Energy Storage
High-Yield Summary
- Triacylglycerols (triglycerides) are the body's main long-term energy store: fatty acid carbons are more reduced than sugar carbons (more energy/unit), and they're hydrophobic (no water weight) — far more compact than hydrated glycogen.
- A triacylglycerol forms by condensation: glycerol + 3 fatty acids → triacylglycerol + 3 H₂O, via ester bonds. Stored in adipocytes (fat cells).
- A free fatty acid is unesterified (not part of a triglyceride or phospholipid) — its intact carboxylate group makes it amphipathic.
- Saponification: base-catalyzed (NaOH/KOH) hydrolysis of ester bonds — triacylglycerol + 3 NaOH → glycerol + 3 soap (fatty acid salts). Chemical, not enzymatic.
- Soap molecules (amphipathic) self-assemble into micelles in water: polar heads face out (water), nonpolar tails face in (trap oil/grease) — this is how soap emulsifies.
Key Terms
- Triacylglycerol
- Triglyceride; three fatty acids esterified to a glycerol backbone; the body's main energy-storage lipid.
- Adipocyte
- Specialized fat cell that stores triglyceride droplets as energy reserves.
- Free fatty acid
- Unesterified fatty acid with an intact carboxylate group; amphipathic.
- Saponification
- Base-catalyzed hydrolysis of ester bonds in a triglyceride, producing glycerol + soap.
- Micelle
- Spherical self-assembly of amphipathic molecules — polar heads out, nonpolar tails in.
- Emulsifier
- A substance (like soap) that lets nonpolar material disperse/be removed in water via micelle formation.
Triacylglycerol Formation and Breakdown
Glycerol + 3 fatty acids ⇌ triacylglycerol + 3 H₂O (ester bonds, condensation forward)
- Glycerol = 3-carbon backbone with 3 hydroxyl groups
- Fatty acid = Long hydrocarbon chain with a terminal carboxylic acid
- Ester bond = Links each fatty acid's COOH to a glycerol OH; releases H₂O when formed
- Saponification runs the reverse (hydrolysis) direction using NaOH/KOH instead of water/acid, yielding glycerol + 3 soap molecules (sodium/potassium fatty acid salts).
Common MCAT Trap
- Saponification is base-catalyzed chemical hydrolysis (NaOH/KOH) — not an enzyme-catalyzed reaction like most other lipid breakdown in the body. Don't assume a lipase is involved.
- Triacylglycerols store more energy per gram than glycogen for two independent reasons — more reduced carbons AND no water weight. Both matter; don't cite only one.
- In a micelle, heads point outward (toward water) and tails point inward — the opposite orientation from a lipid bilayer leaflet's outer-facing surface, but the same amphipathic logic (polar seeks water, nonpolar avoids it).
Quick Recall
Why are triacylglycerols more energy-dense than hydrated glycogen?
What reaction type forms a triacylglycerol from glycerol and fatty acids, and what byproduct does it release?
What base(s) drive saponification, and what two products result?
In a micelle formed by soap, which part of the molecule faces the water?