Download 1 CHAPTER 15. BIOCHEMISTRY: THE CHEMISTRY OF OUR

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CHAPTER 15. BIOCHEMISTRY: THE CHEMISTRY OF OUR BODIES
The Molecules of Life
Some of the most fascinating and challenging problems in chemistry deal with the chemistry of
the molecules that make up our own bodies. How does the brain process and transmit information?
How does the body "know' how to produce a brain cell or a skin cell or a cancer cell? How does the
body generate the energy to maintain a constant body temperature and power all the functions of the
muscles and organs? The final answers to all these questions must be found in an understanding of
how the body works on the molecular level. The study of the molecules of the living organism is called
biochemistry.
Biomolecules can be large and complex, and tracing their interactions within a living system is
an intricate task. Yet the simple chemical principles which we have learned remain the keys to
understanding this labyrinthine world of the molecules of life. The interactions of the functional groups
of these organic molecules are governed by rules that are familiar to us. Intermolecular forces between
these molecules are based on familiar principles, like hydrogen bonding. Three-dimensional modelling
remains an important tool of the chemist in exploring these biologically important molecules. Finally,
the knowledge we have already gained about the structures of macromolecules will help us to
understand this new category of macromolecules.
Carbohydrates- Your Crash Course on “Carbs”
Carbohydrates, made of carbon, hydrogen, and oxygen, are the most abundant organic
molecules in the biosphere. In Chapter 14 we learned that plants use the sun's energy to make the
glucose molecule, a simple carbohydrate, out of carbon dioxide and water. The cellulose which forms
the supporting structure of plants and the starch which serves as stored energy for plant seeds are
complex carbohydrates, or polymeric molecules in which glucose is the monomer unit. Carbohydrates
in the form of sugars and starches provide the energy which powers our bodies. In the form of
glycogen or animal starch, another complex carbohydrate, our bodies store ready energy. Looking at
the chemical structures of the different types of carbohydrates will give us a better understanding of
these important compounds.
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The simplest carbohydrates are the sugars, which can be either monosaccharides or
disaccharides, two monosaccharides linked together. Fig. 15-1 shows the structures of the two
monosaccharides which occur most frequently in our diets, glucose and fructose, along with two
common disaccharides, sucrose and lactose. As in earlier organic ring structures we have seen, the
carbon atoms in the rings are not labeled; the glucose ring, for example, has five carbon atoms in
addition to the oxygen atom that is shown. These structural representations are only one of several
possible ways to draw sugar structures, and, like all two-dimensional drawings, do not fully show the
three-dimensional shapes they represent.
Glucose
Sucrose
Lactose
Fig. 15-1. Chemical structures of some common sugars.
Glucose is also called blood sugar, for it is the simple sugar which is carried by the bloodstream
throughout the body to be metabolized to carbon dioxide and water, releasing energy to the tissues.
Most cells can "burn" fat if glucose is not available, but the brain and other nerve tissues use only
glucose. Intravenous feeding tubes in hospitals carry glucose solutions. Other names for glucose are
dextrose, the name which is usually used on packaged food labels, corn sugar, and grape sugar.
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Fructose means "fruit sugar," and it is often found in fruits, though glucose and sucrose are found in
fruit also. Fructose and glucose are the most abundant sugars in honey.
Ordinary table sugar is sucrose, which is extracted from sugar cane or sugar beets. Sucrose is
also common in other plants, especially fruits. Sucrose, a disaccharide, is made of a glucose molecule
and a fructose molecule linked together as shown in Fig. 15-1. Substances in the body called enzymes
in the body break apart these linkages to give the monosaccharides. Lactose, or milk sugar, which
makes up about 5% by weight of the composition of cow's milk, and almost 7% of human milk, is
another example of a disaccharide. Lactose is made of a glucose unit joined to another monosaccharide
unit called galactose. As part of the digestive process the enzyme lactase breaks up the disaccharide
lactose into these monosaccharide units. Not until the late 1960's did Western medical science realize
that, worldwide, the vast majority of adults are unable to digest milk because their bodies no longer
produce lactase after infancy. In the absence of lactase, lactose passes through the small intestine
undigested. In the colon, bacteria cause fermentation of the lactose, producing carbon dioxide gas and
much accompanying discomfort. About 70% of Americans of African descent are lactose intolerant,
while for those of European descent the figure is about 10%. Some milk products, like cheese and
yogurt, are practically free of lactose because of the action of bacterial fermentation and can be eaten
without ill effects by those who are lactose intolerant.
Polysaccharides: molecules with many sugar units
and cellulose
examples: starch
Starch
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Cellulose
Fig. 15-2: Typical structures of molecules of starch and cellulose.
Starch molecules are polymers of glucose with about 1000 to 4000 glucose units per
molecule, linked together as shown in Fig. 15-2; such a sugar polymer is also called a polysaccharide.
They are found mainly in the seeds of plants, where they serve as a reserve food supply for the newly
sprouted plant. Tubers, like potatoes, which form on the roots of some plants, serve a similar function.
When starch is digested, it is broken down into glucose molecules, which are then circulated through
the blood to the body cells. Most nutrition authorities now recommend that the greatest part of caloric
intake should be in the form of complex carbohydrates, or starches. Starches are preferable to sugars as
a source of food energy because they are broken down more gradually to release glucose into the
system. Sugars are quickly digested and passed into the bloodstream, placing great demands on the
body's regulatory systems that must maintain a relatively constant level of blood sugar. Refined sugar is
especially poor, because it is a source of "empty calories," unaccompanied by the vitamins and other
nutrients present in whole grains and other sources of complex carbohydrates.
Corn syrup is made by treating starch granules, extracted from corn kernels, with enzymes to
produce glucose and the disaccharide maltose made from two glucose units. The enzymes used are
produced by the easily cultured molds Aspergillus oryzae or Aspergillus niger. The more completely
the starch is broken down, the sweeter the corn syrup will be. A higher starch content gives a syrup of
thicker consistency. The corn syrup can be further treated with another type of enzyme derived from
the bacterial genus Streptomyces, which rearranges the glucose molecule to form fructose. Highfructose corn syrup has become a popular sweetener for the soft drink industry because of the
convenience of its liquid form and its low price. The name of fructose may also have some consumer
appeal because of the "healthy" connotation of its association with fruit, though the enzyme-produced
product has had no connection with fruit at all.
We need to apply our knowledge of chemistry to understand why high-fructose corn syrup is
an inexpensive alternative to sucrose. Unlike the glucose molecule, which tastes less sweet than
sucrose, the fructose molecule is almost as sweet to the taste as sucrose. Yet because sucrose is a
disaccharide and fructose is a monosaccharide, a mole of sucrose weighs about twice as much as a
mole of fructose, so it takes about twice as much sucrose by weight to produce the same sweetness
effect. This simple molecular weight effect has meant millions of dollars in increased profits for the soft
drink industry as high-fructose corn syrup has replaced sucrose as the primary sweetener in soft drinks.
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Since the amount of energy supplied by a food, measured in calories, is dependent on the number of
bonds in the food, the smaller fructose molecule also provides about half the calories of the sucrose
molecule.
Cellulose, like starch, is a glucose polymer. As Fig. 15-2 shows, in cellulose the glucose units
are linked together in a different way than in starch. Cellulose molecules are probably at least 1500
glucose units longer, and they may be much longer. For plants, while starch is used for food storage,
cellulose is used as a structural support. It forms tiny linear fibers which reinforce and strengthen the
plant's structure, much as synthetic oriented polymers are used commercially. The long-chain polymer
molecules are tightly packed together, held by hydrogen bonding between the many hydroxyl groups of
these molecules. Wood and plant fibers are largely made of cellulose. Cotton, for example, is almost
entirely cellulose.
