Lipids are a broad group of mostly hydrophobic biomolecules that include fats, oils, waxes, phospholipids, and steroids. They matter because they store large amounts of energy, form cell membranes, cushion organs, and help cells communicate. Unlike carbohydrates and proteins, many lipids are not built from long repeating chains of identical monomers.
Their chemistry is shaped by nonpolar hydrocarbon regions that avoid water and polar regions that can interact with water.
Fatty acids are key lipid building blocks, with a carboxyl group at one end and a hydrocarbon chain that may be saturated or unsaturated. Three fatty acids joined to glycerol form a triglyceride, the main storage form of fat in animals and many plants. Phospholipids have two fatty acid tails and a phosphate-containing head, so they naturally assemble into bilayers in water.
This bilayer structure creates the basic boundary of cells and controls how substances enter and leave.
Key Facts
- A fatty acid has a carboxyl group and a hydrocarbon chain: R-COOH.
- Saturated fatty acids have no C=C double bonds, so their chains are straighter and pack tightly.
- Unsaturated fatty acids have one or more C=C double bonds, often creating bends that lower melting point.
- Triglyceride formation: glycerol + 3 fatty acids -> triglyceride + 3 H2O.
- Phospholipids are amphipathic, with a hydrophilic phosphate head and two hydrophobic fatty acid tails.
- In water, phospholipids form a bilayer with heads facing water and tails facing inward.
Vocabulary
- Lipid
- A lipid is a mostly hydrophobic biomolecule such as a fat, oil, phospholipid, wax, or steroid.
- Fatty acid
- A fatty acid is a molecule with a carboxyl group attached to a long hydrocarbon chain.
- Triglyceride
- A triglyceride is a lipid made from one glycerol molecule bonded to three fatty acid chains.
- Phospholipid
- A phospholipid is an amphipathic lipid with a polar phosphate head and two nonpolar fatty acid tails.
- Bilayer
- A bilayer is a double layer of phospholipids that forms the main structural barrier of cell membranes.
Common Mistakes to Avoid
- Calling all lipids fats is wrong because fats are mainly triglycerides, while lipids also include phospholipids, waxes, and steroids.
- Thinking saturated fats contain more oxygen is wrong because saturated means the hydrocarbon chain has no carbon-carbon double bonds and holds the maximum number of hydrogens.
- Drawing phospholipids with three fatty acid tails is wrong because a typical membrane phospholipid has two fatty acid tails and one phosphate-containing head group.
- Assuming cell membranes are solid walls is wrong because phospholipid bilayers are fluid structures whose lipids and many proteins can move sideways.
Practice Questions
- 1 A triglyceride forms from 1 glycerol molecule and 3 fatty acids. How many water molecules are released when 12 triglycerides form?
- 2 A fatty acid contains 18 carbons and has 0 carbon-carbon double bonds. Another 18-carbon fatty acid has 2 carbon-carbon double bonds. Which one is more likely to have a lower melting point, and why?
- 3 Explain why phospholipids form bilayers in water instead of spreading out with their tails exposed to the surrounding water.