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