Lipids are a second group of macromolecules important to all living organisms. Lipids, just like carbohydrates, are made primarily from atoms of carbon, hydrogen, and oxygen, but the atoms are in different proportions. In particular, while carbohydrates usually have two hydrogen atoms for every oxygen, lipids have more hydrogen atoms for each carbon. As a result, lipids have signifi cantly more C–H bonds and contain signifi cantly more stored energy.

What exactly is a lipid? That’s not as easy to answer as you might expect. Lipids come in a wide variety of structures. They don’t have any unique subunits (such as the simple sugars that make up disaccharides and polysaccharides) or particular ratios of atoms that serve as defi ning features. Consequently, lipids are defi ned based on their physical characteristics. Most notably, lipids do not dissolve in water

and are greasy to the touch—think of salad dressings.

Lipids are insoluble in water because, in sharp contrast to water, they consist mostly of hydrocarbons, which are non-polar. Non-polar molecules (or parts of molecules) tend to minimize their contact with water and are considered hydrophobic (“water-fearing”). Lipids cluster together when mixed with water, never fully dissolving. Molecules that

readily form hydrogen bonds with water, on the other hand, are considered hydrophilic (“water-loving”).

One familiar type of lipid is fat, the type most important in long-term energy storage and insulation (FIGURE 2-28). (Penguins and walruses can maintain relatively high body temperatures, despite living in very cold habitats, due to

56 CHAPTER 2 • CHEMISTRY

Atoms Water Carbohydrates Lipids

4 Lipids store energy for a rainy day.

2 • 12 ------------------------------------------------- Lipids are macromolecules with several functions, including energy storage.

A well-insulated harbor seal, in Alaska.

Why does a salad dressing made with vinegar and oil separate into two layers shortly after you shake it?

Q

57

Proteins Nucleic Acids

their thick layer of insulating fat.) Lipids also include sterols,which include cholesterol and many of the sex hormones that play regulatory roles in animals, and phospholipids, which form the membranes that enclose cells. We examine each type of lipid below.

All fats have two distinct components: they have a “head” region and two or three long “tails” (FIGURE 2-29). The head region is a small molecule called glycerol. It is linked to “tail” molecules known as fatty acids. A fatty acid is simply a long hydrocarbon—that is, a chain of carbon molecules, often a dozen or more, linked together with one or two hydrogen atoms attached to each carbon.

The fats in most foods we eat are triglycerides, which are fats having three fatty acids linked to the glycerol molecule. For this reason, the terms “fats” and “triglycerides” are often used interchangeably. Triglycerides that are solid at room temperature are generally called “fats,” while those that are liquid at room temperature are called “oils.”

Fat molecules contain much more stored energy than carbohydrate molecules. That is, the chemical breakdown of fat

Lipids are insoluble in water and greasy to the touch. They are valuable to organisms for long-term energy storage and insulation, in membrane formation, and as hormones.

TAKE-HOME MESSAGE 2 • 12FIGURE 2-28 The many purposes of lipids.

2 • 13 ------------------------------------------------- Fats are tasty molecules too plentiful in our diets.

Symbol for fat (triglyceride) used in this book

C H O

HC

ENERGY

Fats are composed primarily of three elements:

Head:Glycerol

Tails:Fatty acids

Energy is stored in the many carbon-hydrogen bonds.

STRUCTURE OF FATS (TRIGLYCERIDES)

FIGURE 2-29 Triglycerides have glycerol heads and fatty acid tails.

TYPICAL FEATURES OF LIPIDS• Non-polar molecules that do not dissolve in water• Greasy to the touch• Significant source of energy storage

THREE TYPES OF LIPIDS

FATS PHOSPHOLIPIDSSTEROLS

FUNCTIONLong-term energy storage and insulation

FUNCTIONForm the membranes that enclose cells

FUNCTIONRegulate growth and development

INTRODUCTION TO LIPIDS

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Atoms Water Carbohydrates Lipids

molecules releases signifi cantly more energy. A single gram of carbohydrate stores about 4 calories of energy, while the same amount of fat stores about 9 calories—not unlike the difference between a $5 bill and a $10 bill. Because fats store such a large amount of energy, animals have evolved a strong taste preference for fats over other energy sources (FIGURE 2-30). Organisms evolving in an environment of uncertain food supply will build the largest surplus by consuming molecules that hold the most amount of energy in the smallest mass. This feature helped the earliest humans to survive, millions of years ago, but today puts us in danger from the health risks of obesity now that fats are all too readily available.

An important distinction is made between “saturated” and “unsaturated” fats (FIGURE 2-31). These terms refer to the hydrocarbon chain in the fatty acids. If each carbon atom in the hydrocarbon chain of a fatty acid is bonded to two hydrogen atoms, the fat molecule carries the maximum

In saturated fats, each carbon in the hydrocarbon chain is bound to two hydrogen atoms.

