Energy Storage:

• More than 2× energy per gram vs. carbohydrates
• Efficient long-term storage
• Adipose tissue in animals

2. Phospholipids

Structure:

• Glycerol + 2 fatty acids + phosphate group
• Amphipathic: hydrophilic head + hydrophobic tails

Biological Role:

• Cell membrane structure
• Form bilayers in aqueous solution
• Heads face water, tails face each other
• Selectively permeable barrier

3. Steroids

Structure:

• Four fused carbon rings
• Different functional groups attached

Examples:

• Cholesterol: membrane fluidity, precursor to other steroids
• Sex hormones: testosterone, estrogen
• Cortisol: stress hormone

Key Concepts

1. Lipids are hydrophobic (don't dissolve in water)
2. Triglycerides store energy efficiently
3. Saturated fats have no double bonds, pack tightly (solid)
4. Unsaturated fats have double bonds, don't pack well (liquid)
5. Phospholipids form cell membranes (bilayer structure)
6. Steroids have ring structure, various functions