Absolutely Small: How Quantum Theory Explains Our Everyday World - Michael D. Fayer (2010)
Chapter 16. Fat, It’s All About the Double Bonds
IN THIS CHAPTER, we will use some of the ideas developed previously to talk about some of the large molecules commonly discussed in everyday life. We hear about such things as saturated fats, unsaturated fats, transfats, and cholesterol. What are these things, and how do they differ? What are the relationships between molecular structure and health effects?
WHAT IS A FAT MOLECULE?
When we say fat, you may think of butter, lard, olive oil, or cottonseed oil. Each of these is actually a mixture of different fats. Figure 16.1 shows one particular molecule that is a fat. It is stearic acid. It is a long hydrocarbon chain with an organic acid group on the end. Stearic acid has 18 carbon atoms. The end carbon on the right is the organic acid group. Figure 15.4, bottom, shows acetic acid, which is an acid group with one methyl attached. Figure 15.5 shows tetradecane, a 14-carbon hydrocarbon, which is a component of heating oil. Stearic acid is like acetic acid but with a 17-carbon chain attached to the acid group instead of a single methyl group, or stearic acid is a somewhat longer hydrocarbon than tetradecane with an acid group on the end. Basically, a fat is a long carbon chain with an organic acid group on one end. The acid group forms hydrogen bonds to water. As discussed in Chapter 15, acetic acid is soluble in water because of the strong hydrogen bonding interactions between the acid group and water. Stearic acid, and fats in general, are not soluble in water because of the long hydrocarbon chain. While the acid group is strongly attracted to water (hydrophilic), the long hydrocarbon chain, like hydrocarbons discussed in Chapter 15, does not like to interact with water (hydrophobic). For fats, the long hydrocarbon portion of the molecules wins, and in general fats are not water soluble.
FIGURE 16.1. Ball-and-stick (top) and space-filling models (bottom) of stearic acid. Stearic acid has 18 carbon atoms, 36 hydrogen atoms, and two oxygen atoms. It is a 17-carbon hydrocarbon with an acid group, -COOH, on the end (right side).
SATURATED AND UNSATURATED FATS
Stearic acid is a saturated fat. Each carbon is bonded to one or two other carbons only by single bonds. There are no double bonds connecting carbon atoms. A saturated fat is a fat with only single bonds between carbon atoms.
Figure 16.2 shows a ball-and-stick model of oleic acid. Like stearic acid, oleic acid has 18 carbon atoms with an acid group on one end. However, it has one double bond between carbons 9 and 10, where the carbon atoms are numbered beginning with the carboxylic acid carbon. Oleic acid is a monounsaturated fat. It is unsaturated because it has a double bond. It is monounsaturated because it has only one double bond. Saturated fats have no double bonds between carbon atoms. In stearic acid, except for the carbon in the acid group, all of the carbons use four sp3 hybrid orbitals to form bonds. Carbons not at the ends of the stearic acid molecule use two of the four sp3 hybrid orbitals to form single bonds to the adjacent carbons and two to bond two hydrogens. Each carbon, except for the acid carbon, has a tetrahedral arrangement of bonds to other carbons and hydrogens. Oleic acid’s carbons 9 and 10 use three sp2 hybrids to form three σ bonds, one to a hydrogen and two to the adjacent carbons. Carbons 9 and 10 each use their remaining 2p orbitals, one on each carbon, to form a π bond between them. So carbons 9 and 10 have a double bond, and these carbons have a trigonal, rather than a tetrahedral, bonding geometry. The difference in the geometry can be seen clearly in Figure 14.14 by comparing the model of ethane (carbon-carbon single bond) with ethylene (carbon-carbon double bond). In ethane, the carbon centers are tetrahedral. In ethylene, the carbons are trigonal. Oleic acid has 34 hydrogen atoms in contrast to stearic acid, which has 36 hydrogen atoms. Oleic acid uses two orbitals to form the double bond that is used in stearic acid to bond hydrogens. A saturated fat has as many hydrogen atoms as possible, meaning it has no double bonds.
FIGURE 16.2. Ball-and-stick model of oleic acid. Oleic acid has 18 carbon atoms like stearic acid in Figure 16.1, but it has one carbon-carbon double bond between carbons 9 and 10 counting from the acid group.
