Lipids: Fats, Oils, Waxes, etc.
2D says, “Being able to store fats in our bodies is really important to us butterflies and our caterpillars. Since our pupae are not able to eat while all the changes that turn a caterpillar into a butterfly are going on, the caterpillar must, first, eat enough food to store up enough energy as body fat to last through the whole time we’re in the pupal stage. While we Monarchs travel to Mexico to overwinter, there are other species of butterflies that overwinter here in Cincinnati. Those butterflies must eat enough (plant nectar) to convert lots of sugar to fat and store it to last through the winter. Your human bodies will do the same thing if you consume too much sugar in soft drinks. Also, while we Monarchs can get some food along the way to Mexico, it helps out a lot if we can have a good store of body fat built up before we leave. That both helps to insulate us during the cooler autumn nights and gives us a bit more food to keep our wing muscles going.” Some types of moths, including Cecropia moths, don’t even have functional mouths as adults, and so cannot/do not eat. In those kinds of moths, the caterpillars must eat enough and store up enough body fat to make it through not only their pupal stage, but also their whole adult life!
What Are Lipids?
All Lipids are hydrophobic: that’s the one property they have in common. This group of molecules includes fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.
Structure of Fatty Acids
One Fatty Acid The “tail” of a fatty acid is a long hydrocarbon chain, making it hydrophobic. The “head” of the molecule is a carboxyl group which is hydrophilic. Fatty acids are the main component of soap. where their tails are soluble in oily dirt and their heads are soluble in water to emulsify and wash away the oily dirt. However, when the head end is attached to glycerol to form a fat, that whole molecule is hydrophobic.
Fatty Acids The terms saturated. mono-unsaturated. and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible. Since the fatty acids in these triglycerides contain the maximum possible amouunt of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature. Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturated fats. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature. Many people have heard that the unsaturated fats are “healthier” than the saturated ones. Hydrogenated vegetable oil (as in shortening and commercial peanut butters where a solid consistency is sought) started out as “good” unsaturated oil. However, this commercial product has had all the double bonds artificially broken and hydrogens artificially added (in a chemistry lab-type setting) to turn it into saturated fat that bears no resemblance to the original oil from which it came (so it will be solid at room temperature).
Cis and Trans Bonds In unsaturated fatty acids, there are two ways the pieces of the hydrocarbon tail can be arranged around a C=C double bond. In cis bonds. the two pieces of the carbon chain on either side of the double bond are either both “up” or both “down,” such that both are on the same side
of the molecule. In trans bonds. the two pieces of the molecule are on opposite sides of the double bond, that is, one “up” and one “down” across from each other. Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds. If oil is used only once like when you fry an egg, only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, like in fast food French fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up. The reason this is of concern is that fatty acids with trans bonds are carcinogenic. or cancer-causing. The levels of trans fatty acids in highly-processed, lipid-containing products such as margarine are quite high, and the government now requires that the amounts of trans fatty acids in such products be listed on the labels.
Omega-3 and Omega-6 Fatty Acids Another set of fat-related terms that are “in the news” a lot, lately are “omega-3” and “omega-6” fatty acids. These terms both refer to fatty acids that have at least one unsaturated bond in their chains, and these terms describe where that bond is located. Starting from the end of the carbon chain (that does not contain the carboxyl group and is not bonded on to the glycerol), the carbon atoms before the first double bond are counted. If there are three carbons, it is an omega-3 fatty acid, and if there are are six carbons, it is an omega-6 fatty acid. We need a balanced amount of both in our diets, but the omega 3 fatty acids are not as common, thus harder to obtain, and so many people’s intake of these two types of fatty acids is way out of balance, including way too many omega-6 fats as compared to the amount of omega-3 fats in their diets. Flax seed and chia seed contain significant amounts of omega-3 fatty acids. Certain types of marine algae manufacture lots of omega-3 fatty acids, and those are incorporated into the tissues of fish which eat those algae. Because of that, many people take fish-oil capsules to increase the amount of omega-3 fats in their diets, but especially for vegetarians, consuming algae-derived omega-3 fats is another option.
Structure of Fats and Oils
Triglyceride Fats and oils are made from two kinds of molecules: glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration synthesis. Since there are three fatty acids attached, these are known as triglycerides. “Bread” and pastries from a “bread factory” often contain mono- and diglycerides as “dough conditioners.” Can you figure out what these molecules would look like? The main distinction between fats and oils is whether they’re solid or liquid at room temperature, and this, as we’ll soon see, is based on differences in the structures of the fatty acids they contain.
The fatty acids that make up the fats and oils in our diets and our bodies may also be lumped into groups based upon the number of carbon atoms in their chains. Interestingly, most of the fatty acids in living organisms have an even number of carbons (2, 4, 6. ). Fatty acids with less than 6 carbons in their chains may collectively be called short-chain fatty acids. but usually these are just called carboxylic acids and are not referred to as “fatty” acids. Those with 6 to 12 carbons are medium-chain fatty acids. those with 14 to 22 carbons are long-chain fatty acids. and those with over 22 carbons are the very-long-chain fatty acids. Since, as mentioned above, fats and oils contain three fatty acids and are called triglycerides, those which contain primarily medium-chain fatty acids are referred to as medium-chain triglycerides (MCTs). while those which contain primarily long-chain fatty acids are referred to as long-chain triglycerides (LCTs). Most of the fats and oils in our diets and that we’re used to hearing about in the news are LCTs.