Lipids
The lipids are a very diverse group of molecules, which have very different structures. But they all have one thing in common . . . they are all insoluble in H2O. Lipids have some portion of their structure which is "non-polar", therefore these will not dissolve in "polar" solvents (i.e. H2O).
Recall the nature of "Ionic Bonds" and "Polar Covalent Bonds"
Lipids include the compounds commonly known as fats, oils, and waxes. We will look at three important classes of lipids.
The Triglycerides
Both fats and oils are "triglycerides". These molecules are made up of 3 long chain "fatty acids" attached to a 3 carbon molecule called "glycerol".
The carboxyl and the fatty acids are attached to the -OH groups of the Glycerol via a "dehydration synthesis" reaction to yield an "ester" bond.
Function: storage of energy - "fat" in animals, and "oils" in plants.
Saturated and Unsaturated Fatty Acids
Saturated Fatty Acid: These are fatty acids which contain the maximum possible number of hydrogen atoms. That is each carbon in the chain has two hydrogen atoms attached to it. It is "saturated" with hydrogen atoms.
Unsaturated Fatty Acid: These are fatty acids which contain carbon-to-carbon "double" bonds. Therefore since a carbon atom can have only 4 covalent bonds, there is one less bond available for hydrogen, therefore there is one less hydrogen. (The carbons are not "saturated" with hydrogen atoms.)
Fatty acids with several carbon-carbon double bonds are said to be "polyunsaturated".
Fats containing unsaturated fatty acids are oils and most tend to be liquid at room temperature. These are often produced by plants (corn oil, sunflower oil, etc.). Fats containing saturated fatty acids tend to be solid at room temperature. Butter and animal fat are good examples.
One interesting feature of the carbon-carbon double bond in these unsaturated fatty acid chains is that it puts a "kink" in the chain. These kinked carbon chains tend to take up more room than do the straight chained fatty acids.
Trans Fats: What's up with that?
Fatty Acid Configurations
What are Trans Fats?
Double bonds bind carbon atoms tightly and prevent rotation of the carbon atoms along the bond axis. This gives rise to configurational isomers which are arrangements of atoms that can only be changed by breaking the bonds.
Cis-9-octadecenoic acid
(Oleic acid)Trans-9-octadecenoic acid
(Elaidic acid)These three-dimensional molecular projections show the Cis and Trans configurational isomers of 9-octadecenoic acid with the hydrogen atoms shown in blue. The Latin prefixes Cis and Trans describe the orientation of the hydrogen atoms with respect to the double bond. Cis means "on the same side" and Trans means "across" or "on the other side". Naturally occurring fatty acids generally have the Cis configuration. The natural form of 9-octadecenoic acid (oleic acid) found in olive oil has a "V" shape due to the Cis configuration at position 9. The Trans configuration (elaidic acid) looks more like a straight line.
Cis Configuration Trans Configuration
What is Hydrogenation and Partial Hydrogenation?
Unsaturated fats exposed to air oxidize to create compounds that have rancid, stale, or unpleasant odors or flavors. Hydrogenation is a commercial chemical process to add more hydrogen to natural unsaturated fats to decrease the number of double bonds and retard or eliminate the potential for rancidity. Unsaturated oils, such as soybean oil, which contain unsaturated fatty acids like oleic and linoleic acid, are heated with metal catalysts in the presence of pressurized hydrogen gas. Hydrogen is incorporated into the fatty acid molecules and they become saturated with hydrogen. Oleic acid (C18:1) and linoleic acid (C18:2) are both converted to stearic acid (C18:0) when fully saturated. The liquid vegetable oil becomes a solid saturated fat (shortening with a large percentage of tristearin). By comparison, animal fats seldom have more than 70% saturated fatty acid radicals. In the table above, for example, lard has 54% unsaturated fatty acid radicals.
Hydrogenation ProcessFully saturated fats are too waxy and solid to use as food additives, so manufacturers use partially hydrogenated oils. These oils are also produced at high temperatures with metal catalysts and pressurized hydrogen, but the process is stopped when the oil has the proper consistency for its application. The high temperatures and catalysts used for this chemical reaction weaken the double bonds and, as a side effect, cause a large percentage of the natural Cis double bonds to change to Trans double bonds. Trans fatty acids are present mainly in partially hydrogenated fats, but they are also present in hydrogenated fats because chemical reactions never achieve 100% efficiency.
Metabolism of Fats -- Why are Trans Fats Bad?
Metabolism of natural C20 Cis fatty acids produces powerful eicosanoids.
Metabolism of natural 20-carbon polyunsaturated fatty acids like arachidonic acid results in the biosynthesis of mediators with potent physiological effects such as prostaglandins, prostacyclins, thromboxanes, leucotrienes, and lipoxins. These substances are known collectively as eicosanoids because they contain 20 carbon atoms (Greek eikosi = 20). However, polyunsaturated trans fatty acids cannot be used to produce useful mediators because the molecules have unnatural shapes that are not recognized by enzymes such as cyclooxygenase and lipoxygenase. Although low levels of trans-vaccenic acid occur naturally in some animal food products, partially hydrogenated oils contain a large proportion of diverse trans fatty acids. When large amounts of Trans fatty acids are incorporated into the cells, the cell membranes and other cellular structures become malformed and do not function properly.
