Rucete ✏ Campbell Biology In a Nutshell
Unit 1 THE CHEMISTRY OF LIFE — Concept 5.3 Lipids Are a Diverse Group of Hydrophobic Molecules
Lipids are not true polymers, but they play vital roles in energy storage, membrane structure, and hormone production. They are united by one key trait: they don’t mix well with water.
1. Fats: Long-Term Energy Storage
Fats (triglycerides) consist of:
- One glycerol molecule
- Three fatty acids joined by ester linkages
Fatty acids are long hydrocarbon chains, making fats hydrophobic and energy-rich—storing over twice as much energy as carbohydrates.
Saturated vs. Unsaturated Fats
- Saturated fats: No double bonds, solid at room temp (e.g., butter)
- Unsaturated fats: One or more cis double bonds, liquid at room temp (e.g., olive oil)
Hydrogenated oils convert unsaturated fats into saturated ones, forming trans fats, which are linked to heart disease and now regulated or banned in many countries.
Fat Storage in Animals
Stored in adipose tissue, which:
- Cushions organs
- Provides insulation
- Serves as energy reserve
2. Phospholipids: Cell Membrane Architects
A phospholipid resembles a fat but has:
- Two fatty acids
- One phosphate group (often with choline)
This creates:
- Hydrophilic head
- Hydrophobic tails
In water, phospholipids form bilayers—the foundation of cell membranes.
3. Steroids: The Four-Ringed Regulators
Steroids have a carbon skeleton of four fused rings with various functional groups.
- Cholesterol is:
- A key component of animal cell membranes
- A precursor for sex hormones (e.g., estrogen, testosterone)
- Synthesized in the liver and also obtained from food
Excess cholesterol may contribute to atherosclerosis, narrowing arteries due to plaque buildup.
In a Nutshell
- Lipids are hydrophobic molecules including fats, phospholipids, and steroids.
- Fats store energy and provide insulation.
- Phospholipids form cell membranes.
- Steroids, like cholesterol, function structurally and hormonally.
- Lipids are structurally diverse but all repel water.