Lipid Structure and Function

5.1 Structural Lipids

This section describes structural lipids, including phospholipids, glycerophospholipids, sphingolipids, and waxes, focusing on their structure, saturation, and functional roles in membranes and protection.

• Lipids: insoluble in water.
• Phospholipid bilayer = amphipathic: polar head, hydrophobic tail.
• Head group: phosphate + alcohol, phosphodiester linked to fatty acid tail.
• Glycerol: 3C alcohol → phosphoglycerides/glycerophospholipids.
• Sphingolipids: sphingosine backbone.
• Tail varies by saturation and length:
  • Saturated: single bonds → more stable.
  • Unsaturated: double bonds → liquid at room temp.
• Glycerophospholipids
  • All glycerophospholipids are phospholipids; not all phospholipids are glycerophospholipids.
  • Glycerol backbone + 2 fatty acids.
  • Named for head group:
    • Phosphatidylcholine → choline
    • Phosphatidylethanolamine → ethanolamine
  • Head group charges vary; chains vary in length/saturation.
• Sphingolipids
  • Surface cell antigens (ABO blood typing).
  • Sphingosine/sphingoid backbone.
  • Some are phospholipids (phosphodiester linkage); some glycolipids (glycosidic linkage).
  • Four main subclasses:
    • Ceramide: single H as head group.
    • Sphingomyelins: phosphatidylcholine/ethanolamine head; only phospholipid sphingolipid; no net charge; plasma/myelin component.
    • Glycosphingolipids: sugar head groups; no phosphate; outer membrane surface.
      • Cerebrosides = single sugar
      • Globosides = 2+ sugars
      • Neutral glycolipids: no net charge at pH 7.4
    • Gangliosides: oligosaccharide + N-acetylneuraminic acid (NANA); negative charge; cell interaction, recognition, signal transduction.
• Sphingolipid Accumulation
  • Niemann-Pick disease: intellectual disability, seizures.
  • Sulfatides: sulfated cerebrosides; associated with Alzheimer’s.
• Waxes
  • Esters of long fatty acids + long chain alcohols.
  • Pliable solids at room temp.
  • Functions: protection, prevent evaporation, shield against parasites.
  • Examples: carnauba wax (candies, cars), beeswax (shelter).

5.2 Signaling Lipids

Signaling lipids include terpenes, terpenoids, steroids, prostaglandins, and fat-soluble vitamins, which serve as precursors, hormones, and regulators in the body.

• Terpenes and Terpenoids
  • Terpenes: odiferous metabolic precursors to steroids; built from isoprene (C5H8) units; produced by plants/insects; part of defense and scents.
  • Classification by isoprene units:
    • Monoterpenes: 2 units → essential oils, turpentine
    • Sesquiterpenes: 3 units
    • Diterpenes: 4 units → Vitamin A (retinal)
    • Triterpenes: 6 units → cholesterol, steroids
    • Tetraterpenes: 8 units → carotenoids
    • Polyterpenes: natural rubber (1000–5000 units)
  • Terpenoids: oxygenated derivatives; contribute to steroid biosynthesis, aromas.
• Steroids
  • Metabolic derivatives of terpenes; 4 fused cycloalkane rings (3 cyclohexanes + 1 cyclopentane).
  • Steroid hormones: endocrine secretion, protein carriers (testosterone, estrogen, cortisol, aldosterone).
  • Cholesterol: membrane fluidity, steroid/bile acid/vitamin D precursor.

  

• Prostaglandins
  • 20 C unsaturated carboxylic acids from arachidonic acid; 1 5C ring.
  • Paracrine/autocrine signaling; regulates cAMP, smooth muscle, sleep-wake, body temp.
  • NSAIDs inhibit COX → reduce prostaglandin synthesis.
• Fat-Soluble Vitamins
  • Vitamin A: carotene → retinal (vision); retinol storage; retinoic acid regulates gene expression.
  • Vitamin D: cholecalciferol → calcitriol; ↑ Ca²⁺ & PO₄³⁻ absorption; deficiency → rickets.
  • Vitamin E: tocopherols/tocotrienols; aromatic ring + isoprenoid chain; antioxidant.
  • Vitamin K: phylloquinone (K1), menaquinones (K2); posttranslational modification of prothrombin; Ca²⁺ binding; clotting.

5.3 Energy Storage

Triacylglycerols are hydrophobic energy storage molecules, with properties determined by fatty acid saturation, and participate in processes like saponification and micelle formation.

• Triacylglycerols
  • 3 fatty acids esterified to glycerol; rare for all 3 to be identical.
  • High energy density (more reduced carbons than glycogen); hydrophobic → no water weight.
  • Stored as oily droplets in cytosol (adipocytes under skin; seeds in plants).
  • Travel bidirectionally in blood between liver and adipose tissue.
  • Properties depend on fatty acid saturation.

  

• Free Fatty Acids & Saponification
  • Free fatty acids: unesterified, circulate bound to serum albumin.
  • Saponification: ester hydrolysis of triacylglycerols using strong base (NaOH/KOH).
    • Produces soap (Na salt of fatty acid + glycerol).
    • Soaps act as surfactants: reduce surface tension, emulsify polar/nonpolar mixtures.
• Micelles
  • Nonpolar compounds dissolve in micelle core.
  • Important for fat-soluble vitamin absorption.
  • Formation facilitated by fatty acids + bile salts → increase surface area.

  

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