Trans fats are bad for your heart. Dietary trans fats raise the level of low-density lipoproteins (LDL or "bad cholesterol") increasing the risk of coronary heart disease. Trans fats also reduce high-density lipoproteins (HDL or "good cholesterol"), and raise levels of triglycerides in the blood. Both of these conditions are associated with insulin resistance which is linked to diabetes, hypertension, and cardiovascular disease. Harvard University researchers have reported that people who ate partially hydrogenated oils, which are high in Trans fats, had nearly twice the risk of heart attacks compared with those who did not consume hydrogenated oils. Because of the overwhelming scientific evidence linking Trans fats to cardiovascular diseases, the Food and Drug Administration will require all food labels to disclose the amount of Trans fat per serving, starting in 2006.
Trans fats are bad for your brain. Trans fats also have a detrimental effect on the brain and nervous system. Neural tissue consists mainly of lipids and fats. Myelin, the protective sheath that covers communicating neurons, is composed of 30% protein and 70% fat. Oleic acid and DHA are two of the principal fatty acids in myelin. Studies show that trans fatty acids in the diet get incorporated into brain cell membranes, including the myelin sheath that insulates neurons. These synthetic fats replace the natural DHA in the membrane, which affects the electrical activity of the neuron. Trans fatty acid molecules alter the ability of neurons to communicate and may cause neural degeneration and diminished mental performance. Neurodegenerative disorders such as multiple sclerosis (MS), Parkinson's Disease, and Alzheimer's Disease appear to exhibit membrane loss of fatty acids. Unfortunately, our ingestion of trans fatty acids starts in infancy. A Canadian study showed that an average of 7.2% of the total fatty acids of human breast milk consisted of trans fatty acids which originated from the consumption of partially hydrogenated vegetable oils by the mothers.
What are Omega-3 and Omega-6 fatty acids?
Omega-3 (ω3) and omega-6 (ω6) fatty acids are unsaturated "Essential Fatty Acids" (EFAs) that need to be included in the diet because the human metabolism cannot create them from other fatty acids. Since these fatty acids are polyunsaturated, the terms n-3 PUFAs and n-6 PUFAs are applied to omega-3 and omega-6 fatty acids, respectively. These fatty acids use the Greek alphabet (α,β,γ,...,ω) to identify the location of the double bonds. The "alpha" carbon is the carbon closest to the carboxyl group (carbon number 2), and the "omega" is the last carbon of the chain because omega is the last letter of the Greek alphabet. Linoleic acid is an omega-6 fatty acid because it has a double bond six carbons away from the "omega" carbon. Similarly, alpha-linolenic acid is an omega-3 fatty acid because it has a double bond three carbons away from the "omega" carbon. By subtracting the highest double-bond locant in the scientific name from the number of carbons in the fatty acid we can obtain its classification. For arachidonic acid, we subtract 14 from 20 to obtain 6; therefore, it is an omega-6 fatty acid. This type of terminology is sometimes applied to oleic acid which is an omega-9 fatty acid.
In these simplified structural formulas of unsaturated fatty acids, each angle represents a carbon atom. Notice that all the double bonds have the Cis configuration.
DHA (docosahexaenoic acid) and AA (arachidonic acid) are both crucial to the optimal development of the brain and eyes. The importance of DHA and AA in infant nutrition is well established, and both substances are routinely added to infant formulas. Excessive amounts of omega-6 polyunsaturated fatty acids and a very high omega-6/omega-3 ratio have been linked with pathogenesis of many diseases, including cardiovascular disease, cancer, and inflammatory and autoimmune diseases. The ratio of omega-6 to omega-3 in modern diets is approximately 15:1, whereas ratios of 2:1 to 4:1 have been associated with reduced mortality from cardiovascular disease, suppressed inflammation in patients with rheumatoid arthritis, and decreased risk of breast cancer.
Click hear to learn more about how to increase Omega-3 and Omega-6 fatty acids the in your diet. omega-3info.com
Phospholipids
These molecules are structurally similar to the triglycerides, but they differ in one important respect. Triglycerides have 3 fatty acid chains, but the phospholipids have only 2 fatty acid chains and one phosphate (-) group. The negatively charged phosphate group (and its various end groups) cause this end of the molecule to form a "polar" covalent bond with glycerol. That is this end of the phospholipid molecule is "polar" while the fatty acid chain is "non-polar". Therefore one end of the molecule is charged (-), i.e. polar and the other end of the molecule is not charged (neutral), i.e. non-polar.
Since water is also a polar molecule the polar end of the phospholipid is "attracted" to the + ends of the water molecules. It is said to be "hydrophillic" (or water loving). While the neutral end of the phospholipid molecule is non-polar, i.e. is repelled by the "polar" water molecules, it is said to be "hydrophobic" (water fearing).
This duel nature of the phospholipid molecule makes it very useful as a component of cell membranes.
