Lipid Classification Overview
Three Major Lipid Classes
1. Triglycerides (Neutral Fats)
- Structure: 1 glycerol + 3 fatty acids (via ester bonds)
- Properties: Non-polar, entirely hydrophobic, insoluble in water
- Function: Long-term energy storage in adipose tissue
- Location: Adipose tissue, liver, muscle
2. Phospholipids
- Structure: 1 glycerol + 2 fatty acids + 1 phosphate group (+ head group)
- Properties: AMPHIPATHIC (hydrophilic head + hydrophobic tails)
- Function: Structural component of all biological membranes (bilayer)
- Self-assembly: Spontaneously form bilayers in water (hydrophobic effect)
- Example: Phosphatidylcholine (lecithin)
3. Steroids
- Structure: Four fused carbon rings (cyclopentanoperhydrophenanthrene backbone)
- Properties: Hydrophobic, derived from cholesterol
- Function: Membrane component (cholesterol), hormones (testosterone, oestrogen, cortisol), bile salts
- NOT glycerol-based (unlike triglycerides and phospholipids)
Saturated vs. Unsaturated Fatty Acids
| Feature | Saturated | Unsaturated |
|---|---|---|
| C=C double bonds | None | One or more |
| Chain shape | Straight (no kinks) | Kinked at double bonds |
| Physical state | Solid at room temp | Liquid at room temp |
| Examples | Palmitic, stearic (animal fats) | Oleic, linoleic (plant oils) |
| Packing | Tight packing possible | Loose packing (kinks prevent it) |
Membrane Fluidity Rule
Cholesterol = membrane fluidity buffer. More unsaturated FA = more fluid membrane. More saturated FA = less fluid membrane.