Phospholipids & Glycolipids

Phospholipids & Glycolipids

These are complex lipids forming the structural backbone of all biological membranes. They are amphipathic molecules with hydrophilic head groups and hydrophobic fatty acid tails.

Phospholipids

Structure: Glycerol backbone + 2 fatty acids (esterified at C1 and C2) + Phosphate + Polar head group (at C3).

  • Phosphatidylcholine (Lecithin): Most abundant phospholipid. Major component of lung surfactant (dipalmitoylphosphatidylcholine — DPPC). Absent in premature infants → IRDS. Also major component of cell membranes and HDL particles.
  • Phosphatidylethanolamine (Cephalin): Found on inner leaflet of plasma membrane. Important in blood clotting.
  • Phosphatidylserine: Inner leaflet; when flipped to outer leaflet of apoptotic cells, signals phagocytosis.
  • Phosphatidylinositol: Minor component but extremely important in cell signaling. PIP2 cleaved by Phospholipase C → DAG (activates PKC) + IP3 (releases Ca²⁺ from ER).
  • Cardiolipin (diphosphatidylglycerol): Unique to inner mitochondrial membrane; also found in bacteria. Antigen for Wassermann reaction (syphilis test).
  • Plasmalogens: Vinyl ether at C1 instead of ester; abundant in heart, brain, skeletal muscle. Important in myelin.
  • PAF (Platelet Activating Factor): Ether at C1, acetyl at C2; potent mediator of anaphylaxis and inflammation.

Sphingolipids (Sphingomyelin)

Backbone: Sphingosine (not glycerol). Sphingosine + Fatty acid → Ceramide. Ceramide + Phosphocholine → Sphingomyelin. Found abundantly in myelin sheath (protects nerves) and brain. Deficiency of Sphingomyelinase → Niemann-Pick disease (sphingomyelin accumulation in liver, spleen, brain).

Glycolipids

Lipids with one or more sugar residues attached to Ceramide.

  • Cerebrosides: Ceramide + 1 sugar (Glucose → Glucocerebroside; Galactose → Galactocerebroside). Galactocerebroside is major glycolipid of myelin. Gaucher's disease: Glucocerebrosidase deficiency → glucocerebroside accumulates.
  • Sulfatides: Sulfated galactocerebroside; in myelin. Metachromatic leukodystrophy: Arylsulfatase A deficiency.
  • Gangliosides: Ceramide + oligosaccharide chain + sialic acid (N-acetylneuraminic acid, NANA). Found on neuronal cell surfaces; important for cell recognition. Tay-Sachs: Hexosaminidase A deficiency → GM2 ganglioside accumulates.

Phospholipid Metabolism

  • Phospholipase A1: Cleaves C1 fatty acid
  • Phospholipase A2: Cleaves C2 fatty acid (releases Arachidonic acid for eicosanoid synthesis). Inhibited by glucocorticoids (via Lipocortin/Annexin).
  • Phospholipase C: Cleaves head group releasing DAG + Phosphate head
  • Phospholipase D: Cleaves head group from phosphate, releasing PA (phosphatidic acid)

Cell Membrane Structure

Fluid mosaic model: Phospholipid bilayer with embedded proteins. Cholesterol modulates fluidity (too little = too fluid; too much = too rigid). Membrane is asymmetric: Phosphatidylcholine and sphingomyelin on outer leaflet; phosphatidylserine and PE on inner leaflet (maintained by flippases).

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

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❓ Frequently Asked Questions

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