THE SHAPES OF FAT MOLECULES
Stearic acid, as shown in Figure 16.1, is in the all-trans conformation. Figure 14.13 shows butane in the all-trans configuration, with rotation around one of the bounds to give a gauche conformation. Stearic acid can assume many conformations besides the all-trans conformation shown. The all-trans conformation is the most linear, which is the lowest energy conformation for a saturated hydrocarbon or a saturated fat. Because saturated fats only have carbon-carbon single bonds, they are constantly interconverting from one conformer to another. Figure 15.7 displays heptadecaneacetate not in an all-trans configuration. In contrast to stearic acid, oleic acid (Figure 16.2) is not naturally produced in an all-trans configuration. In Figure 16.2, the angles made by carbons 8, 9, and 10 and by carbons 9, 10, and 11 are 120° (trigonal) and not 109.5° (tetrahedral). Therefore, under normal biological conditions adding one double bond locks in a particular shape around the double bond.
SATURATED, MONOUNSATURATED, AND POLYUNSATURATED FATS
Figure 16.3 is a ball-and-stick model of α-linolenic acid. Like stearic acid and oleic acid, α-linolenic acid has 18 carbons, but it has three double bonds. α-linolenic acid is said to be polyunsaturated in that it has more than one double bond. In linolenic acid, six of the carbon atoms, specifically 9, 10, 12, 13, 15, and 16, are trigonal, with 120° angles formed by three carbon atoms, such as 8, 9, and 10, rather than with 109.5° tetrahedral angles. Under normal conditions, these additional double bonds force the shape further away from an all-trans configuration. While α-linolenic acid has three double bonds, linolenic acid, which is similar, has only two double bonds. A fat with no double bonds is saturated. A fat with one double bond is monounsaturated, and a fat with two or more double bonds is polyunsaturated.
FIGURE 16.3. Ball-and-stick model of α-linolenic acid, which has 18 carbon atoms and three carbon-carbon double bonds.
DOUBLE BONDS IN FATS MATTER
Why does it matter whether or not a fat has double bonds? This is a question that divides into two types of issues, those for chemically modified fats and those for nonchemically modified fats contained in oils and other sources of fat like butter. First, let’s look at fats that have not been chemically modified. Fats have been correlated with either increasing or decreasing cholesterol levels. Myristic acid, which is a saturated fat with 14 carbons, is believed to increase cholesterol significantly in a deleterious manner. Palmitic acid, which is a saturated fat with 16 carbons, is thought to increase cholesterol to some extent. In contrast, linolenic acid (18 carbons with two double bonds) and other polyunsaturated fats decrease cholesterol levels. However, some saturated fats, such as stearic acid (Figure 16.1), do not appear to have much influence on cholesterol levels, which is also true of monounsaturated fats such as oleic acid (Figure 16.2).
In common oils used in food, the fractions of important unsaturated and polyunsaturated fats vary widely. Butter fat and coconut oil contain large amounts of myristic acid and palmitic acid and very little linolenic acid. Olive oil contains no myristic acid, but it has a significant amount palmitic acid. It also has some linolenic acid. Canola oil has no myristic acid and almost no palmitic acid. It has a substantial amount of linolenic acid. Grape seed oil, safflower oil, and sunflower oil (the later two when not processed for high-temperature cooking, see below) have no myristic, small amounts of palmitic, but very large amounts of linolenic acid. The chemical composition of these fats indicate that butter fat and coconut oil will have a deleterious effect on blood cholesterol, olive oil is approximately neutral, canola oil has a positive effect, and grape seed oil, safflower oil, and sunflower oil (the last two not for high-temperature cooking) have a very positive effect on blood cholesterol.
CHEMICALLY MODIFIED FATS
Fats are chemically modified for several reasons. Unsaturated fats are fats with double bonds. Double bonds are chemically very reactive. The double bonds in unsaturated fats can react with oxygen. When this occurs to a significant extent, the smell and taste of the oil becomes unpleasant; we say the oil has become rancid. The rates of reactions of oxygen with unsaturated fats are exacerbated by light.
