Fatty Acid Comparison:

(a) Structural Differences:

Saturated Fatty Acids:

• No C=C double bonds
• All carbon-carbon bonds are single (C-C)
• General formula: CH₃(CH₂)ₙCOOH
• Maximum number of hydrogens ("saturated" with H)
• Example: Palmitic acid (16:0), Stearic acid (18:0)

Unsaturated Fatty Acids:

• One or more C=C double bonds
• Monounsaturated: 1 double bond (oleic acid, 18:1)
• Polyunsaturated: 2+ double bonds (linoleic acid, 18:2)
• Each double bond creates a "kink" in the chain

(b) Physical Properties:

Saturated:

• Straight chains pack tightly together
• Strong van der Waals forces between molecules
• Higher melting point → solid at room temperature
• Examples: butter, lard, coconut oil

Unsaturated:

• Kinked chains prevent tight packing
• Weaker intermolecular forces
• Lower melting point → liquid at room temperature
• Examples: olive oil, fish oil, vegetable oils

(c) Health Implications:

Saturated Fats:

• ⚠️ Raise LDL ("bad") cholesterol
• Associated with increased cardiovascular disease risk
• Recommendation: limit to <10% of daily calories

Unsaturated Fats:

• ✓ Lower LDL cholesterol, raise HDL ("good") cholesterol
• Omega-3 fatty acids (EPA, DHA): anti-inflammatory, heart-protective
• Omega-6 fatty acids (linoleic): essential, but balance with omega-3
• Recommendation: replace saturated with unsaturated fats

Trans Fats (Special Case):

• Artificially saturated (hydrogenation)
• Worst for health: raise LDL, lower HDL
• Should be avoided completely

$\boxed{\text{Unsaturated (kinked) → liquid, healthier; Saturated (straight) → solid, limit intake}}$

Phospholipid Structure:

Components:

1. Hydrophilic "head":

   • Glycerol backbone
   • Phosphate group (PO₄³⁻) - negatively charged
   • Often additional molecule (choline, serine, ethanolamine)
   • Polar → attracted to water

2. Hydrophobic "tails":

   • Two fatty acid chains (usually 14-18 carbons)
   • One typically saturated (straight)
   • One typically unsaturated (kinked)
   • Nonpolar → repelled by water

Structure Diagram:

        Choline
           |
    Phosphate group  ←  Hydrophilic head
           |              (polar, charged)
        Glycerol
          / \
    Fatty acid chains  ←  Hydrophobic tails
         |   |            (nonpolar)
         |   |
         |   | (kinked if unsaturated)

Bilayer Formation:

In aqueous solution, phospholipids spontaneously arrange into a bilayer:

1. Hydrophilic heads face outward toward water
2. Hydrophobic tails face inward, away from water
3. Forms continuous lipid bilayer membrane

Cross-section:

Water | ○○○○○○○○ | ← Heads (exterior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ~~~~~~~~ |
      | ~~~~~~~~ | ← Tails (interior)
      | ○○○○○○○○ | ← Heads (interior/exterior)
Water

Thermodynamic Favorability:

Entropy-driven process:

1. Hydrophobic effect:

   • Water molecules form ordered "cages" around nonpolar tails
   • Decreases entropy (unfavorable)
   • Bilayer minimizes water-tail contact
   • Releases ordered water → increases entropy ✓

2. Hydrogen bonding:

   • Polar heads interact with water via H-bonds
   • Maximizes favorable interactions

3. Van der Waals forces:

   • Tails interact with each other in bilayer interior
   • Stabilizes structure

4. Self-sealing:

   • Exposed edges are unfavorable
   • Bilayer spontaneously forms closed vesicles (liposomes)
   • Minimizes edge effects

Gibbs Free Energy:

$\Delta G = \Delta H - T\Delta S$

• ΔS is positive (entropy increases) → large negative TΔS term
• ΔG < 0 → spontaneous process

$\boxed{\text{Bilayer formation is spontaneous due to hydrophobic effect (entropy-driven)}}$

Biological Significance:

• Forms basis of all cell membranes
• Selectively permeable barrier
• Fluid mosaic model: proteins embedded in fluid lipid bilayer
• ~50% of membrane mass in typical cell