The Organic Molecules of Life
Chapter
3
Outline
	3.1	Organic Molecules
	•	The characteristics of organic molecules depend on the chemistry of carbon.30
	•	Variations in the carbon skeleton and the attached functional groups account for the great diversity of organic molecules.31
	3.2	The Organic Molecules of Cells
	•	The four classes of organic molecules in cells are carbohydrates, lipids, proteins, and nucleic acids.32
	•	Large organic molecules called polymers form when their specific monomers join together.32 
	•	Glucose, a carbohydrate, is an immediate energy source for many organisms.33
	•	Some carbohydrates (starch and glycogen) function in short-term energy storage.34
	•	Other carbohydrates (cellulose and chitin) function as structural components of cells.34
	•	Fats and oils function as long-term stored energy sources.35
	•	Cellular membranes, including the plasma membrane, are a bilayer of phospholipid molecules.36
	•	Steroids are derived from cholesterol, a complex ring compound.37
	•	Proteins serve many and varied functions, including metabolism, support, transport, defense, regulation, and motion.38-–39
	•	Each protein has levels of structure that result in a particular shape.40
	•	Genes are composed of DNA (deoxyribonucleic acid). DNA specifies the correct ordering of amino acids in proteins, with RNA serving as an intermediary.41
Without cholesterol your body would not function well. What is it about cholesterol that can cause problems in our bodies? Although blood tests now distinguish between “good” cholesterol and “bad” cholesterol, in reality there is only one molecule called cholesterol. Cholesterol is absolutely essential to the normal functioning of our cells. It is a part of the cell’s outer membrane, and it also serves as a precursor to hormones such as the sex hormones testosterone and estrogen. Without cholesterol, your cells would be in major trouble. 
So when does cholesterol become “bad”? Your body packages cholesterol in high-density lipoproteins (HDL) or low-density lipoproteins (LDL), depending on such factors as genetics, your diet, your activity level, and whether you smoke. Cholesterol packaged as HDL is primarily on its way from the tissues to the liver for recycling. This reduces the likelihood of deposits called plaques forming in the arteries; thus, HDL cholesterol is “good.” LDL is carrying cholesterol to the tissues; high levels of LDL-cholesterol contribute to the development of plaque that can result in heart disease, making LDL the “bad” form. When cholesterol levels are measured, the ratio of LDL to HDL is determined. Exercising, improving the diet, and not smoking can lower LDL. In addition, a variety of prescription medications lower LDL levels.
The molecules that make up our bodies include carbohydrates, lipids, proteins, and nucleic acids. The building blocks for nearly all of these molecules come from our diet. These molecules give cells the ability to perform many of their complex duties. In this chapter you will learn about the structure and function of the major molecules of cells.
3.1		Organic Molecules
If you decide to take a chemistry course, you will most likely have a choice of either inorganic -chemistry or organic chemistry. Inorganic chemistry is often thought of as the chemistry of the nonliving world, and organic chemistry is the chemistry of the living world (Fig. 3.1). But water, the molecule that makes up 70–90% of a cell, is an inorganic molecule. Why should that be? For a molecule to be organic, it must contain carbon and hydrogen; otherwise, it is an inorganic molecule. The chemistry of carbon accounts for the molecules that make up the structure of cells and organisms.
The Carbon Atom
A bacterial cell contains some 5,000 different organic molecules, and a plant or animal cell has twice that number. What is there about carbon that makes organic molecules so diverse and also so complex? Carbon, with a total of six electrons, has four electrons in the outer shell. In order to acquire four electrons to complete its outer shell, a carbon atom almost always shares electrons with—you guessed it—CHNOPS, the elements that make up most of the weight of living things (see p. 16).
Because carbon is small and needs to acquire four electrons, carbon can bond with as many as four other elements, and this spells diversity. But even more significant to the shape of biomolecules, and therefore their function, is the fact that carbon often shares electrons with another carbon atom. The C–C bond is stable, and the result is that carbon chains can be quite long. Hydrocarbons are chains of carbon atoms that are bonded only to hydrogen atoms. Any carbon atom of a hydrocarbon molecule can start a branch chain, and a hydrocarbon can also turn back on itself to form a ring compound (Fig. 3.2). Carbon can form double bonds with itself and other atoms. Some carbon molecules, called isomers, have the same number and kinds of atoms but a different arrangement of atoms. Isomers are another example of how the chemistry of carbon leads to variations in organic molecules. 
The Carbon Skeleton and Functional Groups 
The carbon chain of an organic molecule is called its skeleton, or backbone. This terminology is appropriate because just as your skeleton accounts for your shape, so does the carbon skeleton of an organic molecule account for its shape. The reactivity of an organic molecule is largely dependent on the attached functional groups (Fig. 3.3). A functional group is a specific combination of bonded atoms that always reacts in the same way, regardless of the particular carbon skeleton. As in Figure 3.3, it is even acceptable to use an R to stand for the rest of the molecule because only the functional group is involved in the reaction. 