Polyunsaturated fats have more double bonds. With more double bonds available to react with oxygen, the polyunsaturated oil will become rancid more readily. Oils containing unsaturated fats should be refrigerated. Lowering the temperature slows the chemical reactions that cause rancidity. If such oils are not refrigerated, it is better to keep them in a cool, dark place. Some oils come in dark bottles. The dark bottle may improve their shelf life in a store where they are exposed to light. Oils that are composed of almost all saturated fats will keep for a long time without refrigeration. The result is that many oils are chemically processed to reduce or eliminate the double bonds. Such processing also changes the physical properties of the oils, raising the temperatures at which they melt and boil. These changes can be useful in cooking, in preparing baked goods, and in other processed food applications.
PARTIALLY HYDROGENATED AND HYDROGENATED FATS
The oils that are chemically modified to reduce or eliminate double bonds are said to be partially hydrogenated or hydrogenated. A carbon atom makes four bonds. In saturated fats (no double bonds), each carbon is bonded to two other carbons and to two hydrogens, except for the carbons at the ends of a molecule. A double bond eliminates two hydrogen atoms. This can be seen by comparing stearic acid (Figure 16.1) and oleic acid (Figure 16.2). Formation of the double bond in oleic acid between carbons 9 and 10 uses up one bond on each of the two carbon atoms that in stearic acid form bonds to hydrogen atoms. Therefore, the process of eliminating double bonds from a fat increases the number of hydrogen atoms, and we say the oil has been hydrogenated.
HYDROGENATION OF FATS
Double bonds are very stable, and it is difficult to break a double bond. The hydrogenation process turns a carbon-carbon double bond into a carbon-carbon single bond with a hydrogen added to each carbon. The process requires high temperature, a metallic catalyst, and hydrogen. A catalyst is a material that makes a chemical reaction occur faster, but the catalyst is not used up in the process. Qualitatively, the process of hydrogenation works as follows. One of the carbon atoms of a double bond binds to the metal, essentially eliminating the double bond. The binding leaves the other carbon atom with an unpaired electron. That carbon atom is now single bonded to two other carbons and one hydrogen. As we know, a carbon atom wants to make four bonds to have enough electrons to obtain the neon closed shell configuration. The carbon grabs a hydrogen atom. The other carbon atom breaks its bond to the metal catalyst, and it grabs another hydrogen atom. The carbon-carbon double bond has been converted to a single bond and two hydrogen atoms have been added to the fat.
For a polyunsaturated fat, this process can be repeated at all of the double bonds or only at some of them. If it occurs at all of the double bonds, the polyunsaturated fat has been converted to a saturated fat; we say the fat has been hydrogenated. If hydrogenation occurs at some but not all of the double bonds, we say the fat has been partially hydrogenated. The resulting fat may be monounsaturated or still polyunsaturated but with fewer double bonds. The degree of hydrogenation is controlled to produce a resulting fat with the desired properties, particularly whether it is a solid or liquid at room temperature, the melting temperature, and the boiling temperature.
READ THE LABEL
As discussed above, polyunsaturated fats can be beneficial. As they occur naturally, both sunflower oil and safflower oil have very large percentages of polyunsaturated fats. However, many brands of sunflower oil and safflower oil have been partially hydrogenated, so they are more usable for high-temperature cooking. It is possible to tell by reading the nutrition label if sunflower oil and safflower oil have been partially hydrogenated. If the label shows that the amount per serving of monounsaturated fat is greater than the amount of polyunsaturated fat, then the oil has been partially hydrogenated. If these oils have not been partially hydrogenated, then the amount of polyunsaturated fat will greatly exceed the amount of monounsaturated fat. So to get the benefits of significant quantities of polyunsaturated fat from sunflower or safflower oils, the oils should not be partially hydrogenated. Reading the label will tell.
All of this hydrogenation of oils sounds fine except for one thing; the big elephant in the room is trans fats. What is a trans fat? Figure 16.4 shows oleic acid in both its cis and trans geometries. Both molecules have 18 carbons, 34 hydrogens, and one carbon-carbon double bond. The atoms are connected one to another in the same order. The difference is the geometry around the double bond.
