Lipids
Structure and Function
Explore the hydrophobic macromolecules that build membranes, store energy, and signal between cells.
AP Biology
What Are Lipids?
Lipids are a diverse group of hydrophobic or amphipathic biomolecules. Unlike proteins, carbohydrates, and nucleic acids, lipids are not true polymers: they are not built from repeating monomer subunits linked by condensation reactions.
Lipids are united by their physical behavior (hydrophobicity) rather than a shared chemical structure: this makes them the most structurally diverse class of biological macromolecules.
Major Categories of Lipids
Fats and Oils
Triglycerides: glycerol + 3 fatty acids. Long-term energy storage.
Phospholipids
Glycerol + 2 fatty acids + phosphate group. Form cell membranes.
Steroids
Four fused carbon rings. Include cholesterol, testosterone, estrogen.
Waxes
Long-chain fatty acid + long-chain alcohol. Waterproof coatings.
Why Lipids Matter
1
Energy storage: Fats store about \( 9 \text{ kcal/g} \), more than twice that of carbohydrates (\( \approx 4 \text{ kcal/g} \)).
2
Membrane structure: Phospholipid bilayers form the selectively permeable barrier of every cell.
3
Signaling: Steroid hormones, prostaglandins, and lipid-soluble vitamins (A, D, E, K) regulate body processes.
4
Insulation and protection: Adipose tissue cushions organs and reduces heat loss.
Glycogen and fat store the same energy in different physical packages
Using approximate nutritional energy densities of 4 and 9 kcal/g, 100 kcal requires 25 g of dry glycogen or 11.1 g of fat. If glycogen carries 2 g of water per gram, its total storage mass is 75 g; hydration varies.Compare the same energy target
Using approximate nutritional energy densities of 4 and 9 kcal/g, 100 kcal requires 25 g of dry glycogen or 11.1 g of fat. If glycogen carries 2 g of water per gram, its total storage mass is 75 g; hydration varies.Fatty Acid Structure
Saturated vs. Unsaturated
Triglycerides: Energy Storage
The Phospholipid Bilayer
Steroids: Four-Ring Framework
Fats vs. Phospholipids
| Triglycerides (Fats) | Phospholipids | |
|---|---|---|
| Glycerol | Yes | Yes |
| Fatty acid tails | 3 | 2 |
| Phosphate group | No | Yes |
| Polarity | Entirely nonpolar | Amphipathic |
| Primary function | Energy storage | Membrane structure |
| In water | Clump together | Form bilayers / micelles |
How to Identify a Lipid
1
Check solubility: Does the molecule dissolve in nonpolar solvents but NOT in water?
2
Look for long hydrocarbon chains: Fatty acids have chains of 12+ carbons with mostly C-H bonds.
3
Identify the backbone: Glycerol backbone indicates a fat or phospholipid. Fused ring structure indicates a steroid.
4
Count the components: 3 fatty acids = triglyceride. 2 fatty acids + phosphate = phospholipid.
5
Check for double bonds: No C=C = saturated. One or more C=C = unsaturated (mono- or polyunsaturated).
Example: Dehydration Synthesis
A glycerol molecule reacts with three fatty acids. How many water molecules are released, and what type of bond forms?
Example: Predicting State
Butter contains mainly saturated fatty acids. Olive oil contains mainly unsaturated fatty acids. Which is solid at room temperature, and why?
Butter
Saturated chains are straight. They pack tightly via van der Waals interactions. Solid at room temp.
Olive Oil
Unsaturated chains have cis kinks. They cannot pack tightly. Liquid at room temp.
Example: Membrane Fluidity
A cell membrane at low temperature becomes too rigid. Which modifications would increase fluidity?
1
More unsaturated fatty acid tails: Kinks prevent tight packing, keeping the membrane fluid.
2
Shorter fatty acid tails: Fewer van der Waals interactions between neighboring phospholipids.
3
Cholesterol: At low temperatures, cholesterol inserts between phospholipids to prevent tight crystalline packing, acting as a fluidity buffer.
Common Mistakes
✕ Lipids are polymers
Lipids are NOT true polymers. They do not consist of repeating monomer subunits like proteins or nucleic acids.
✕ Unsaturated = unhealthy
Unsaturated fats (especially omega-3 and omega-6) are generally beneficial. Trans fats, not all unsaturated fats, are harmful.
✕ Cholesterol is only bad
Cholesterol is essential: it stabilizes membranes and serves as a precursor for steroid hormones and vitamin D.
Lipid Summary
| Lipid Type | Building Blocks | Function | Example |
|---|---|---|---|
| Triglyceride | Glycerol + 3 fatty acids | Energy storage | Body fat, oils |
| Phospholipid | Glycerol + 2 FA + phosphate | Cell membranes | Bilayer |
| Steroid | 4 fused carbon rings | Signaling, structure | Cholesterol |
| Wax | Fatty acid + long alcohol | Waterproofing | Cuticle, beeswax |
Question 1 of 5
Which structural feature makes phospholipids amphipathic?
Amphipathic means having both a hydrophilic (water-loving) region and a hydrophobic (water-fearing) region. The phosphate head is polar, while the two fatty acid tails are nonpolar.
Question 2 of 5
During the formation of a triglyceride, how many water molecules are released?
Each of the three fatty acids bonds to glycerol via dehydration synthesis, releasing one \( \text{H}_2\text{O} \) per ester bond: \( 3 \) bonds = \( 3 \) water molecules.
Question 3 of 5
An unsaturated fatty acid differs from a saturated fatty acid because it contains:
Unsaturated means the hydrocarbon chain has at least one C=C double bond, which introduces a kink. "Saturated" means every carbon is bonded to the maximum number of hydrogen atoms (no double bonds).
Question 4 of 5
At low temperatures, cholesterol in a cell membrane primarily functions to:
At low temperatures, cholesterol wedges between phospholipid tails, preventing tight crystalline packing. This maintains membrane fluidity. At high temperatures, it has the opposite effect, reducing fluidity.
Question 5 of 5
Why are lipids not classified as true polymers?
True polymers (proteins, nucleic acids, polysaccharides) consist of repeating monomer units joined by condensation reactions. Lipids are assembled from varied components (glycerol, fatty acids, phosphate groups, ring structures) and lack a single repeating monomer.