For many animals, cellulose can serve as food just as starch does. Their digestive tracts harbor
bacteria that release enzymes which can break the linkages of the cellulose polymer, freeing the glucose
units. Humans lack these bacteria. That is the reason why cows and sheep are able to eat grass, hay,
and cornstalks, while humans cannot.
Though cellulose is useless to us as food, it is important to us in numerous ways. In the diet,
cellulose serves as fiber, or roughage, providing bulk to the bowel contents. In addition to improving
the musculature of the bowel wall, a high-fiber diet reduces the time that food spends in the bowel, thus
reducing the contact time with potential carcinogens derived from food. Research shows that if you eat
whole grains rather than refined grains, the starch is broken down more slowly into its glucose units,
perhaps because it is mixed up with the cellulose. Thus, blood sugar levels (and the insulin levels that
regulate blood sugar) stay more constant, and you have more energy and are less hungry over a longer
period of time, and By serving as food for animals which then serve as sources of meat and milk
products, cellulose makes an indirect contribution to our diets. We make use of the structural strength
of the cellulose polymer in the wood products for building materials and furniture as well as in paper
and in cotton fibers for textiles. Cotton is particularly comfortable for use in clothing because its many
alcohol groups hydrogen bond well to water; it absorbs sweat, keeping the skin comfortably dry.
Cellulose was the basis for the earliest synthetic polymers. In 1845 a German-Swiss chemist
named Christian Schönbein used his wife's cotton apron to mop up an accidental spill of a mixture of
nitric acid and sulfuric acid. When the apron dried out it exploded! The nitric acid had reacted with the
alcohol groups of the cellulose in the cotton apron, forming nitrocellulose, a powerful explosive which
also came to be called guncotton. Further chemical experiments with cellulose produced a partially
nitrated cellulose which the American inventor John Wesley Hyatt used to make celluloid, the first
synthetic plastic. Wyatt's celluloid plastic won a contest to find a substitute for expensive ivory in
making billiard balls, thus changing billiards from a rich man's sport to a popular pastime. In 1884
George Eastman of Eastman-Kodak first used celluloid as a photographic film to replace glass plate.
Then, by replacing the nitrate groups with acetate groups, he made a new compound called cellulose
acetate which was less flammable than celluloid for use as photographic film. Cellulose acetate became
the basis of a new industry not only for still photography but for the motion picture industry. In 1884
the French chemist Louis Bernigaud, Count of Chardonnay, first made a synthetic fiber from cellulose
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by dissolving partially nitrated cellulose and forcing it through tiny holes to make a fiber he called
rayon. Cellulose acetate can also be formed into a fiber. Most rayon fiber is made today by the viscose
process,in which cellulose is dissolved in a mixture of aqueous sodium hydroxide and carbon disulfide.
The resulting solution is forced through a spinneret into an acid bath, and fibers of regenerated
cellulose are formed. Thus, unlike nylon and dacron, which we learned in Chapter 13 are made from
fossil fuel feedstocks, rayon is made from renewable material which is a form of biomass. The polymer
chains of the regenerated cellulose in rayon are much shorter than the original cellulose. Hence,
although rayon fabric is absorbent like cotton, its fibers are not as strong.
The human body can synthesize yet another form of carbohydrate called glycogen. Like starch
and cellulose, glycogen is a polysaccharide, though it is more highly branched and has shorter chains, of
about 12 to 18 glucose units. The body of an average adult contains about 3/4 pound of glycogen, or
half a day's energy, both in the liver, where it is used to maintain the blood glucose level, and in muscle
tissue. When the body's glycogen reserves are filled, any additional excess calories consumed are stored
in the form of fat. When marathon runners practice "carbohydrate loading" before a race, they are
maximizing glycogen stores in order to postpone as long as possible the need to metabolize fat, which
is less easily metabolized.
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Proteins, Our Own Structural Polymers
Much of our own bodies' structures, our skin, muscles, hair and fingernails are made of the
polymers called proteins. The monomer molecules of which these macromolecules are constructed are
called amino acids. Each amino acid molecule, as shown in Fig. 15-3, has both an amine group and a
carboxylic acid group attached to the same carbon atom.
Fig. 15-3. Amino acids have both an amine group and a carboxylic acid group attached to
the same carbon.
As we have learned in our study of the organic functional groups (Chapter 12), carboxylic acids behave
like acids, and amines behave like bases; so, how would an amino acid behave? At neutral pH values
the removable proton of the carboxylic acid is transferred to the basic amine group, forming a dipolar
ion called a zwitterion. The zwitterion form of glycine is shown in Fig. 15-4.
Fig. 15-4. The zwitterion form of amino acids
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A list of amino acids found in proteins is given in Table 15-1. Each of these has an amine group and a
carboxylic acid groups attached to the same carbon like glycine. Unlike glycine, which has two
hydrogen atoms on this carbon, the other amino acids have different types of groups attached. These
groups, like the methyl group on alanine, can be thought of as side chains as they hang off the long
protein chain.
Table 15-1. The Common Amino Acids (http://click4biology.info/c4b/3/aat.htm)
Of these twenty amino acids listed in Table 15-1, nine are called the essential amino acids because
they must be obtained from the food we eat. Other amino acids can be synthesized as needed within
the body.
What kinds of foods can provide us with the essential amino acids? A simple way of providing
the essential amino acids for protein synthesis is by eating meat, since it is composed of animal protein
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similar to our own. Plants also can provide amino acids, although no single plant provides all the
required amino acids in adequate amounts. Vegetarians must be careful to assemble meals that will
provide complete protein ingredients by combining foods in such a way as to provide the eight essential
amino acids. As we learned in Chapter 14, legumes like beans, peas, and lentils carry nitrogen-fixing
bacteria on their root systems, so it is not surprising that they are particularly good sources of amino
acids, which bear the nitrogen-containing amine group.
The relative proportions of the essential amino acids by molar percent in some common foods
are shown in Fig. 15-4. Since the nutritional needs of a human infant correspond well to the
composition of human milk, the amino acid composition of human milk is used as a nutritional
standard, shown as a horizontal dotted line on each bar graph. The graph shows that wheat protein is
low in lysine, while peas are low in methionine compared with human nutritional needs. Eating wheat
products and peas at the same meal helps to satisfy the body's need for complete protein, as each helps
to supplement the other's amino acid deficiency. Over the centuries different cultures have developed
diets which combine lysine-deficient cereals with legumes, which are deficient in the sulfur-containing
amino acids, to make complete proteins. The Central American diet of corn tortillas and beans and the
Asian diet of rice and soybean products are examples of traditional food combinations which are
nutritionally sound protein complements.
Fig. 15-4. The essential amino acid content of some food proteins. The relative proportions of
each amino acid are shown expressed as the molar percentage of the total
essential amino acids. From "Food, the Chemistry of Its Components, 2nd ed.,
by T.P. Coultate.
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An additional argument for a plant-based diet is given by studies of green house gases
produced by meat production accounts for between 14 and 22% of all greenhouse gases that cause
climate change.
http://www.sciam.com/article.cfm?id=the-greenhouse-hamburger&SID=mail&sc=emailfriend
The bonds linking the amino acids to form the protein polymer chains are called peptide
bonds. A peptide bond is identical to the amide group listed in Chapter 12 as one of the fundamental
organic functional groups. It is formed by the condensation reaction of a carboxylic acid with an amine
group:
Figure 15-5 . The carboxylic acid group of one amino acid reacts with the amino acid group of
another amino acid to form a peptide bond.
Unlike the polysaccharides, which are formed from one type of monomer, glucose, proteins are more
complex in that so many combinations of the many possible amino acids are possible. Table 15-2 shows
the amino acid composition of some common proteins. The three-letter codes are abbreviations for the
amino acid names.