In the cis conformation, the two hydrogens bonded to carbons 9 and 10 are on the same side of the molecule. They point at an angle toward the top of the page. The molecule is drawn with the double bond horizontal. The angle between the double bond and one of the H atoms is 120° because trigonal sp2 hybrids are used to make the σ bonds. So the angles made by the bonds from carbons 9 and 10 to the hydrogens are 30° from the vertical direction. For the cis molecule, the two chains of atoms connected to carbons 9 and 10 on either side of the double bond angle downward, again making angles that are 30° from the vertical line that is perpendicular to the double bond.
FIGURE 16.4. Ball-and-stick models of cis-oleic acid and trans-oleic acid. Both have 18 carbons and one double bond. However, their geometries differ.
In the trans conformation, the two hydrogens bonded to carbons 9 and 10 are on opposite sides of the molecule. One points almost straight up toward the top of the page and the other points almost straight down toward the bottom of the page. The two chains of carbon atoms that are bound to carbons 9 and 10 come out in opposite directions relative to the double bond. The net result is that the cis molecule is “bent” around the double bond, while the trans molecule is “straight” relative to the double bond.
Rotation around a carbon-carbon double bond cannot occur under normal conditions. The inability of rotation to occur around double bonds is fundamentally important. Figure 14.13 shows the gauche and trans conformations of n-butane, which has only single bonds. Rotation around single bonds can occur readily at room temperature. Therefore, in the example of n-butane, the gauche and trans conformations are not fixed. In fact, in liquid solution at room temperature, the n-butane gauche and trans conformations will interconvert by rotation around the middle carbon-carbon single bond in approximately 50 ps (50 trillionth of a second), which is a very short time. In contrast, the cis and trans conformations of oleic acid shown in Figure 16.4 are locked in. They will not interconvert without very high temperature and a catalyst.
To understand why rotation around a carbon-carbon single bond occurs readily while rotation about a double bond does not, we need to look again at the hybrid orbitals used by carbon to make single and double carbon-carbon bonds. Figure 14.9 illustrates the hybrid orbitals used in ethane to form the carbon-carbon single bond. Each carbon bonds to the other atoms using four sp3 hybrid orbitals. The middle part of Figure 14.9 shows schematically the formation of the carbon-carbon single bond by the overlap of one sp3 orbital on each carbon. The two orbitals, one from each carbon, point right at each other. Rotating one of the carbons does not change the overlap of the orbitals. There are favored configurations because the hydrogens on the two carbons want to avoid each other as much as possible, but the molecule can readily rotate from one favored configuration to another without changing the carbon-carbon sp3 orbital overlap. This is in contrast to the situation for ethylene, which has a carbon-carbon double bond. Figure 14.15 shows the orbitals used to form the double bond in ethylene. Each carbon uses three sp2 hybrids to make σ bonds to two hydrogens and the other carbon as shown in the top portion of Figure 14.15. The three sp2 orbitals on each carbon are formed by superpositions of the 2s, 2px, and 2py orbitals. These orbitals and the σ bonds are in the plane of the page, which is taken to be the xy plane. That leaves one 2pz orbital on each carbon atom, which will point perpendicular to the plane of the page. As shown in the bottom portion of Figure 14.15, the two 2pzorbitals overlap side to side to formaπ bond. If you could grab one of the carbons and rotate it, you would be rotating that 2pz orbital away from the z direction toward the xy plane. Such a rotation will decrease the overlap between the two 2pz orbitals, breaking the π bond. As shown in the table that follows the discussion of Figure 13.9, a double bond is much stronger than a single bond. Therefore, it will take a great deal of energy to rotate around a carbon-carbon double bond because it is necessary to break the π bond for the rotation to occur. The large energy penalty that would be paid prevents rotation.
