Structural Lipids
High-Yield Summary
- Lipids are insoluble in water, soluble in nonpolar solvents. Structural lipids are amphipathic (hydrophilic head + hydrophobic fatty acid tail) — that duality builds the phospholipid bilayer.
- Saturated fatty acids (no double bonds) pack tightly, solid at room temp. Unsaturated fatty acids (double bonds kink the chain) pack loosely, liquid/fluid — more unsaturation = more membrane fluidity.
- Glycolipids: sugar head group via glycosidic linkage. Phospholipids: phosphate + alcohol head group via phosphodiester linkage.
- Phospholipids split by backbone: glycerophospholipid (glycerol, 2 fatty acids via ester bonds) vs. sphingophospholipid (sphingosine backbone, e.g. sphingomyelin).
- Ceramide (sphingosine + H head group) is the sphingolipid core — precursor to sphingomyelin (phospholipid) and glycosphingolipids: cerebroside (1 sugar) → globoside (2+ sugars) → ganglioside (+ sialic acid, net negative charge).
- Waxes = long-chain fatty acid + long-chain alcohol ester. Protective/waterproofing role (plant cuticles, animal skin/fur, earwax), not membrane-building.
Key Terms
- Amphipathic
- Has both a hydrophilic region and a hydrophobic region — what lets lipids form bilayers.
- Glycerophospholipid
- Phospholipid with a glycerol backbone; two fatty acid tails via ester bonds, phosphate head via phosphodiester bond.
- Sphingophospholipid
- Phospholipid with a sphingosine backbone and phosphate-containing head group (e.g. sphingomyelin).
- Ceramide
- Simplest sphingolipid (sphingosine + hydrogen head group); precursor to sphingomyelin and glycosphingolipids.
- Ganglioside
- Most complex glycosphingolipid — oligosaccharide + sialic acid head, net negative charge; key in cell recognition and neuronal signaling.
- Wax
- Ester of a long-chain fatty acid and a long-chain alcohol; nonpolar, solid, protective/waterproofing.
Glycerophospholipid vs. Sphingophospholipid
| Feature | Glycerophospholipid / Sphingophospholipid |
|---|---|
| Backbone | Glycerol / Sphingosine |
| Fatty acid attachment | Two fatty acids via ester bonds / built into the sphingosine backbone |
| Head group | Phosphate + alcohol (phosphodiester) / phosphate-containing, e.g. phosphocholine |
| Example | — / Sphingomyelin |
Sphingolipid Family, Built Stepwise from Ceramide
- 1Ceramide: sphingosine backbone + hydrogen head group (neutral; not a phospholipid or glycolipid on its own).
- 2+ phosphocholine or phosphoethanolamine → Sphingomyelin (a sphingophospholipid; major component of the myelin sheath).
- 3+ single sugar residue (e.g. glucose, galactose) → Cerebroside (a glycosphingolipid, neutral).
- 4+ two or more sugars → Globoside (glycosphingolipid, neutral).
- 5+ oligosaccharide + at least one sialic acid → Ganglioside (glycosphingolipid, negatively charged).
Common MCAT Trap
- Glycolipids and phospholipids are distinguished by head group + linkage (sugar/glycosidic vs. phosphate+alcohol/phosphodiester) — not by backbone. Backbone (glycerol vs. sphingosine) is a separate axis, used to split phospholipids specifically.
- Ceramide itself is neither a phospholipid nor a glycolipid — it's the shared precursor. Don't classify it as one or the other.
- Sialic acid is what makes gangliosides negatively charged; cerebrosides and globosides are neutral despite also being glycosphingolipids.
Quick Recall
What linkage attaches the head group in a glycolipid vs. a phospholipid?
Why does an unsaturated fatty acid increase membrane fluidity?
What's the precursor molecule for both sphingomyelin and the glycosphingolipids?
What two building blocks form a wax?