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Table 15-2. Amino Acid Composition of a Few Proteins (from "Biochemistry," by J. David Rawn.
Amino acid
Number per molecule of protein
Insulin
Hemoglobin, β
Wool
(bovine)
(human, gorilla)
(sheep)
Ala
Val
Leu
Ile
Pro
Met
Phe
Trp
Cys
Gly
Ser
Thr
Tyr
Asn
Gln
Asp
Glu
Lys
Arg
His
3
4
6
1
1
0
3
0
6
4
3
1
4
0
0
3
7
1
9
2
15
18
18
0
7
1
8
2
2
13
5
7
3
6
3
7
8
11
3
9
27
14
38
12
2
2
4
0
15
14
27
15
8
17
25
17
26
18
31
2
Total
54
146
312
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The structure of a protein is determined not only by the types of amino acids it contains, but
also by the order in which they appear on the chain. The linear sequence of the amino acids on the
chain is called the primary structure of the protein. Fig. 15-6 shows the primary structure of just one
protein, the beta chain of hemoglobin. Changing even a single amino acid can change the nature of the
resulting protein. People with the hereditary disease sickle-cell anemia have molecules of β-hemoglobin
in which the glutamic acid at position 6 is replaced by valine. This minuscule change affects the shape
and hence the functioning of the red blood cell.
Fig. 15-6. Structure of human hemoglobin. The protein's α and β subunits are in red and blue, and the
iron-containing heme groups in green. From PDB 1GZX Proteopedia Hemoglobin
The arrangement of the protein chain in space is called its secondary structure. Just as the
covalent peptide bonds hold together the primary structure of the chain, hydrogen bonds between
groups on different parts of the chain hold together the secondary structure. The protein in silk assumes
a relatively flat secondary structure called a pleated sheet. This structure, first proposed by Linus
Pauling, is shown in Fig. 15-7. The dotted lines indicate hydrogen bonds between two silk protein
chains. In each case the hydrogen bond is between the carbonyl group oxygen and a hydrogen attached
to a nitrogen on the other chain. Although this protein contains 15 different kinds of amino acids, 46%
are glycine, which has no side chain, and 38% are either alanine or serine with the side chains -CH3 and
-CH2OH, respectively. The pleated sheet secondary structure seems to work best with proteins that
have small side groups.
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Fig. 15-7. An example of secondary structure in proteins, this is the pleated sheet structure proposed
by Pauling for silk fibroin. Hydrogen bonding is indicated by dotted lines between the two
amino acid chains. The hydrogen-bonded secondary structure extends in each direction to
form a flat sheet. (thanks, Wikipedia!)
When side chains are bigger, the preferred secondary structure is a chain coiled like a spring
called a helix. Fig. 15-8 is a simple sketch of a right-handed helix. The discovery of this structure, called
the alpha helix, was a major step in understanding the nature of proteins. Fig. 15-9 shows the alpha
helix structure, proposed by Linus Pauling in the early 1950's, for α-keratin, a protein found in wool,
hair, horn, and nails. Hydrogen bonding across the helix, indicated by dotted lines, holds the structure
in place. When wool is stretched, the hydrogen bonds within the helix are broken, and bonds form
between chains as in silk to form a pleated sheet called β-keratin.
Fig. 15-8. The alpha helix structure.
Other forms of bonding besides hydrogen bonding are important in some secondary protein structures.
The protein in hair, for example, has sulfur-containing amino acids in which the sulfur atoms in different
parts of the chain can link to one another. Depending on the amino acids present, polar forces between
charged species and weak dispersion forces between nonpolar parts of the protein molecule may
influence secondary structure.
The secondary structure can be folded or otherwise arranged in space. The shape of the folded
secondary structure is called its tertiary structure. Tertiary structure, though it may appear untidy and
random, is actually determined very precisely by the primary and secondary structure. Each twist and
turn accommodates a bulky side chain or optimizes a bonding interaction between two groups. Fig. 1510 is a sketch which shows the tertiary structure of myoglobin, a protein which forms an alpha helix as
its secondary structure. The helix, instead of appearing as a long, thin chain, is folded and twisted over
itself. The disk-shaped structure shown nestled in the folds of the myoglobin is called heme. Heme, a
flat structure with an iron atom in the middle, is able to bind to an oxygen atom, and is also found in
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hemoglobin, a related protein which carries oxygen through the body. The overall shape of myoglobin
is more or less spherical in outline; proteins of this general shape are called globular proteins.
Globular proteins are usually folded in such a way that polar, hydrophilic (water-loving) groups are on
the outside of the molecule, making the protein water-soluble.
Fig. 15-10. Tertiary structure of myoglobin.
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Quaternary structure is important in some proteins, and describes the way several protein
molecules group together. Hemoglobin, for example, is made up of four protein chains, each with its
own primary, secondary, and tertiary structure, and each bearing a heme group in the middle.
Structure of human hemoglobin. The protein's α and ¶ subunits are in red and blue, and the ironcontaining heme groups in green. From PDB 1GZX Proteopedia Hemoglobin
Fig. 15-11. Quaternary structure of hemoglobin.
Fats
Biochemists classify fats in a category called lipids; the distinguishing characteristic of lipids is
solubility in nonpolar solvents and a lack of solubility in water. Fats are the most abundant lipids in the
human body, where they serve several different functions. Subcutaneous fat (fat under the skin) serves
as an insulator. Fat deposits help to support the body organs. Fat molecules are a structural component
of cell membranes. Perhaps most importantly, fat serves as a reservoir of stored energy. Carbohydrates,
as we have learned, store energy also, in the form of carbohydrates in plants and glycogen in animals.
Fat is much more efficient, however, in storing energy. One gram of carbohydrate provides 4
kilocalories of energy, while one gram of fat provides 9 kilocalories. From the standpoint of efficiency
in animal structure, storage of energy as fat makes sense. A typical woman's body, for example, is
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about 25% fat. In order for a 120-pound woman to store an equal amount of energy in the form of
carbohydrate instead of fat, she would have to weigh about 150 pounds. A typical man's fat supply is
about 15% of body weight, but the principle remains the same for all animal species: unlike plant life,
animal life needs maximum mobility for survival, and benefits from the lesser weight of a compact
form of fat storage. From the standpoint of diet, the calorie density of fats has become a matter of
concern in recent years. The high fat content of the American diet has resulted in a high-calorie dietary
intake, resulting in increased levels of obesity as the excess calories are stored as body fat.
The common feature in the chemical structure of fats is that they are all esters of glycerol, the
three-carbon chain with an alcohol group on each carbon. The carboxylic acids that combine with the
alcohol groups to form the ester linkages are called fatty acids. Three fatty acids are required to make
three ester linkages by reacting with the three alcohol groups in glycerol ; the fats thus formed are often
referred to as triglycerides. Fig. 15-12 shows the structure of tristearin, a typical triglyceride, along
with the structures of glycerol and the fatty acid called stearic acid from which it is formed.
Fig. 15-12. Glycerol reacts with three molecules of a fatty acid to form a fat, or triglyceride.
The fatty acids have long-chain hydrocarbon groups attached to the carboxylic acid group; most fatty
acids range from three to fifteen carbon atoms long. Except for those with three and five carbon atoms,
the fatty acids usually have an even number of carbons. Biochemists have learned that in the body the
fatty acids are formed stepwise, adding on two-carbon fragments in each step, and that this biosynthesis
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biosynthesis process accounts for the even number of carbons in the fatty acid. A fat can be made of
three identical fatty acids, or different ones. The properties of the fat are determined by these three fatty
acids.
If all the carbon atoms in the hydrocarbon chain of a fatty acid are linked by single bonds, the
compound is said to be saturated; a fat made of these single-bonded fatty acid chains is called a
saturated fat. If double bonds occur in the fatty acid hydrocarbon chains, an unsaturated fat results.