NATURE PRODUCES CIS FATS BUT CHEMICAL PROCESSING PRODUCES TRANS FATS
Unsaturated fats, both monounsaturated and polyunsaturated, are produced in nature virtually exclusively in cis conformations. Small amounts of trans fats are found in the meat and milk of cattle, sheep, goats, and other ruminants. However, large amounts of trans fats are present in partially hydrogenated oils, and trans fats are also found in hydrogenated oils because the chemical processing does not result in an oil that is 100% saturated. Unprocessed monounsaturated and polyunsaturated vegetable oils have only cis conformations about their double bonds. Partial hydrogenation of the naturally occurring oils generates large quantities of trans fats. This transformation from cis to trans occurs during the hydrogenation process. As discussed above, one of the carbons of a carbon-carbon double bond binds to a metal catalyst in the reaction vessel, which is at very high temperature. When bound to the catalyst, the carbon-carbon bond is effectively a single bond, and rotation from cis to trans can occur. The oil can break its bond to the catalyst before hydrogenation occurs, so the double bond is not hydrogenated, and it reforms. So, rotation from cis to trans can take place before the oil is released from the catalyst. If this occurs, the result is the conversion of a cis conformation to a trans conformation without hydrogenation of the double bond. The processing is intended to reduce the number of double bonds, not eliminate all of them. But a substantial number of double bonds are converted from cis to trans. The result is that partially hydrogenated oils can contain substantial fractions of trans double bonds.
TRANS FATS CAN BE DELETERIOUS
Trans fats have been shown to have a number of deleterious health effects. The basic reason for the harmful effects of trans fats arises from the fact that biological systems have developed to deal with cis fats, and shape matters. Enzymes are proteins (large biological molecules) that act as very specific chemical factories. They can convert fats into other useful molecules, as well as break down fats for elimination. However, an enzyme that will work on a cis fat will, in general, not produce the same chemical reactions or any reaction at all for the trans fat, although it has the identical chemical formula. So two fats that have the identical numbers of carbons, hydrogens, and oxygens, all connected to each other in the same order, will be treated very differently biochemically depending on whether they are cis or trans. Our bodies have not evolved to deal with substantial amounts of trans fats.
Trans fats have been strongly linked to heart disease because of their effect on cholesterol levels in the blood. Trans fats may also have a harmful effect on the nervous system. Myelin is a covering that protects neurons. It is composed of about 30% protein and about 70% fat. The two main fats are oleic acid (see Figures 16.2 and 16.4) and docosahexaenoic acid (DHA, see below). Trans fats replace DHA in brain cell membranes and in myelin. Trans fat alters the electrical signals that comprise the messages in the nervous system, affecting communication between neurons. It is remarkable that a change in shape of a molecule, without changing its chemical composition, can make a beneficial food a harmful food.
WHEN ZERO IS ZERO
Because of the mounting evidence that trans fats are harmful to our health, they should be avoided. Consumer advocacy groups have encouraged the required removal of trans fats from cooking oils used in fast food restaurants and in various commercial products. Because of the bad publicity surrounding trans fats, manufacturers try to avoid alerting consumers to their presence. Now that most people know that partial hydrogenation will produce trans fats, some food labels use the term modified instead of the phrase “partially hydrogenated.” Even more amazing is the U.S. government’s definition of 0% trans fat. Government regulations permit a product label to say that an oil contains 0% trans fat if one serving contains less than 0.5 grams of trans fat, but the manufacturer is permitted to define the size of a serving. Let’s say that there are 0.6 grams of trans fat in a tablespoon of oil. One tablespoon is three teaspoons. So, the manufacturer defines a serving as two teaspoons, which contain 0.4 grams of trans fat. Thus, by changing the definition of a serving, the oil has 0% trans fat. This type of labeling is not permitted in Europe and other countries. Minimizing the amount of partially hydrogenated oil that you consume will reduce your exposure to trans fats.
OMEGA 3 FATTY ACIDS
Figure 16.5 is a ball-and-stick model of docosahexaenoic acid (DHA). As mentioned, DHA is an important component of the lining of nerves. It has 22 carbons and six carbon-carbon double bonds. It is highly unsaturated. All of the double bonds are in a cis configuration. DHA is one of a class of unsaturated fatty acids (fats) popularly called omega-3 (ω-3) fatty acids. These fats are thought to be beneficial to human health.
FIGURE 16.5. Ball-and-stick model of docosahexaenoic acid (DHA). DHA is a polyunsaturated fat with 22 carbon atoms and six carbon-carbon double bonds all in the cis conformation.
