Lipids

🧈 Lipids: The Hydrophobic Family

Lipids are a diverse group of biomolecules united by one shared trait: they are mostly hydrophobic (water-fearing). Unlike carbohydrates and proteins, lipids are not true polymers — they don't form by linking identical monomers.

Why hydrophobicity matters

Hydrophobic molecules:

• Cluster away from water (hydrophobic effect)
• Spontaneously form bilayers in aqueous environments → cell membranes
• Pack densely → 2× more energy per gram than carbohydrates

The major lipid families

Concept Check 🎯

Fatty Acids: The Common Tail

A fatty acid is a long hydrocarbon chain (the "tail") with a carboxyl group ($-COOH$) at one end (the "head").

Saturated vs Unsaturated

Why this matters

Straight saturated chains pack tightly → high melting point → solid at room temp.

Kinked unsaturated chains can't pack as well → lower melting point → liquid at room temp.

This same packing logic also determines membrane fluidity (covered in part 3).

Concept Check 🎯

Fill in the Blanks 🔍

Lipids: Fats, Phospholipids & Steroids

Triglycerides (Fats & Oils)

Trigleride = glycerol + 3 fatty acids (linked by ester bonds)

Fats store 2× more energy per gram than carbohydrates (9 kcal/g vs 4 kcal/g).

Phospholipids

Phospholipid = glycerol + 2 fatty acids + phosphate group

• Amphipathic: hydrophilic head + hydrophobic tails
• Form the basis of cell membranes (phospholipid bilayer)
• Spontaneously form bilayers in water

Steroids

Four fused carbon rings (e.g., cholesterol, estrogen, testosterone). Cholesterol is a component of animal cell membranes.

🎯

What Lipids Do for Cells

Why fats beat sugars for storage

Triglycerides yield about 9 kcal/g vs 4 kcal/g for carbohydrates. They are also stored without water, making them compact for migrating birds, hibernating mammals, and seeds.

Concept Check 🎯

Phospholipids and the Cell Membrane

A phospholipid has:

• A hydrophilic head (phosphate + glycerol) — likes water
• Two hydrophobic tails (fatty acids) — flee water

In water, phospholipids spontaneously arrange into a bilayer: heads outward toward the aqueous environment, tails buried inside. This bilayer is the foundation of every cell membrane.

🎯 AP Review: Lipids

Must-know synthesis points

1. Lipids are not true polymers — TGs, phospholipids, and steroids share hydrophobicity, not a repeating monomer.
2. Saturation determines packing — saturated tails = straight = solid; unsaturated tails = kinked = liquid; this also drives membrane fluidity.
3. Phospholipid amphipathicity (hydrophilic head + hydrophobic tails) is the engineering basis of every cell membrane.
4. Cholesterol is a fluidity buffer — restricts movement at high temp, prevents stiffening at low temp.
5. Steroid hormones (cortisol, estrogen, testosterone) are lipid-soluble and cross membranes directly to bind intracellular receptors.

Workshop Problem 📐

Workshop Problem 📐

AP Synthesis 🔬