Fig. 15-13 shows the structures of two saturated fats, palmitic acid and stearic acid, and two
unsaturated fats, oleic and linoleic acid.
Fig. 15-13. Saturated fats have all single bonds in the carbon chain.
Unsaturated fats have some double bonds in the hydrocarbon chain.
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Without knowing the molecular structures, it is easy to identify a saturated fat when you see one,
because they are invariably solids at room temperature. The saturated hydrocarbon chains on adjacent
fat molecules can pack together snugly, maximizing intermolecular attraction and resulting in a
relatively high-melting solid. If double bonds are present in the fatty-acid chains, the molecules do not
fit together as snugly, and intermolecular forces are not as strong. Less heat energy is required to
loosen attractive forces between molecules, and so the melting point is lower. Beef tallow and lard are
typical saturated fats, solid at room temperature, and corn oil and safflower oil are examples of
unsaturated fats, liquid at room temperature. Unsaturated fats can be converted to saturated ones by
reacting them with hydrogen in the presence of platinum, palladium, or nickel; that is how solid
vegetable shortening is made in the process called hydrogenation Notice that not all the double bonds
are reacted; the hydrogenation process is carefully controlled to give a product with a creamy texture,
not a hard one like that of lard, a fully saturated fat.
Oleic acid
acid is a cis unsaturated fatty acid that
Elaidic acid
Elaidic acid is a trans unsaturated fatty acid
Stearic acid
Stearic acid is a saturated fatty acid fou
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comprises 55–80% of olive oil.
often found in partially hydrogenated vegetable
oils.
animal fats and is the intended produc
hydrogenation.
fatty acids are geometric isomers (chemically identical except for the arrangement of the This fatty acid contains more hydrogen
double bond).
not isomeric with the previous.
The trans fatty acid elaidic acid has different chemical and physical properties owing to the slightly
different bond configuration. Notably, it has a much higher melting point, 45 °C rather than oleic
acid's 13.4 °C, due to the ability of the trans molecules to pack more tightly, forming a solid that
is more difficult to break apart.[25] This notably means that it is a solid at human body
temperatures.
The proportion of fat in the diet and the type of fat consumed, saturated or unsaturated, are
both important factors in the development of coronary artery disease, because they affect the formation
on the walls of the blood vessels of plaque deposits which contain cholesterol, another important body
lipid. The formation of plaque deposits on the walls of blood vessels results in arteriosclerosis, or
coronary artery disease. Eventually, the artery may become completely blocked. Blockage of the
arteries leading to the heart (coronary arteries) can cause heart attack. Blocked arteries leading to the
brain can result in a stroke. The biochemical interaction of fats, cholesterol, and other substances in the
body is a complex one, and a variety of dietary and other factors are implicated in arteriosclerosis.
Dietary fat is an important factor. On the average, Americans get about 35 to 40% of their calories as
fat, much of it in the form of saturated fat. Experts agree that a diet lower in fats, particularly in
saturated fats, would be healthier. Partially hydrogenated fats were developed as an alternative to the
saturated animal fats in butter and meat in the hope that they would be healthier. Recently it was
discovered that the trans arrangement around the double bond that results from hydrogenation is very
unhealthy. Not only does it result in greatly elevated levels of coronary artery disease, but there are
possible associations with breast cancer, infertility, and Altzheimer’s disease. Several cities have banned
trans fats in restaurants and fast food outlets.
To meet the need for a lower-fat diet, fat substitutes have been developed which have a "mouth
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feel" similar to familiar fats, but lack their caloric content, and are metabolized differently in the body
because their molecular structure is different. Simplesse, developed by the G. D. Searles Co., is made
from egg white or milk proteins, and is best suited for frozen desserts or other foods which do not
require cooking. Olestra, a product from Proctor and Gamble Co., is a sucrose polyester made from
fatty acids and sugar which has no calorie value because, lacking enzymes which can break its bonds,
the body is unable to digest it.
A totally fat-free diet, however, is not desirable. Rats given a diet totally free of all fats are
found to exhibit serious symptoms involving the skin, vascular system, reproductive organs, and lipid
metabolism. Further investigation has shown that one particular fatty acid, linoleic acid, is an essential
dietary component. Research is making clearer to us the many ways that this compound is involved in
the complex biochemical systems of the body.
Other Lipids
Other lipids, though less abundant than fats, play an important role in our biochemistry.
Prostaglandins, for example, being fat-soluble, are classified as lipids. The fat-soluble vitamins: vitamin
A, vitamin D, vitamin E, and vitamin K, are also lipids. Another lipid category of importance is the
steroids. Chapter 12 shows the structures of some important steroids which are found in our bodies.
Notice the "backbone" of four attached rings common to all steroid molecules, including the two
methyl groups which are attached to the ring structure. Slight changes in the functional groups on this
basic steroid structure can have a major effect on how the molecule acts within the body. For example,
as we saw in the problem set in Chapter 12, testosterone, the male hormone, and progesterone, one of
the female hormones, differ very slightly, with testerone having an alcohol (or hydroxyl) functional
group and progesterone having a ketone group. Cholesterol in the diet has become a recent concern in
the United States because high blood cholesterol levels are associated with heart disease. It would be
both undesirable and impossible, however, to have a zero cholesterol level in the blood, because the
body needs this steroid as material for making important molecules like the sex hormones, and
cholesterol can be synthesized within the body as needed. In Chapter 18 we will learn about the role of
both fat and cholesterol in the diet, using the knowledge we have gained about lipid structure and
function. In Chapter 19 we will see how some effective drugs have been synthesized to mimic the
actions of these potent natural biochemicals.
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DECODING CHEMISTRY: THE OLIVE OIL BOTTLE
The label on a bottle of olive oil lists the following:
Saturated fat
2g
Polyunsaturated fat
Monounsaturated fat
Cholesterol
1.5 g
10 g
0g
Using the chemical information in this chapter, we can decode the following words from
this label: fat, saturated fat, polyunsaturated fat, monounsaturated fat, cholesterol.
Fat: Fats are triglycerides, with three ester functional groups formed from glycerol and three fatty
acids; the structures of typical triglycerides are shown in figures 15-14 and 15-16.
Saturated fat: A saturated fat has no double bonds in the hydrocarbon chains of the fatty acids
from which it is formed. Fig. 15-12 shows an example of a saturated fat from beef tallow,
made from the fatty acid stearic acid. The structure of stearic acid is found in Fig. 15-13.
(Studies have linked excessive consumption of saturated fats to the formation of a
substance called "plaque" in the arteries, plaque formation is associated with both heart
disease and stroke. Chapter 18 includes a discussion of fat as a dietary component.)
Polyunsaturated fat: A polyunsaturated fat has more than one double bond in the hydrocarbon
chains of the fatty acids from which it is formed. In Fig. 15-13, linoleic acid is an example
of a polyunsaturated fatty acid that might form a polyunsaturated triglyceride ester, or fat,
such as is found in soybean oil. (Though unsaturated fats were once thought to be the
best type of fats from a health standpoint, recent studies seem to favor monounsaturated
fats.)
Monounsaturated fat: A monounsaturated fat has one double bond in the hydrocarbon chains of
the fatty acids from which it is formed. In Fig. 15-13, oleic acid is an example of such a
monounsaturated fatty acid, which forms the triglyceride, or fat, in olive oil.
Cholesterol: Cholesterol is not a fat, but belongs in the same general biochemical classification
with fats (the lipids) because it is oil-soluble. Cholesterol is important in animal
biochemistry. It is not, however, a plant product. Therefore, no vegetable-based fat or oil
contains cholesterol. Since fat intake has been linked to cholesterol levels in the human
body (see Chapter 18), consumers can confuse fat and cholesterol as food components.