The better name for this class of fats is n-3 fatty acids, with n referring to the number of carbons. The carbons are numbered beginning with the carbon that forms the carboxylic acid. This carbon is labeled 1. So, for DHA counting around the chain, the last carbon at the opposite end from the carboxylic acid group is carbon 22 (see Figure 16.5). This is n, the number of carbons in the chain. Carbon n-3 is the carbon number that results when 3 is subtracted from the total chain length, n. For DHA, that number is 19 as indicated in Figure 16.5. The fat is an ω-3 fatty acid if the n-3 carbon is double bonded as shown in the figure. α-linolenic acid displayed in Figure 16.3 is also an ω-3 fatty acid. It has 18 carbons, so n-3 is 15. As seen in Figure 16.3, there is a double bond between carbons 15 and 16.
The fats we have discussed so far involve single chains. However, a large class of fats commonly found in the body contains three fatty acid molecules tied together into one molecule. These are called triglycerides. Capric acid is a saturated fat with 10 carbons. Capric acid triglyceride is shown in Figure 16.6. There are three capric acid chains, with the first carbon in each chain labeled 1 and the last carbon labeled 10. There is a short carbon chain of three carbons labeled A, B, and C. In a stand-alone capric acid, the acid carbon (labeled 1) is doubled bonded to one oxygen and single bonded to a hydroxyl group,—OH. In a triglyceride, the H of the hydroxyl group is replaced with one of the carbons in the three-carbon chain. In Figure 16.6, the top chain is bonded to carbon A, the middle chain to carbon B, and the bottom chain to carbon C. Capric acid triglyceride is a medium chain-length triglyceride. The medium chain-length triglycerides, or MCTs, have chains from 6 to 12 carbons. Long-chain triglycerides (LCTs) have chain lengths greater than 12.
When talking about foods and fat, you frequently hear that it is unwise to eat too much fat because it will increase your cholesterol level. Therefore, many people have the mistaken impression that cholesterol is a fat. They think that eating a lot of fat means eating a lot of cholesterol. However, cholesterol is not a fat. Rather, it is an alcohol, as indicated by the suffix, ol. The ol indicates that a molecule is an alcohol, as for example, in ethanol (see Figure 15.1). The structure of cholesterol is shown in Figure 16.7. The top is a diagram of cholesterol, the middle portion is a ball-and-stick model, and the bottom shows a space-filling model. The alcohol—OH group is on the left side of the diagram and is the bottom left group in the ball-and-stick and space-filling models. The molecule has four carbon rings labeled 1 to 4. In the figure, carbons are at all of the vertices, and each carbon will make four bounds. Hydrogens are not shown in the figure except where necessary to indicate if a hydrogen is pointing into or out of the plane of the page. A dashed triangle is pointing into the plane, and a solid triangle is pointing out of the plane. If there is no H at the end of the triangle, then the group is a methyl,—CH3. The top portion of the figure makes it easy to see which atoms are connected to each other. The ball-and-stick model provides a more detailed three-dimensional illustration of the molecular structure. The space-filling model gives a more representative picture of the molecule’s three-dimensional structure. The space-filling model reflects the regions of space where most of the electron probability is located. It is important to keep in mind that molecules are not balls and sticks but rather delocalized electron clouds surrounding the atomic centers, the positively charged atomic nuclei.
FIGURE 16.6. Ball-and-stick model of capric acid triglyceride, which is composed of three capric acid chains. Each chain is a saturated fatty acid with 10 carbon atoms labeled 1 to 10. These are attached to three carbons labeled A, B, and C.
Comparing the structure of cholesterol in Figure 16.7 to any of the models of fats (fatty acids) displayed above shows that cholesterol is a very different type of molecule from fats. For example, the space-filling model of stearic acid (Figure 16.1) is very different from the space-filling model of cholesterol in Figure 16.7. Clearly, at the molecular level, cholesterol has little relationship to fatty acids. Yet it is frequently linked to discussions of fat in food, and the cholesterol molecule has a very negative aura associated with it.
FIGURE 16.7. Cholesterol. Top: Diagram of cholesterol. Middle: Ball-and-stick model. Bottom: Space-filling model. Cholesterol is an alcohol (—OH group) composed of four carbon rings, labeled 1 to 4, and a hydrocarbon chain.
