Fat
Definition and meaning of Fat in chemistry.
Fat is a triglyceride (triacylglycerol), an ester formed from one glycerol molecule and three fatty acid chains, that serves as a concentrated energy-storage molecule in living organisms. At room temperature, fats are solid, distinguishing them from oils, which are liquid triglycerides.
In more detail
Each fatty acid attaches to one of glycerol's three hydroxyl groups through an ester (condensation) bond, releasing water; hydrolyzing these bonds back with base (saponification) regenerates glycerol and fatty acid salts (soap). Fats tend to be solid because they contain mostly saturated fatty acids, whose straight hydrocarbon chains pack tightly together, whereas oils contain unsaturated fatty acids with kinked, cis double bonds that pack loosely. Because fatty acid chains are highly reduced (rich in C-H bonds), oxidizing fat yields roughly 9 kilocalories per gram, more than double the energy released from carbohydrates or proteins, making fat an efficient long-term energy reserve.
Key facts
| Field | Biochemistry |
|---|---|
| Chemical class | Triglyceride (triester of glycerol + 3 fatty acids) |
| Energy density | ~9 kcal/g (38 kJ/g) |
| Physical state | Solid at room temperature (mostly saturated fatty acids) |
Tristearin, formed from glycerol and three molecules of stearic acid (a saturated 18-carbon fatty acid), is a solid fat found in animal tissue and used industrially to make soap and candles.
Frequently asked questions
What is the chemical difference between a fat and an oil?
Both are triglycerides, but fats contain a higher proportion of saturated fatty acids with straight chains that pack tightly, making them solid at room temperature, while oils contain more unsaturated fatty acids with bent chains, keeping them liquid.
Why does digesting fat release more energy than digesting sugar?
Fatty acid hydrocarbon chains are more reduced than the carbon skeletons in carbohydrates, so oxidizing them releases more energy per gram, about 9 kcal/g versus about 4 kcal/g for carbohydrates.