Sometimes vegetable oils are labelled prominently "contains no cholesterol," even though
this is not a useful criterion for choosing one vegetable oil over another.
22
Enzymes: Natural Catalysts
In learning about the hydrogenation of fats we encountered the concept of catalysis: a catalyst
facilitates a reaction without being consumed. This phenomenon is observed frequently in biochemical
systems. A biochemical catalyst is called an enzyme. Lactase, for example, is the enzyme that makes
possible the conversion of milk sugar, or lactose, into a form that the body can digest by breaking up
the disaccharide lactose into the monosaccharides. We have already encountered in our study of dietary
sugars how lactase deficiency, common in adults in many parts of the world, can effect the ability to
digest milk products. Lactase is now available commercially, marketed under the name LactAid. Those
who lack a natural supply of the enzyme in their bodies can add a few drops to a glass of milk or take a
lactase-containing tablet. Milk containing lactase or reduced in lactose content is also available.
A much more serious enzyme deficiency is the condition called PKU, or phenylketonuria. PKU
is a genetic error of metabolism in which the enzyme phenylalanine hydroxylase is lacking. This enzyme
catalyzes the conversion of the amino acid phenylalanine to tyrosine, another amino acid. If untreated,
PKU results in severe mental retardation. In most states testing is required at birth to detect this
condition, which must be treated with a specialized diet. Since an amino acid is involved, concentrated
sources of protein like milk products and meat must be eliminated from the diet, and a special
supplement provided which contains exactly the right amount of phenylalanine to ensure normal
growth. The artificial sweetener aspartame contains phenylalanine as a portion of its molecule, and
hence is dangerous to those with PKU.
Lactase and phenylalanine hydroxylase are examples of the myriad of enzymes acting
constantly to make possible the daily functions of the body. Each enzyme has a specific function: for
example, just as lactase is required to break up the disaccharide lactose, sucrase is required for sucrose,
and maltase is required for maltose. These enzymes which act upon carbohydrates are called
carbohydrases. Enzymes which act upon proteins to break them up into smaller units are called
proteases. Lipids are acted upon by lipases. These digestive enzymes are only a beginning of the list of
enzymes which make possible chemical reactions within the body. Taking in oxygen from the lungs,
releasing carbon dioxide, and oxidizing glucose to provide energy are all examples of processes which
require enzymes. Though both the structures and the actions of enzymes can be highly complex,
recognizing them by their names is usually simple: they usually end in "ase."
How do enzymes work their magic within the body? Since enzymes are catalysts, the principles
by which they work are like those of other chemical catalysts. In the hydrogenation of fats, for
example, we learned that the metallic catalysts worked in two ways. First, they held the reacting
molecules close together so that they could react. Secondly, they loosened the bonds within the
reacting molecules so that they could more easily break up to form new molecules. Biochemical
catalysts work in the same ways. For biochemical molecules the importance of the first of these two
functions is understandable. After all, biochemical molecules are typically large and cumbersome. As
with other organic chemicals, they tend to react only with one functional group at a time.
Consequently, getting together two molecules in such a way that the atoms on the reactive sites can
interact could be a major problem. For instance, in order for hydrolysis to occur, a water molecule
23
might bump at random against a larger molecule in hundreds of ways, only one of which might result in
encountering the reactive group on the molecule.. This type of consideration is an entropy effect; we
have already encountered entropy as we discussed order and disorder in Chapter 2. In enzyme catalysis
the reacting molecule, called the substrate, nestles into a site on the enzyme, attracted by
intermolecular forces like hydrogen bonding or other electrostatic attraction. While bound to the
enzyme the substrate is positioned properly for a reaction to occur; moreover, interactions with the
functional groups of the enzyme can serve to weaken key bonds in the substrate, easing the process of
forming new bonds. Fig. 15-17 shows an example of enzyme-substrate interaction.
Fig. 15-17 An example of enzyme-substrate interaction. (thanks, Wikipedia!)
Structurally, enzyme molecules are proteins. Most are globular proteins; their molecular
weights vary from about 10,000 to one million. In some cases in addition to the globular protein
another species, called a cofactor, must be present in order for catalytic action to occur. The cofactor
may be a metal ion like Fe3+ or Mg2+, or it may be an organic molecule, called a coenzyme. Many of
the vitamins serve as coenzymes.
DNA and RNA: the Messengers of Life
Perhaps the most exciting and history-making discoveries of molecular structure in our
lifetimes are those involving the molecules DNA and RNA. Encoded in these molecules is the genetic
information that determines the makeup of each human and of each living creature. As our
understanding of these molecules becomes deeper, new possibilities for eliminating diseases and even
creating life forms will arise, and, inevitably, new moral issues as well. DNA molecules are huge; their
molecular weights range from about 6 million to 16 million. RNA molecules are considerably smaller,
but with molecular weights ranging from about 20,000 to 40,000, they are still very large molecules.
These DNA or RNA biopolymers are called nucleic acids. As with other large, polymeric molecules
we have studied, nucleic acid molecules are formed from repeating units of similar structure. By
24
looking at the structures of these units and applying the chemical principles we have learned, we can
gain an understanding of these important molecules.
The monomers of the nucleic acids are called nucleotides. Each nucleotide is made up of
three parts:
1. A phosphoric acid molecule, H3PO4
2. A sugar molecule with five carbons
3. An organic base molecule with nitrogen atoms
In RNA molecules the sugar component is always ribose, hence the name ribonucleic acid, for which
RNA is the abbreviation. In DNA molecules, the sugar is always deoxyribose; hence the name
deoxyribonucleic acid, or DNA. Fig. 15-18 shows the structures of these two sugars. Notice that they
differ only in that deoxyribose is lacking one oxygen atom found in ribose.
Fig. 15-18. Structures of ribose and deoxyribose, the two sugars found in nucleic acids.
http://www.elmhurst.edu/~chm/vchembook/543ribose.html
25
Fig. 15-19. The base molecules found in nucleic acids.
A GC base pair demonstrating three intermolecular hydrogen bonds
An AT base pair demonstrating two intermolecular hydrogen bonds
.
Fig. 15-20 shows the structure of the nucleotide formed from phosphoric acid, the sugar deoxyribose
and the base adenine. Notice that in the condensation reaction that has joined the phosphoric
acid molecule and the sugar, the phosphoric acid has lost one hydrogen and the sugar has lost
one OH. Similarly, in joining together, the
base adenine loses a hydrogen and the sugar an OH group.
26
Fig. 15-20. Structure of a nucleotide.
To form the DNA or RNA polymer, nucleotide units are joined together by condensation reactions
between the phosphoric acid group of one nucleotide with the sugar unit of another.
In a DNA molecule two polymeric strands consisting of nucleotide units are wound together in
a structure called a double helix. Figure 15-22 shows a model of the double helix structure, with two
helical strands wrapping around each other. The "backbone" of each strand is the chain of alternating
phosphoric acid and sugar units.
Fig. 15-22. Three-dimensional model showing the shape of the DNA double helix.
27
http://en.wikipedia.org/wiki/DNA has a beautiful rotating structure of the DNA molecule.
Within the helix coil the nitrogen bases of the two strands pair up, attracted by hydrogen bonding
interactions. Only certain base pairs fit together in such a way as to allow these interactions. As shown
in Fig. 15-19, thymine and adenine fit together, as do cytosine and guanine.
In order for the base pairs to fit together properly within the double helix, thymine must always be
paired with adenine, and cytosine must always be paired with guanine. These hydrogen-bonded base
pairs form a pattern like the rungs of a ladder inside the double helix, as shown in Fig. 15-24. The
abbreviations C, G, A, and T for the four bases are a common usage.