Cholesterol Is Good, Contrary to Public Perception
Well, cholesterol is getting a bad rap. Cholesterol is a fundamentally important biological molecule. Cells are surrounded by membranes. Inside the cell are all of the complex molecular machines necessary to perform the chemistry responsible for life. Outside the cell are a large number of other chemicals, including oxygen, salts, and large biological molecules. The cell membrane that separates the inside from the outside permits certain molecules to go in and out of the cell while others remain only on the outside or inside. The principal components of these cell membranes are phospholipids. A phospholipid is composed of two hydrocarbon chains, typically 16 carbons long, connected at one end by a head group structure that contains both a positive and a negative charge. The charges of the head group make the head groups highly hydrophilic (attracted to water). The hydrocarbon chains are very hydrophobic (repelled by water). Cells are surrounded by water and also contain a great deal of water on the inside. The charged head groups want to be in water, while the hydrocarbon tails want to avoid water. To satisfy both the hydrophilic-charged head groups and the hydrophobic hydrocarbon tails, the phospholipids arrange themselves into a bilayer, as shown schematically in Figure 16.8.
The figure shows a section of a phospholipid bilayer membrane that completely surrounds and encloses cells. The balls are the charged head groups, and the wavy lines represent the hydrocarbon chains. An actual cell membrane is much more complex than illustrated in Figure 16.8. The membrane contains a large number of proteins that perform specific functions, such as allowing certain ions or molecules to pass into the cell while preventing others from entering.
FIGURE 16.8. Schematic of a portion of a phospholipid bilayer with two cholesterol molecules. The head groups (balls) are charged and want to be in water. The hydrocarbon tails avoid water by formation of the bilayer. The cholesterol hydroxyl is at the water interface.
In addition to phospholipids, a major component of cell membranes is cholesterol. Cholesterol comprises as much as 30% of cell membranes. Figure 16.8 shows a schematic of two cholesterol molecules replacing two of the phospholipids. Cholesterol is important because it controls the mechanical properties of the bilayers. Without cholesterol, cell membranes would not function. Therefore, cholesterol is essential. The human body produces a great deal of cholesterol, and only a limited fraction of the necessary cholesterol is taken in through food. The take-home message is that if you could eliminate all of the cholesterol from your body, you would die.
The Problem with Cholesterol
The problem with cholesterol is not that you take some in when you eat, but rather how it behaves in the body. Harmful effects on health from cholesterol are related to fat, but not because cholesterol is a fat or even because fatty foods may contain cholesterol. Cholesterol moves through the bloodstream associated with very large biomolecular aggregates called lipoproteins. These are composed of very large proteins, phospholipids, fats, cholesterol, and other molecules. The lipoproteins can be divided into at least two classes: low-density lipoproteins (LDL) and high-density lipoproteins (HDL). They are somewhat egg shaped, with an approximate diameter of 200 Å (200 × 10-10meters). The volume of these particles is ap proximately 5,000,000 Å3. In contrast, the volume of a cholesterol Å3 molecule is approximately 200 Å3. So, an LDL or HDL particle is about 20,000 times larger than a cholesterol molecule and carries many cholesterol molecules in the bloodstream. High levels of LDL relative to HDL are strongly correlated with coronary artery disease and atherosclerosis. The mechanism is not well understood, but LDL-carrying cholesterol leads to deleterious deposits in the arteries, while HDL does not. These high levels of LDL, when compared to HDL (a high LDL to HDL ratio), are produced by saturated fats and, even worse, trans fats. Saturated fats increase the level of LDL. Trans fats not only increase LDL levels, but they also decrease HDL levels, exacerbating the problem. So eating fat does matter, but not because fatty foods can contain cholesterol. What matters is the nature of the fat we eat. Oils that are high in polyunsaturated fats, which have not been processed in a manner that produces large quantities of trans fats, are desirable.
In Chapter 14, we discussed carbon-carbon single and double bonds. The types of hybrid atomic orbitals used to form molecular orbitals were explicated. Quantum theory allows us to understand the details of bonding and the effects of the nature of bonding on the shapes of molecules and the strengths of the bonds that hold atoms together. In this chapter, fats have been used to illustrate how the seemingly small details of the molecular bonding, such as single bonds versus double bonds, the number of double bonds, and cis versus trans structure about a double bond, play fundamentally important roles in biology. The geometry of double bonds may literally be a life-and-death issue.