In unsaturated fats, at least one carbon in the hydrocarbon chain is bound to just one hydrogen, causing the fatty acid to have a crooked shape.

Straight fatty acids can be packed together tightly. As a result, saturated fats are solid at room temperature.

Crooked fatty acids cannot be packed together tightly. As a result, unsaturated fats are liquid at room temperature.

SATURATED FATS UNSATURATED FATS

Double bond

FIGURE 2-30 Animals (including humans!) prefer the taste of fats.

Because fats store such large amounts of energy, animals have evolved a strong taste preference for fats over other energy sources.

FIGURE 2-31 Degrees of saturation. Fatty acids (and thus the fats that contain them) can be unsaturated or saturated; unsaturated fats can be saturated artifi cially, making them tastier but less healthful.

59

Proteins Nucleic Acids

number of hydrogen atoms and is said to be a saturated fat. Most animal fats, including those found in meat and eggs, are saturated. They are not essential to your health and, because they accumulate in your bloodstream and can narrow the vessel walls, they can cause heart disease and strokes.

An unsaturated fat is one in which some of the carbon atoms are bound to only a single hydrogen. Most plant fats are unsaturated. Unsaturated fats may be mono-unsaturated (if a fatty acid hydrocarbon chain has only one pair of neighboring carbon atoms in an unsaturated state—that is, has only one double bond) or polyunsaturated (if more than one pair of carbons is unsaturated—there’s more than one double bond). Unsaturated fats are still high in calories, but because they can lower cholesterol, they are generally preferable to saturated fats. Foods high in unsaturated fats include avocados, peanuts, and olive oil. Relative to other animals, fi sh tend to have less saturated fat.

The shapes of unsaturated fat molecules and saturated fat molecules are different. When saturated, the hydrocarbon tails of the fatty acids all line up very straight and the fat molecules can be packed together tightly. The tight packing causes the fats, such as butter, to be solid at room temperature. When unsaturated, the fatty acids have kinks in the hydrocarbon tails and the fat molecules cannot be packed together as tightly (see Figure 2-31). Consequently, unsaturated fats, such as canola oil and vegetable oil, do not solidify as easily and are liquid at room temperature.

The ingredient list for many snack foods includes “partially hydrogenated” vegetable oils. The hydrogenation of an oil means that a liquid, unsaturated fat has had hydrogen atoms added to it so that it becomes more saturated. This can be useful in creating a food with a more desirable texture, since increasing a fat’s degree of saturation changes its consistency and makes it more solid at room temperatures. By attaining just the right degree of saturation, it is possible to create foods, such as chocolate, that are near the border of solid and liquid and “melt in your mouth” (FIGURE 2-32). Unfortunately, hydrogenation also makes the food less

healthful because saturated fats are less reactive—your body is less likely to break them down—and so are more likely to

accumulate in your blood vessels, increasing the risk of heart disease.

Hydrogenation of unsaturated fats is problematic from a health perspective because it creates trans fats, the “trans” referring to the unusual orientation of the added hydrogen atoms, which differs from that in other dietary fats—which have their hydrogens in an orientation called “cis.” Trans fats cause your body to produce more cholesterol, further raising the risk of heart

disease, and they also reduce your body’s production of a type of cholesterol that protects against heart disease.

Unsaturated fatty acid(cis form)

Partially hydrogenated trans fatty acid

(trans form)

H

H

Hydrogenation is the artificial addition of hydrogen atoms to an unsaturated fat. This can improve a food’s taste, texture, and shelf-life.

HYDROGENATION

Hydrogenated trans fats increase your risk of heart disease.

Chocolate chip cookie recipes call for some lipids. How will the “chewy-ness” of the cookies differ depending on whether you use butter or vegetable oil as the lipid? Which cookies will be better for your health?

Q

Many snack foods contain “partially hydrogenated” vegetable oils. Why might it be desirable to add hydrogen atoms to a vegetable oil?

Q

FIGURE 2-32 Hydrogenation improves a food’s taste, texture, and shelf-life (but at a cost).

Because of the well-documented links between dietary fats and heart disease, many people are trying to reduce their fat intake. “Fake fats” make this possible. They are designed to be similar to fats in taste and texture, but have one big difference: they cannot be digested by humans. One such “fake fat” is olestra. Olestra, instead of being a

triglyceride fat with three fatty acid tails linked to a glycerol molecule, has eight separate hydrocarbon fatty acids attached to a molecule of sucrose. This octopus-like molecule allows the fatty acids to stimulate their usual taste buds on your

tongue, telling your brain that you are eating a fat. The complex shape of the molecule, however, prevents your body’s digestive chemicals from grabbing onto it and breaking it down. As a consequence, it passes through your digestive system without being digested. It’s not a perfect solution, however. Olestra reduces absorption of some vitamins, and in some people causes abdominal cramping.