The complementary fit of the two base pairs A-T and C-G is an important concept which is the key to
understanding how the genetic code functions, and how genetic information is passed on from one
generation to another.
Within each cell of a human body, with the exception of egg and sperm cells, is a nucleus which
contains all the genetic information for that person packaged into 23 pairs of chromosomes. Each
chromosome contains thousands of genes; there are between 50,000 to 100,000 genes in total. Each
gene has a specific task in determining the structure of a protein which forms part of a cell or regulates
a cell function. In determining these protein structures, genes or combinations of genes determine
individual characteristics such as eye color or body functions such as insulin production. Chemically,
what is a gene? Each gene is simply a sequence of nucleotides in a DNA molecule. Since it is the
identity of the base that varies from one DNA nucleotide to another, it is the base sequence that
determines the structure of the gene and, hence, carries the genetic code. For example, Fig. 15-25
shows the DNA base sequence which is the genetic information needed to produce a single protein
found in hemoglobin.
28
Fig. 15-25. The DNA base sequence for one of the proteins in hemoglobin. From The New Republic,
July 9 and 16, 1991.
How does a DNA molecule reproduce itself, making it possible to carry on the genetic code to
a new generation? And how does a DNA molecule produce new protein using this genetic information
in the base code? The key to the answers to these questions lies in the double helix structure. Aided by
enzymes which can break the hydrogen bonds holding the base pairs together, the two strands of the
molecule can "unzip", becoming partially unconnected, as shown in Fig. 15-26.
29
Fig. 15-26. DNA replication. As the DNA unzips, free nucleotides in the cytoplasm, or cell fluid, pair
with a complementary base on each separated strand, thus forming two identical DNA
molecules. A always pairs with T, and C always pairs with G.
As the DNA unzips, free nucleotides present in the cell pair with a complimentary base on each
separated strand. As before, A always pairs with T, and C always pairs with G. The result is two
identical DNA molecules. Because of this process, when a cell divides, forming two cells, each cell can
carry an identical DNA molecule. In reproductive cells only one strand of a DNA molecule is present.
Our inherited traits come from both parents because each has contributed one strand of a DNA
molecule.
The DNA strands are also separated during the process of protein manufacture. RNA
molecules play the role of taking the information encoded in the base sequences of the DNA molecules
and using this information to form proteins in a stepwise process. There are three kinds of RNA
molecules, all of which are smaller than DNA molecules. Messenger RNA, the largest of the three
kinds, has molecular weights of about 25,000 to 1 million. Transfer RNA molecules have molecular
weights of about 23,000 to 30,000. Ribosomal RNA molecules are intermediate in size, between
messenger RNA and transfer RNA.
When first looking at the chemical structures of the nucleic acids, we noted that RNA and
DNA were composed of slightly different collections of nitrogen bases. Both DNA and RNA
molecules contain adenine (A), guanine (G), and cytosine (C). Instead of thymine (T), RNA contains
uracil (U), which differs from thymine only in that it lacks a methyl group (Fig. 15-19). In the first step
30
of protein synthesis, a molecule of messenger RNA is formed in the nucleus, with part of an unzipped
DNA molecule serving as a template. Each base on the DNA strand results in the formation of a
complementary base opposite itself on the forming RNA molecule. The complementary base pairs are
the same as those involved in bonding across the double helix, except that adenine pairs with uracil
instead of thymine (Fig. 15-27).
31
Thus RNA "transcribes" the base code and carries it from the nucleus to other parts of the cell, where,
with the aid of other forms of RNA, it uses the base code to form protein by assembling amino acids in
the proper order. A group of three bases on messenger RNA corresponds to one amino acid unit on a
protein. For example, AUG is the three-base code for the amino acid methionine.
Today in laboratories all over the world the details are being filled in on the fascinating story of
the DNA molecule and how its structure translates into the structure and functioning of our bodies.
Some diseases have a genetic basis. For example, in sickle-cell anemia, glutamic acid, one of the 146
amino acids on one of the proteins of hemoglobin, is replaced by the amino acid valine because of a
tragic mistake in the DNA coding for this protein in the carriers of this trait. The resulting effect on the
tertiary protein structure results in malformed, or "sickled" red blood cells, interfering with oxygen
transport to the body. Another disease known to have a genetic basis is cystic fibrosis. An estimated
30,000 people have this disease, which causes abnormally thick mucous in the lungs, and most of these
will die before their thirtieth birthday. In 1989 a heroic effort by research groups in Toronto, Ontario,
and Ann Arbor, Michigan culminated in the discovery of the cystic fibrosis gene. The protein encoded
by the gene, they found, contains 1480 amino acids. Further research into how this protein functions
will be a first step to developing an effective treatment for the lethal disease. A screening procedure has
been developed which makes it possible to determine whether individuals carry the cystic fibrosis gene,
and thus are at risk for having a child with the disease.
Diseases which can be traced to a single genetic flaw are relatively rare, affecting on the order
of tens of thousands of victims each year. More common diseases like heart disease and diabetes have
much more complex causes. Increasingly, however, researchers are able to identify genes which may
be involved in the processes which lead to these diseases. Some types of breast cancer are associated
with certain genes. Hence, treatment options can be better understood after a genetic test of a woman
diagnosed with breast cancer, and predictions can be made about the likelihood of a woman getting
certain types of breast cancer through genetic screening.
A wide variety of commercial applications has already arisen based on DNA technology.
Chapter 18, "Chemistry in the Crime Lab," discusses the use of DNA typing, which analyzes the DNA
present in traces of blood and other body materials to identify criminals. Recombinant DNA
technology, in which a gene from one organism is incorporated into the genetic material of another
organism, has led to a new industry called biotechnology. The first successful commercial product
from recombinant DNA technology was human insulin, made by introducing the human gene for
insulin production into a common bacterium called E. coli. The bacteria thus modified produced
human insulin in quantity, making possible its commercial use for diabetics, who produce insufficient
amounts of insulin in their own bodies. The human insulin made through recombinant DNA
technology replaced animal insulin, which had previously been the only available commercial product.
Similarly, human growth hormone has been produced through recombinant DNA technology, making
this previously rare substance available for medical treatment of children whose bodies produce
insufficient amounts of the substance. A microbe genetically altered to improve its capacity to break
down crude oil has been created, and was the subject of the first biotechnology patent in 1980.
Recombinant DNA research now being conducted with plants have resulted in genetically altered
32
varieties that are more disease resistant or more nutritious. These GMO plants, however, have been
criticized for their unknown effects on an ecosystem or even on the human organism. An experimental
technique called gene therapy offered hope to the victims of genetic disease who lack a gene for
producing an essential protein, like hemoglobin. The missing gene can be introduced into the body by
means of a virus which "infects" cells with the new genetic material; experimental results so far,
however, have been disappointing. Each of these new technologies presents a spectrum of possibilities
for changing our world.
As yet our knowledge of the human genetic code is incomplete. Less than 1% of the human
genome, the full list of human genes, is known. In 1989 the Human Genome Research Project, funded
by the National Institutes of science and the Department of Energy, was founded, with the ambitious
objective of identifying and mapping each human gene. Some experts estimate that the project may
essentially be completed in fifteen years. Certainly our knowledge of the human genome will have
advanced considerably in that span of time. New issues in medical and scientific issues that have already
begun to arise will become more pressing. Will those known to carry defective genetic material be
discriminated against? How far should we go in "perfecting" the genetic makeup of human beings?
Who will be empowered to make these decisions for individuals and for society? Answers to these
questions will be inescapable in the twenty-first century, and a knowledge of the basic science involved
will be essential in making informed decisions.
33
CONCEPTS TO UNDERSTAND FROM CHAPTER 15
The study of the molecules of the living organism is called biochemistry.
