| 1.5 Lipids |
|---|
Summary
Lipids are nonpolar, hydrophobic molecules whose diverse functions depend on how their subcomponents are assembled.
Fatty Acids
- Are the building blocks of many lipids
- Are classified as saturated (straight chains with only single bonds) or unsaturated (kinked chains due to one or more double bonds)
The degree of unsaturation directly determines the physical state of a lipid: more double bonds mean more kinks, less tight packing, and a more liquid consistency at room temperature. This structure-to-property relationship is a central concept tested on the AP exam.
Different classes of lipids carry out distinct biological roles.
Fats
- Store energy efficiently
- Can provide insulation in mammals
Steroids
- Function as hormones regulating growth, metabolism, and homeostasis
Cholesterol
- Stabilizes animal cell membranes by moderating fluidity
Phospholipids
- Have an amphipathic structure, with a hydrophilic phosphate head and two hydrophobic fatty acid tails
- Spontaneously form the lipid bilayers that make up all cell membranes
At a Glance
| Lipid Class | Biological Role | Structural Detail |
|---|---|---|
| Fats | Store energy efficiently and can provide insulation in mammals | A glycerol backbone bonded to three fatty acid tails (a triglyceride); tails may be saturated or unsaturated |
| Steroids | Function as hormones regulating growth, metabolism, and homeostasis | Four fused carbon rings with various attached functional groups |
| Cholesterol | Stabilizes animal cell membranes by moderating fluidity | A steroid built from four fused carbon rings with a hydroxyl group |
| Phospholipids | Spontaneously form the lipid bilayers that make up all cell membranes | Amphipathic: a hydrophilic phosphate head and two hydrophobic fatty acid tails |
In every case, the biological function of a lipid traces directly back to its molecular structure — a theme that recurs throughout AP Biology.