Lipids are a diverse group of biological molecules that include fats, oils, phospholipids, and steroids. They matter because they store large amounts of energy, form the membranes around cells, and help organisms send chemical signals. Unlike carbohydrates and proteins, most lipids are nonpolar and do not mix well with water.
This property explains many of their biological roles, from waterproofing leaves to building flexible cell boundaries.
A common lipid called a triglyceride is made from glycerol bonded to three fatty acids, making it useful for long-term energy storage. Phospholipids have a hydrophilic head and two hydrophobic tails, so they naturally arrange into a bilayer in water. Steroids have four fused carbon rings and include molecules such as cholesterol and several hormones.
The structure of each lipid type determines whether it stores energy, forms membranes, or acts as a signaling molecule.
Key Facts
- Lipids are mostly nonpolar molecules, so they are hydrophobic and do not dissolve well in water.
- A triglyceride forms from glycerol + 3 fatty acids by dehydration synthesis.
- Triglycerides store about 9 kcal/g, while carbohydrates and proteins store about 4 kcal/g.
- Saturated fatty acids have no carbon-carbon double bonds, while unsaturated fatty acids have one or more C=C bonds.
- Phospholipids have hydrophilic heads and hydrophobic tails, causing them to form bilayers in water.
- Steroids contain four fused carbon rings and include cholesterol, testosterone, estrogen, and cortisol.
Vocabulary
- Lipid
- A lipid is a mostly hydrophobic biological molecule, such as a fat, oil, phospholipid, or steroid.
- Fatty acid
- A fatty acid is a long hydrocarbon chain with a carboxyl group at one end.
- Triglyceride
- A triglyceride is a lipid made of one glycerol molecule bonded to three fatty acids.
- Phospholipid
- A phospholipid is a membrane lipid with a hydrophilic phosphate-containing head and two hydrophobic fatty acid tails.
- Steroid
- A steroid is a lipid with a structure built from four fused carbon rings.
Common Mistakes to Avoid
- Calling all lipids fats is wrong because fats are only one type of lipid, while phospholipids and steroids have different structures and functions.
- Assuming saturated fats contain double bonds is wrong because saturated fatty acids have only single carbon-carbon bonds and are fully saturated with hydrogen.
- Drawing a phospholipid bilayer with tails facing water is wrong because the hydrophobic tails avoid water and point inward toward each other.
- Thinking lipids are not important because they do not dissolve in water is wrong because their hydrophobic nature is exactly what makes membranes, energy storage, and waterproof barriers possible.
Practice Questions
- 1 A student eats 12 g of fat. Using 9 kcal/g for lipids, how many kilocalories of energy does the fat provide?
- 2 A triglyceride contains 1 glycerol molecule and 3 fatty acids. How many fatty acid molecules are needed to build 25 triglyceride molecules?
- 3 Explain why phospholipids form a bilayer in water instead of forming a single straight chain or dissolving evenly throughout the water.