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 (also called triglycerides) consist of:
One glycerol molecule
Three fatty acids attached via dehydration reactions, forming ester linkages
Each fatty acid is a long hydrocarbon chain, making fats hydrophobic and energy-dense—storing more than twice the energy of carbohydrates.
Saturated vs. Unsaturated Fats
Saturated fats: No double bonds, packed tightly, solid at room temperature (e.g., butter)
Unsaturated fats: One or more cis double bonds, create kinks, liquid at room temperature (e.g., olive oil)
Hydrogenated oils turn unsaturated fats into saturated ones—creating trans fats, which are linked to heart disease. Due to health concerns, many countries have banned or regulated trans fats.
Fat Storage in Animals
Stored in adipose tissue, which:
Cushions organs
Provides insulation (especially in marine mammals)
Serves as a long-term energy reserve
2. Phospholipids: Cell Membrane Architects
A phospholipid is similar to a fat but has:
Two fatty acids
One phosphate group (with an additional charged group like choline)
This gives the molecule two regions:
A hydrophilic head
Hydrophobic tails
In water, phospholipids spontaneously form bilayers—the foundation of all biological membranes—creating barriers between cells and their environment.
3. Steroids: The Four-Ringed Regulators
Steroids are lipids with a carbon skeleton of four fused rings. They differ based on their functional groups.
Cholesterol is:
A key component of animal cell membranes
A precursor for sex hormones like estrogen and testosterone
Synthesized in the liver and obtained from the diet
While cholesterol is essential, high levels may contribute to atherosclerosis, a condition where plaque builds up in arteries.
In a Nutshell
Lipids are hydrophobic biological molecules, including fats, phospholipids, and steroids.
Fats store energy efficiently and insulate the body.
Phospholipids form the bilayer membranes that enclose cells.
Steroids, like cholesterol, serve structural and hormonal roles.
Lipids are diverse in structure and function but united by their water-avoiding nature.