Chem 336 - Spring 2000
Organic Chemistry III
Dr. Carl C. Wamser

Chapter 25 - Lipids

Lipids

• general term for biomolecules that are water-insoluble but soluble in organic media
  (in biological systems this usually means membrane-bound)
  
  
• categories of lipids:
  • fatty acids - long chain carboxylic acids (C₁₂ - C₂₀)
    
  • fats and oils - triesters of fatty acids and glycerol (triglycerides)
    (monoglycerides are monoesters, diglyecrides are diesters)
    (function as energy storage materials)

• phosphatides - phosphate esters of glycerol (+ fatty acids)
  (membrane constituents)

• steroids - polycyclic structure with specific substituents
  (hormones, special functions)

• prostaglandins - cyclic structure with long chains
  (hormones, special functions)

Fats and Oils

• fats are solids, oils are liquids (same basic triglyceride structure)
• serve as insoluble deposits that can be retrieved for energy

Fatty Acids

• all natural fatty acids have an even number of carbon atoms, since they are synthesized from acetate (C₂) subunits
• natural unsaturated fatty acids have cis double bonds
  cis-double bonds lead to a lower melting point
  unsaturated and polyunsaturated fats are also more easily digestible

Soaps

• basic hydrolysis of fats (saponification) generates soaps
  (the sodium salts of fatty acids)

• the ionic head group is water-soluble, the nonpolar tail insoluble
• soaps tend to aggregate in micelles, where nonpolar dirt dissolves

Phospholipids

• the natural analog of the soap structure:
  a polar phosphate head group
  two nonpolar tails

Bilayer Lipid Membranes

• phospholipids with two tails tend to make planar bilayer aggregates

• other lipids are found in the nonpolar environment of membranes
• proteins often have a nonpolar and polar part, and are membrane-bound

Steroids

• cholesterol is the precursor to most other steroids: vitamin D, bile salts, hormones

• the sex hormones and birth control pills are steroids