Carbohydrates, made of carbon, hydrogen, and oxygen, can take several forms: monosaccharides;
disaccharides, made of two monosaccharide units linked together; or the polysaccharides, starch,
cellulose, and glycogen, which are polymers formed from monosaccharide units.
Proteins, the structural polymers of animal life, are made from amino acid units linked together by the
peptide linkage, an amide functional group.
The order in which amino acids appear in the protein chain is called its primary structure.
The arrangement of the protein chain in space is called its secondary structure. Examples of
secondary structure are the pleated sheet of silk protein and the alpha helix in wool and hair. The
secondary structure is held together by intermolecular forces like hydrogen bonding, dipole-dipole
attraction, and dispersion forces, and by covalent bonds between sulfur atoms.
Tertiary structure of a protein chain describes the way the secondary structure is folded in space. If
the protein folds up to form a more or less spherical shape, the protein is called a globular protein.
Lipids are oil-soluble biomolecules. They include fats, which are esters made from glycerol and longchain carboxylic acids called fatty acids; steroids; prostaglandins; and the oil-soluble vitamins A, D, E,
and K.
Saturated fats have single bonds between the carbon atoms on the fatty acid chains. Unsaturated fats
have one or more double bonds between the carbon atoms in the fatty acid chain. Unsaturated fats can
be changed to saturated fats by hydrogenation, a reaction in which a hydrogen molecule is added to
the double bond with the aid of a catalyst.
A catalyst enables a reaction to occur without being consumed in the reaction. A catalyst can act by
weakening bonds in the reactants or by holding reactants in a good position for the reaction.
An enzyme is a biochemical catalyst. Different types of biochemical reactions require different
catalysts.
The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biopolymers which
carry genetic information in the form of the order in which their units occur.
The monomers of the nucleic acids are called nucleotides. Each nucleotide is made up of three parts: a
phosphoric acid molecule, H3PO4; a five-carbon sugar; and an organic base molecule. The polymer
chain consists of alternating phosphoric acid and sugar units, with base units hanging off the chain.In
DNA the sugar units are deoxyribose; in RNA they are ribose.
34
The DNA structure consists of two polymeric strands made of nucleotide units. Each strand is in the
shape of a helix; one is a right-hand helix and the other is a left-hand helix. The two helical strands wrap
around each other to form a shape called a double helix. Hydrogen bonding between base units on the
two strands holds the two helices together.
Because of their different shapes, only certain base pairs fit together in the middle of the double helix
structure. In DNA, thymine (T) and adenine (A) fit together, as do cytosine (C) and guanine (G). In
RNA, adenine (A) fits with uracil (U), and cytosine (C) with guanine (G).
The molecular structures that make up an individual organism are determined by 23 pairs of
chromosomes found in the nucleus of each cell. The thousands of genes which are found in each
chromosome, and which determine individual characteristics, contain sequences of nucleotides. The
order of the bases as they appear in the nucleotides makes up a code for synthesizing a given protein.
Messenger RNA carries the base code from the nucleus to other parts of the cell, where, with the aid
of other forms of RNA, it uses the base code to form protein by assembling amino acids in the proper
order. A group of three bases on messenger RNA corresponds to one amino acid unit on a protein.
Some diseases, like sickle-cell anemia and cystic fibrosis, result from abnormalities of the genes which
have been identified. Others, like heart disease and diabetes, are more complex, but may be associated
with genes which increase susceptibility to them.
DNA typing, sometimes called "DNA fingerprinting" analyzes the DNA present in traces of blood and
other body materials to identify criminals.
In recombinant DNA technology a gene from one organism is incorporated into the genetic material
of another organism.
Biotechnology develops and produces new commercial products using recombinant DNA technology.
35
SKILLS TO ACQUIRE FROM CHAPTER 15
Biomolecules are usually large and complex, but the principles which govern them and the structural
units which make them up are the same as for smaller molecules. Learn to apply basic principles you
have already learned to these biomolecules:
Look for the basic functional groups, as in Chapter 12.
In biopolymers like carbohydrates, proteins, and DNA and RNA, look for the repeating unit (the
monomer), as in Chapter 13.
Sometimes with biomolecules even the names need decoding.
Long names, like complex structures, can often be broken down into smaller units to help you
understand what they mean. Deoxyribonucleic acid, for example, is a polymer of alternating
units of phosphoric acid and the sugar deoxyribose which is found in the cell nucleus.
Sometimes types of biomolecules can be identified by their names: sucrose, lactose, fructose are all
recognizable as sugars because their names end in -ose. Lactase, maltase, and phenylalanine
hydroxylase are all recognizable as enzymes because their names end in -ase. Glycine,
phenylalanine, and leucine are all amino acids which end in -ine. Amino acids are not always
so predictable. Glutamic acid, for example, is an amino acid.
As we have seen in this chapter, the names of biochemicals appear frequently in the grocery store and
the health food store as well as in newspapers, in magazines, and on television. Using the decoding
skills you have learned in this chapter, you should be able to recognize the names of the important
biochemicals and understand their basic functions in the body.
36
Name ___________________________________
Date __________________
PROBLEMS TO SOLVE USING CONCEPTS, FACTS, AND SKILLS
FROM CHAPTER 15
15-1. Match the chemical names with the correct descriptive phrases.
Disaccharide
Monosaccharide
Lipid
Complex carbohydrate
Nucleic acid
Amino acid
Enzyme
A. Cholesterol
B. Lactase
C. Sucrose
D. Starch
E. Glycine
F. Fructose
G. DNA
15-2. Identify the type of chemical compound represented by these chemical structures by matching
them with the correct descriptive names.
Monosaccharide
Disaccharide
Steroid
Unsaturated fat
Saturated fat
Fatty acid
Nucleotide
Amino acid
15-3. Match each of the following carbohydrates with the appropriate descriptive phrase.
Glucose
Lactose
Sucrose
Glycogen
Cellulose
Starch
Maltose
Fructose
A. Stores energy for plants
B. Stores energy for animals
C. Strengthen plant structure
D. Disaccharide with two glucose units
E. Very sweet monosaccharide
F. Milk sugar
G. Blood sugar
H. Table sugar
37
Fill in the blanks in problems 15-4 through 15-23.
.
15-4. The study of the molecules of the living organism is called
15-5. A starch is a polymer in which the monomer units are called
.
.
15-6. A protein is a polymer in which the monomer units are called
15-7. Lactase, which is able to break up the disaccharide lactose into the monosaccharides glucose and
galactose, is in the class of biochemicals called
.
15-8. Silk, a natural polymer, is made of protein which assumes a pleated sheet structure. The pleated
sheet is an example of
protein structure. The amino acid chains form these sheets because
they are attracted to one another by
bonding.
15-9. Proteins, like myoglobin, which fold up to form a shape more or less spherical in outline, are
called
proteins. The shape of the folded secondary structure of a protein is called its
structure.
15-10. Although both plants and animals can store energy in the form of carbohydrates, a more
.
efficient form of energy storage is provided by the class of chemical compounds called
15-11. A substance which enables a reaction to occur by participating in the reaction without itself
being consumed is called a
.
15-12. A biochemical catalyst is called an
.
15-13. The three parts of a nucleotide are
,
, and
15-14. Nucleotides are the units which make up the biochemical polymers called
.
.
15-15. In DNA molecules the sugar units are always made of the sugar called
.
15-16. In RNA molecules the sugar units are always made of the sugar called
.
15-17. In a DNA molecule two polymeric strands are wound together in a structure called a
.
15-18. In DNA the base thymine must always be paired with the base
.
15-19. In DNA the base cytosine must always be paired with the base
.
15-20. In RNA the base uracil must always be paired with the base
15-21. In DNA and RNA the base pairs are held together by
.
bonding.
38
15-22. One amino acid group on a protein corresponds to
bases on messenger RNA.
