NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
Fat Facts: Comparing the Structure and Function of Lipids
by Ling Chen and Lalitha S. Jayant Science Department Borough of Manhattan Community College / City University of New York
This case study focuses on the structure and function of different fat molecules, such as fatty acid, triacylglycerol, glycerophospholipid, HDL, and LDL cholesterols. The story also compares the structures and melting points of the paired fatty acids, the saturated with unsaturated, monounsaturated with polyunsaturated, cis- with trans-isomers. The structure of omega-3 polyunsaturated fatty acid is also explained. • It’s noon on Mother’s Day. Dolly, a busy ...view middle of the document...
Dolly: But I need the oil now; should I use canola oil instead? Mom: Calm down, sweetie. Just leave the solidified olive oil at room temperature for a while, or immerse the bottle in warm water until it clears up again. Dolly: Interesting. But your canola oil and the fish oil look crystal clear although they’re kept in the same fridge. And yet butter is solid even after it’s warmed up to room temperature. Why do they behave differently? Mom: Good observations! Have you ever wondered what the difference is between fats and oils? Well, both are forms of energy stored for living organisms—for example, plants and animals. They share similar structures, too. Fat and oil are triacylglycerols (a lipid), i.e. they are triesters of fatty acids (another kind of lipid) and glycerols (a trialcohol). The differentiation is really arbitrary; we call them oils if they are liquid at room temperature and
“Fat Facts” by Chen and Jayant Page 1
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE fats if they are solid at room temperature. Dolly: Whoa! Mom you’re using big words. Please explain words like triacylglycerol, fatty acids, and glycerol. What do you mean by triester and trialcohol? I have read in my chemistry class that alcohols are carbon compounds with an –OH group attached to the carbon. But what’s trialcohol? Mom: Glycerol contains three carbon atoms in its chain and a –OH group is attached to each carbon. That’s why it’s called trialcohol. Dolly: Oh! It means three alcohols. Now what are fatty acids? I know if a –COOH, carboxyl, group is present it’s an acid. Mom: Good Dolly. Fatty acids are long chains of carbon that end with that –COOH group. In nature, these long chains usually have only an even number of carbon atoms, typically between 12 and 20. Dolly: I also know from my organic chemistry class that when an alcohol and an acid react they lose a molecule of water to form an ester linkage. Since there are three alcohol groups in glycerol, it will take three fatty acids to react with each one of them and so it will form three ester linkages or a triester. Have I got it right? Mom: Hey Dolly! You’re good. You should enroll with me in my class. Dolly: But if fats and oils are both triacylglycerols, why are they different? Mom: A good question! The difference can be found in the fatty acid portions of the triacylglycerols. Fats contain higher amounts of saturated fatty acids, mostly from animal sources; consequently, fats are solids at room temperature, like butter and lard. On the contrary, oils contain higher amounts of unsaturated fatty acids, mostly from plants; hence oils are liquids at room temperature, like olive oil and corn oil. Dolly: What do saturated and unsaturated mean? Mom: They refer to the bonding of carbon atoms. In a saturated fatty acid, all carbon atoms are linked to another carbon atom by a single bond. As a result, the carbon atoms are connected with the maximum numbers of hydrogen atoms. Dolly: So there are less hydrogen...