60 CHAPTER 2 • CHEMISTRY

Atoms Water Carbohydrates Lipids

Not all lipids are fats, nor do lipids necessarily function in energy storage. A second group of lipids, called the sterols,plays an important role in regulating growth and development (FIGURE 2-33). This group includes some very familiar lipids:

Fats, including the triglycerides common in the food we eat, are one type of lipid. Characterized by long hydrocarbon tails, fats effectively store energy in the many carbon-hydrogen and carbon-carbon bonds. Their caloric density is responsible for humans’ preferring fats to other macromolecules in the diet, and is responsible for their association with obesity and illness in the modern world.

TAKE-HOME MESSAGE 2 • 13

2 • 14 ------------------------------------------------- Cholesterol and phospholipids are used to build sex hormones and membranes.

cholesterol and the steroid hormones such as testosterone and estrogen. These molecules are all modifi cations on one basic structure formed from four interlinked rings of carbon atoms.

Cholesterol is an important component of most cell membranes. For this reason, it is an essential molecule for living organisms. It has a bad reputation in most Western cultures, though, that is mostly well deserved. When we ingest too much cholesterol (present in animal-based foods such as egg yolks, red meat, and cream) and high levels of cholesterol circulate in our bloodstream, the cholesterol can attach to blood vessel walls and cause them to thicken. In turn, this thickening can lead to high blood pressure, a major contributor to strokes and heart attacks. For these reasons, nutritionists advise limiting the consumption of foods high in cholesterol.

Surprisingly, however, most of the cholesterol in our blood doesn’t come directly from the food we consume. Instead, cells in our liver produce almost 90% of the circulating cholesterol. How do they do it? By transforming the saturated fats in our diet. The best way to reduce cholesterol levels, then, may be to consume fewer saturated fats.

The steroid hormones estrogen and testosterone are built through slight chemical modifi cations to cholesterol. These

Sterols are all based on a structure featuring four carbon rings.

Estrogen

Testosterone

CHOLESTEROL• Important component of cell membranes in animals• Can attach to vessel walls and cause them to thicken, which may lead to high blood pressure, stroke, and heart attack

STEROID HORMONES• Regulate sexual development, maturation, and sex cell production• Estrogen influences memory and mood• Testosterone stimulates muscle growth

STEROLS

FIGURE 2-33 Not all lipids are for energy storage. Cholesterol,estrogen, and testosterone are all lipids.

Olestra is a recently developed “fake fat” chemical that gives foods the taste of fat, without adding the calories of fats. What chemical structure might make this possible?

Q

61

Proteins Nucleic Acids

hormones are among the primary molecules that direct and regulate sexual development, maturation, and sperm and egg production. In both males and females, estrogen infl uences memory and mood, among other traits. Testosterone has numerous effects, one of which is to stimulate muscle growth. As a consequence, athletes often take synthetic variants of testosterone to increase their muscularity. But the use of these supplements is often accompanied by dangerous side effects, including extreme aggressiveness (“ ’roid rage”), high cholesterol, and, following long-term use, cancer. As a consequence, nearly all athletic organizations have banned their use (FIGURE 2-34).

Phospholipids and waxes are also lipids. Phospholipids(FIGURE 2-35) are the major component of the membrane that surrounds the contents of a cell and controls the fl ow of chemicals into and out of the cell. They have a structure similar to fats, but with two differences: they contain a phosphorus atom (hence phospholipids) and they have two fatty acid chains rather than three. We will explore the

signifi cant role of phospholipids in cell membranes in the next chapter.

Waxes resemble fats but have only one long-chain fatty acid linked to the glycerol head of the molecule. Because the fatty acid chain is highly non-polar, waxes are strongly hydrophobic; that is, these molecules do not mix with water but repel it. Their water resistance accounts for their presence as a natural coating on the surface of many plants and in the outer coverings of many insects. In both cases, the waxes prevent the plants and animals from losing the water essential to their life processes. Many birds, too, have a waxy coating on their wings, keeping them from becoming water-logged when they get wet.

FIGURE 2-34 Dangerous bulk. Steroids can increase muscularity, but with serious health consequences.

P

Fatty acids

Phosphategroup

Hyd

roph

ilic

head

(attr

acte

d to

wat

er)

Hyd

roph

obic

tai

ls(n

ot a

ttrac

ted

to w

ater

)

N

PHOSPHOLIPIDS

Symbol for phospholipid used in this book

Water

PHOSPHOLIPIDS IN WATER

Hydrophilic heads

Hydrophobic tails

Phospholipids align so that their hydrophilic heads extend toward the water, while their hydrophobic tails are directed away from the water.

FIGURE 2-35 Lipid versatility. Phospholipids have important roles in many organisms.

Cholesterol and phospholipids are lipids that are not fats. Both are important components in cell membranes. Cholesterol also serves as a precursor to steroid hormones, important regulators of growth and development.

TAKE-HOME MESSAGE 2 • 14