15-23. One gene from an organism can be transplanted into the genetic material of another organism by
DNA technology.
the use of
15-24. What is an essential amino acid? Name two menu-planning strategies to ensure a diet which
contains appropriate amounts of the essential amino acids.
15-25. Suppose that a friend tells you that milk products seem to be causing her indigestion. What
explanation could you give about the likely chemical basis of this complaint? What suggestions could
you give about ways to solve the problem?
15-26. How do saturated fats differ chemically from unsaturated fats? When choosing cooking fats in
the grocery store, how can you identify the saturated fats?
15-27. Why are starches considered to be preferable to sugars as a source of food energy? Why are
whole grains better at giving energy than refined grains?
15-28. Name a product found in grocery stores which contains a chemical compound in each of the
following categories. If possible, name the chemical compound.
A. Lipid
B. Starch
C. Monosaccharide
39
D. Disaccharide
E. Protein
F. Enzyme
G. Cellulose
15-29. How is high-fructose corn syrup made? Why do you think it has become a popular commercial
sweetener?
15-30. How is rayon fiber made? Compare rayon with nylon (Chap. 3) with respect to
a. The nature of the starting materials used in the process.
b. The effect of the chemical structure of the polymer on the ability of the fabric to absorb water.
15-31. Find andcircle all the sugars on the following ingredients list of a cereal that is labelled on the
front of the package as "made with whole wheat/less sugar than most kids' cereals":
INGREDIENTS: WHOLE WHEAT, SUGAR, RICE FLOUR, NATURALLY HYDROGENATED
SOYBEAN OIL, FRUCTOSE, MALTODEXTRIN, SALT, INVERT SYRUP*,
DICALCIUM PHOSPHATE, DEXTROSE, CINNAMON, SOY LECITHIN, TRISODIUM
PHOSPHATE, FRESHNESS PRESERVED BY BHT.
*Note: invert syrup is made by breaking up sucrose into its component monosaccharides.
15-32. Referring to what you have learned about the secondary structure of α-keratin, a protein found
in wool, explain why it is easier to stretch a wool sweater when it is wet.
40
15-33. The label on a container of sunflower oil lists the following:
Saturated fat
Polyunsaturated fat
Monounsaturated fat
Cholesterol
1.5 g
1.5 g
2.5 g
0g
Using information you have learned from this chapter explain the terms
saturated fat, polyunsaturated fat, monounsaturated fat, and cholesterol. What health issues are
involved with each of these?
41
DECODING CHEMISTRY: THE POWER BAR
A student who works at a ski resort asked her chemistry professor: are these "energy
bars" that cost $1.80 at the snack bar of the ski shop really worth that much money? What are
their special ingredients?"
Here are the ingredients listed on the label: Fructose syrup (from grapes, corn, and/or
pears), oat bran, maltodextrin (complex carbohydrate), milk protein (lactose removed), rice
crisps, brown rice, almond butter, natural apple flavors (no msg), cinnamon, citric acid.
First, bear in mind that ingredients are listed in order of their amount in the product, so
that the ingredient listed first is present in greatest amount. The first ingredient listed is fructose, a
monosaccaride; hence, this product contains a great deal of sugar. We know that sugars are good
for "quick energy," though they lack the sustained energy release of complex carbohydrates.
(Would as many people buy these health bars if they were prominently labelled "contains lots of
sugar?") The label is ambiguous as to the source of the fructose. It is possible that all the fructose
is present as high-fructose corn syrup; if so, as we have seen, this is one of the cheapest food
ingredients. Complex carbohydrate is present in the form of maltodextrin, which is a starch
product made by dry-heating corn starch, potato starch, or tapioca starch. Oat bran is a form of
cellulose, not digested in the body but providing bulk. None of these ingredients is expensive. So
far, the ingredients of this product seem to be neither costly nor particularly remarkable
nutritionally, though they show the bar provides food energy.
The fourth ingredient listing, "milk protein with lactose removed" offers no explanation
about why the lactose has been removed. Presumably, a lactose-intolerant individual could eat this
product without suffering digestive upset. Others wuld notice no difference. Removing this small
amount of sugar would be of no special nutritional benefit, since this is a high-sugar product
anyway.
Rice crisps, brown rice, almond butter, natural apple, and cinnamon are well-known as
food ingredients, with the possible exception of almond butter, a source of fats derived from
almonds, and the only expensive ingredient on the list. Citric acid, found naturally in fruits, is a
common food additive which acts as a preservative and gives a tart flavor.
Listed after "ingredients" is a separate list of "minerals, essential amino acids, and
vitamins." It is not made clear whether these are present in the foodstuffs listed, or if they are
additives, in which case they would add to the product's cost.
Having decoded the list of ingredients, do you believe the cost of the bar is justified?
42
DECODING CHEMISTRY: "HIGH POTENCY AMINO ACID TABLETS"
A chemistry student brought in to her professor some tablets labelled "High Potency
Branched Chain Amino Acid Formula" that she had purchased at a health foods store. The
label recommended "Take 4 to 6 tablets approximately 1 1/2 hour before and immediately after
workouts." "Are these any good?" she asked. "What's in them?"
According to the label, 2 of these tablets provide:
Leucine, 800 mg
Isoleucine, 100mg
Valine, 100 mg
Vitamin B-6, 1 mg
Checking against the list of amino acids in Table 15-1, we see that these are indeed
among the essential amino acids that are needed for building protein. If, however, we look also
at Fig. 15-4, which gives the essential amino acid content of some food proteins, we see that
any of the foods featured here will provide these amino acids, and others as well. Wheat, for
example, is about 24% leucine, 12% isoleucine, and 18% valine, expressed as mole percentage
of total amino acids. Approximating weight percent to be the same as mole percent (this is only
approximate, since amino acid molecular weights do differ), if a slice of whole wheat bread
contained about 4 grams of protein, it would have:
Leucine, 960 mg
Isoleucine, 480 mg
valine, 720 mg
or more than two "high-potency" amino acid pills. What's more, if 20 slices of bread cost
$1.89, and 180 amino acid tablets cost $22.95, each slice of bread costs less than ten cents,
while two amino acid tablets cost more than 25 cents. (We will address the issue of vitamins
later, but bread does provide B vitamins as well.)
What about the "branched chain" referred to in the label for the "high-potency" pills? If
you look at the structures of the amino acids in Table 15-1, you will see that the hydrocarbon
portion of each of these amino acids, leucine, isoleucine, and valine, is indeed branched rather
than straight-chain. This is true of the molecular structure of these amino acids no matter what
source they come from.
Would the three amino acids in these pills be sufficient to build body protein? After
decoding the label of the "High Potency Amino Acid Pills," what do you conclude about these
pills as "high-potency" muscle builders to be taken before and after workouts?
43
DECODING CHEMISTRY: THE BREAST CANCER GENE
On September 15, 1994, researchers announced that the long-awaited discovery of a gene
whose mutation causes hereditary breast cancer had been accomplished. About 10 per cent of all
cases of breast cancer are believed to be linked to inherited traits, with about half of these
accounted for by this newly discovered gene. Thus about 600,000 women in the United States
probably carry this gene, and hence have about an 80 per cent risk of breast cancer.
The gene was found to be exceptionally long, much longer than had been expected. Said
the New York Times, "the gene extends far more than 10,000 subunits, or bases of DNA, making
it as much as ten times larger than the average gene."
a) Can you name the "bases of DNA" the article is referring to?
b) When DNA is used as a template to make protein, how many of these bases are needed to
code for a single amino acid unit?
c) If mutation occurs and one of the bases in the DNA sequence is changed, how will that affect
the primary structure of the protein made using that DNA molecule?
All of the information needed to answer these questions is found in Chapter 15 and in its chapter
summary.