrate).
  • Phospholipids: Amphipathic; $PO_4^{3-}$-containing. Membrane lipids.
    • Glycerophospholipids: Glycerol, 2 FAs, $PO_4^{3-}$, alcohol. Parent: Phosphatidic acid (DAG + $PO_4^{3-}$). Types: Lecithin (lung surfactant), Cephalin, Phosphatidylserine (apoptosis), Phosphatidylinositol (PIP2), Cardiolipin (inner mito. membrane).
    • Sphingophospholipids: Sphingosine, 1 FA, $PO_4^{3-}$, choline. Type: Sphingomyelin (ceramide + phosphocholine - myelin).
  • Glycolipids (Glycosphingolipids): Sphingosine, FA, carbohydrate. No $PO_4^{3-}$. Cell recognition.
    • Ceramide: Sphingosine + FA.
    • Cerebrosides: Ceramide + monosaccharide (e.g., Galactocerebroside).
    • Gangliosides: Ceramide + oligosaccharide + NANA (e.g., GM1).

      ⭐ Sphingomyelin: only major membrane phospholipid not from glycerol.

Steroids & Properties - Ring Leaders & Reactions

• Derived lipids, built on cyclopentanoperhydrophenanthrene (CPPP) or gonane nucleus (17C, 4 fused rings).
• Cholesterol: Key animal sterol; C3-OH group (amphipathic), vital precursor.
• Properties:
  • Generally solid alcohols (sterols if C3-OH present).
  • Amphipathic: polar -OH head, nonpolar hydrocarbon tail & ring system.
• Key Reactions (Cholesterol):
  • Esterification: C3-OH + Fatty Acid $\rightarrow$ Cholesterol Ester.
  • Salkowski Test: Conc. $H_2SO_4$ $\rightarrow$ Red/Purple layers.
  • Liebermann-Burchard Test: Acetic anhydride + Conc. $H_2SO_4$ $\rightarrow$ Emerald green color.

⭐ Cholesterol is the parent compound for all steroid hormones (e.g., cortisol, aldosterone, testosterone, estrogen), bile acids, and Vitamin D.

• 📌 CPPP Ring: 17C, 4 fused rings (3 Hex: A,B,C; 1 Pent: D).

High‑Yield Points - ⚡ Biggest Takeaways

• Simple lipids: Fatty acid esters with glycerol (fats) or other alcohols (waxes).
• Complex lipids: Fatty acid esters with alcohols plus additional groups (e.g., phosphate in phospholipids, carbohydrate in glycolipids).
• Derived lipids: Include fatty acids, cholesterol, steroid hormones, and eicosanoids.
• Fatty acids: Saturated (no double bonds, e.g., palmitic acid) vs. Unsaturated (with double bonds, e.g., oleic acid).
• Essential fatty acids: Linoleic acid (ω-6) and α-linolenic acid (ω-3) are indispensable dietary components.
• Phospholipids (e.g., lecithin, sphingomyelin) are key structural components of cell membranes.
• Cholesterol: Vital for membrane fluidity; precursor to bile acids, steroid hormones, and Vitamin D.