triacylglycerols
Summary
Triacylglycerols (triglycerides) are glycerol joined to three fatty acids by ester bonds. They are the body's main long-term energy store, kept in adipocytes, and give more energy per gram than carbohydrate because their carbons are more reduced.
Detail
Meaning: A triacylglycerol (triglyceride) is glycerol with each of its three hydroxyl groups joined to a fatty acid by an ester bond. With no charged head group, it is very nonpolar and is stored as water-free droplets in adipocytes. Fatty acid carbons are more reduced than carbohydrate carbons, so fat gives about 9 kcal/g versus about 4 kcal/g. Pancreatic lipase digests it after bile salts emulsify it. During fasting, hormone-sensitive lipase releases fatty acids (β-oxidized to acetyl-CoA) and glycerol.
Equation: glycerol + 3 RCOOH → triacylglycerol + 3 H₂O, where RCOOH is a fatty acid (R is the hydrocarbon chain) and amounts are in mol. Hydrolysis by lipase runs the reaction in reverse.
On the MCAT: Expect questions that ask you to find the ester bonds, to predict that more cis double bonds in the fatty acids lower the melting point, to compare energy per gram of fat and carbohydrate, or to predict hormone effects. Insulin favors storage, while glucagon and epinephrine favor mobilization.
Common confusion: Triacylglycerols are not phospholipids. They have no phosphate group and do not form bilayers. Also, animals generally cannot make net glucose from the fatty acid portion (acetyl-CoA), only from the glycerol.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.