Lipids

Lipids

Lipids are a heterogeneous group of naturally occurring organic compounds that are insoluble in water but soluble in non-polar solvents (ether, chloroform). They provide the highest caloric value: 9 kcal/g.

Classification

  • Simple Lipids: Esters of fatty acids with alcohols. Fats (glycerol ester), Waxes (long-chain alcohol ester).
  • Complex Lipids: Contain additional groups. Phospholipids (phosphate group — amphipathic, form membranes), Glycolipids (sugar group — in brain/myelin).
  • Derived Lipids: Steroids (cholesterol, bile acids, hormones), Fat-soluble vitamins (A, D, E, K), Eicosanoids.

Fatty Acids

Long hydrocarbon chains with a terminal carboxyl group. Saturated fatty acids (no double bonds) are solid at room temperature (e.g., Palmitic C16:0, Stearic C18:0). Unsaturated have double bonds in cis configuration (e.g., Oleic C18:1, Linoleic C18:2). Essential fatty acids: Linoleic (ω-6) and Linolenic (ω-3) — cannot be synthesized by humans.

Phospholipids

Backbone of all biological membranes. Amphipathic — hydrophilic head (phosphate + alcohol) and hydrophobic tails (2 fatty acids). Lecithin (phosphatidylcholine) is the most abundant; Surfactant (dipalmitoylphosphatidylcholine) prevents alveolar collapse — deficient in premature infants (Respiratory Distress Syndrome).

Steroids

All have a characteristic 4-ring sterane nucleus. Cholesterol is precursor for: steroid hormones (cortisol, aldosterone, testosterone, estrogen), bile acids, and Vitamin D. Statins inhibit HMG-CoA Reductase (rate-limiting step of cholesterol synthesis in liver).

Eicosanoids

20-carbon signaling molecules derived from Arachidonic acid (from membrane phospholipids via Phospholipase A2). Types: Prostaglandins (inflammation, pain), Thromboxanes (platelet aggregation), Leukotrienes (asthma, bronchospasm). NSAIDs inhibit COX enzymes blocking prostaglandin synthesis.

Clinical Significance

  • Tay-Sachs: Deficiency of Hexosaminidase A → GM2 ganglioside accumulation → neurodegeneration
  • Niemann-Pick: Deficiency of Sphingomyelinase → Sphingomyelin accumulates
  • Gaucher's: Deficiency of Glucocerebrosidase → most common lysosomal storage disease

Key Facts

  • Iodine number measures degree of unsaturation
  • Saponification number measures average fatty acid chain length
  • Trans fats raise LDL and lower HDL (atherogenic)
  • Brown adipose tissue — uncoupling protein (thermogenin) generates heat instead of ATP

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.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

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Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

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Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.