Lipids in Biology

Lipids

• Lipids are molecules containing hydrocarbons.
• They are a major component in the structure and function of living cells.
• Lipids are the main molecules for energy storage (triglycerides/triacylglycerols).
• These lipids are non-polar.
• Polar lipids are crucial components of biological membranes, forming lipid bilayers.
• Steroids, chlorophyll, carotenoids, and retinol are also types of lipids.
• Some lipids function as fat-soluble vitamins (A, D, E, and K).
• Lipids like ubiquinone (UQ or CoQ) and plastoquinone (PQ) act as electron transport carriers in chloroplasts and mitochondria.

Classification of Lipids

• Fatty acids (FA): Long hydrocarbon chains (14-30 carbons), typically even-numbered, with a carboxyl group (COOH) at one end and a methyl group (CH3) at the other.

  • Amphipathic (both hydrophilic/polar head and hydrophobic/non-polar tail)
  • In nature, they are typically esterified to glycerol, preventing toxicity if free in their un-esterified form.
  • Usually have C-C single bonds.
  • Saturated or unsaturated (double bonds). -Most unsaturated FAs have cis configuration of double bonds and are unconjugated.
    • FAs with fewer than 12 carbons are soluble in water.
    • FAs are soluble in dilute NaOH/KOH to produce soap (via saponification).

• Triglycerides/Triacylglycerols (TG): Three fatty acid molecules esterified to one glycerol molecule.

  • Stored as fats and oils in animals and plants.
  • Not soluble in water.
  • Provide energy reserve.
  • Can be simple or mixed.

• Phospholipids: Glycerol, two fatty acids, a phosphate group, and a polar group (like choline, ethanolamine, serine, inositol)

  • Also known as phosphoglycerides or glycerophospholipids
  • Many polar groups can be esterified to the phosphate group at C3
  • Key components of biological membranes.
  • Have hydrophilic/polar head and hydrophobic/non-polar tail.

• Sphingolipids: Sphingosine (18 carbon alcohol) substitutes glycerol

• Contain a fatty acid and a polar head group -Ceramides: fatty acid attached to sphingosine -Sphingomyelins: a ceramide with a phosphocholine or phosphoethanolamine head group -Glycosphingolipids: include cerebrosides (contain a sugar, galactose or glucose) and gangliosides (contain more complex sugar chains).

• Waxes: Esters of long-chain fatty acids and long-chain alcohols, highly insoluble.

  • Found in animal skins and the surface of certain fruits.
  • Provide waterproofing.
  • Include lanolin and spermaceti.

• Steroids: Important group of terpenes, performing various functions in cells, including hormone production.

  • Includes androgens (e.g., testosterone), estrogens (e.g., estradiol), progestogens (progesterone), glucocorticoids (e.g., cortisol), mineralocorticoids, and bile acids (e.g., cholic acid).
  • Cholesterol: A crucial steroid.
  • Four interconnected hydrocarbon rings.
  • Important component in cell membranes and the precursor to many other steroid hormones.

• Lipoproteins: Complexes of lipids and proteins that transport lipids through the blood.

  • Different types (chylomicrons, VLDL, IDL, LDL, HDL).
  • Associated with cholesterol transport and cardiovascular health

• Trans Fats: Unsaturated fats with trans-isomer fatty acids.

  • Produced by partially hydrogenating unsaturated fats.
  • Increase in blood cholesterol levels, contributing to heart disease.