# Lipids
Lipids are a diverse group of biological molecules that are insoluble in water but soluble in organic solvents. They include triglycerides (fats and oils) and phospholipids, both crucial for energy storage and membrane structure.
1. Triglycerides
Structure: glycerol + 3 fatty acids joined by ester bonds via condensation reactions.
Saturated vs Unsaturated Fatty Acids
| Type | Bonds | Shape | State | Examples |
|---|---|---|---|---|
| Saturated | No C=C double bonds | Straight chains; pack tightly | Solid (fat) | Butter, lard |
| Unsaturated | One or more C=C | Kinked chains; pack loosely | Liquid (oil) | Olive oil, fish oil |
Functions of Triglycerides
- Energy store: more energy per gram than carbohydrates (~2×)
- Insulation: subcutaneous fat (thermal); myelin sheath (electrical)
- Protection: cushions organs
- Waterproof: sebum on skin; waxy cuticle on leaves
- Metabolic water: respiration of fat produces water (important for desert animals)
2. Phospholipids
Structure: glycerol + 2 fatty acids + 1 phosphate group
- Hydrophilic head (phosphate — polar)
- Hydrophobic tails (fatty acids — nonpolar)
- Amphipathic (both hydrophilic and hydrophobic)
In water: form bilayer (heads face out, tails face in) → basis of cell membranes.
3. Cholesterol
- Small, hydrophobic molecule
- Sits between phospholipids in cell membrane
- Regulates membrane fluidity: prevents too fluid (high T) or too rigid (low T)
- Precursor for steroid hormones
4. Testing for Lipids: Emulsion Test
- Dissolve sample in ethanol
- Pour into water
- Positive: milky-white emulsion forms
- Negative: solution stays clear
5. Practice Questions
- Draw and label a triglyceride.
- Explain why fats contain more energy per gram than carbohydrates.
- Describe the arrangement of phospholipids in a cell membrane.
- Compare saturated and unsaturated fatty acids.
- What is the role of cholesterol in cell membranes?
Want to check your answers and get step-by-step solutions?
Summary
- Triglycerides: glycerol + 3 fatty acids; ester bonds; energy storage
- Saturated: no C=C, solid; unsaturated: C=C, liquid
- Phospholipids: 2 fatty acids + phosphate; form bilayer membranes
- Cholesterol: regulates membrane fluidity
- Emulsion test: ethanol + water → milky white = lipid present