Notice that functional groups occur in certain types of compounds. For example, sugars contain many polar –OH groups. Thus, although all hydrocarbons are nonpolar and hydrophobic (not soluble in -water), glucose with several –OH groups is hydrophilic (soluble in water; see Chapter 2). Because cells are mainly composed of water, the ability to interact with and be soluble in water profoundly affects the function of organic molecules in cells.
Organic molecules containing carboxyl groups (–COOH) are both polar (hydrophilic) and acidic. They tend to ionize and release hydrogen ions in solution:
	–COOH–COOH
3.2		The Organic Molecules of Cells
Despite their great diversity, organic molecules in living things are grouped into only four categories: carbohydrates, lipids, proteins, and nucleic acids. You are very familiar with these molecules because certain foods are known to be rich in carbohydrates, lipids, and proteins, as illustrated in Figures 3.4, 3.5, and 3.6. When you digest these foods, they break down into subunit molecules. Your body then takes these subunits and builds from them the large macromolecules that make up your cells. Many different foods also contain nucleic acids, the type of molecule that forms your genes.
Macromolecules are constructed by linking together a large number of the same type of subunits, called monomers. Linking many monomers results in a polymer. A protein can contain hundreds of amino acid monomers, and a nucleic acid can contain hundreds of nucleotide monomers. How can polymers get so large? Just as a train increases in length when boxcars are hitched together one by one, so a polymer gets longer as monomers bond to one another. 
A cell uses the same type of reaction to synthesize any type of macromolecule. It is called a dehydration reaction because the equivalent of a water molecule, that is, an –OH (hydroxyl group) and an –H (hydrogen atom), is removed as the reaction occurs (Fig. 3.7a). To break down a macromolecule, a cell and also the digestive tract uses an opposite type of reaction: During a hydrolysis reaction, an –OH group from water attaches to one subunit, and an –H from water attaches to the other subunit (Fig. 3.7b). In other words, water is used to break the bond holding monomers together. 
Carbohydrates
Carbohydrates are almost universally used as an immediate energy source in living things, but they also play structural roles in a variety of organisms. The term carbohydrate refers to either a single sugar molecule or two sugar molecules bonded together. Typically, the sugar glucose is a monomer for carbohydrate polymers. 
Monosaccharides: Ready Energy
Monosaccharides (mono, one; saccharide, sugar) have only a single sugar molecule; therefore, they are simple sugars. A simple sugar can have a carbon backbone of three to seven carbons. The word carbohydrate might make you think that every carbon atom is bonded to an H and an –OH. This is not strictly correct, as you can see by examining the structural formula for glucose (Fig. 3.8). Still, sugars do have many polar –OH groups, which make them soluble in water. 
Glucose, with six carbon atoms, has a molecular formula of C6H12O6. Glucose has two important isomers, called fructose and galactose, but even so, we usually think of glucose when we see the formula C6H12O6. That’s because glucose has a special place in the chemistry of organisms. Glucose is transported in the blood of animals, and it is also the molecule that is broken down in nearly all types of organisms as an immediate source of energy. In other words, cells use glucose as the energy source of choice.
Ribose and deoxyribose, with five carbon atoms, are significant because they are found in the nucleic acids RNA and DNA, respectively. RNA and DNA are discussed later in this chapter. 
Disaccharides: Varied Uses
A disaccharide (di, two; saccharide, sugar) contains two monosaccharides bonded together. The brewing of beer relies on maltose, a disaccharide usually derived from barley. During the production of beer, yeast breaks down maltose to two units of glucose and then uses glucose as an energy source. Because yeasts are using a process, called fermentation, ethyl alcohol is produced (Fig. 3.9). 
Sucrose, a disaccharide acquired from sugar beets and sugarcane, is of special interest because we use it at the table and in baking as a sweetener. Our body digests sucrose to its components, glucose and fructose. (Later, the fructose is changed to glucose, our usual energy source.) If the body doesn’t need more energy at the moment, the glucose is metabolized to fat! That’s why eating sugary desserts can make you fat. 
When you make lemonade at home, you add sucrose. But drinks made commercially often contain high-fructose corn syrup (HFCS). In the 1980s, a commercial method was developed for converting the glucose in corn syrup to the much sweeter-tasting fructose. Nutritionists are not in favor of eating highly processed foods that are rich in sucrose, HFCS, and white starches. They say these foods provide “empty” calories, meaning that although they supply energy, they don’t supply any of the vitamins, minerals, and fiber needed in the diet. In contrast, minimally processed foods provide glucose, starch, and many other types of nutritious molecules. 
Polysaccharides as Energy Storage Molecules
Polysaccharides are polymers of monosaccharides. Some types of polysaccharides function as short-term energy storage molecules because they are much larger than a sugar and are relatively insoluble. Polysaccharides cannot easily pass through the plasma membrane and are kept (stored) within the cell.
Plants store glucose as starch. The cells of a potato contain granules where starch resides during winter until -energy is needed for growth in the spring. Notice in -Figure 3.10a that starch exists in two forms—one is nonbranched and the other is branched. Animals store glucose as glycogen, which is even more branched (Fig. 3.10b). Branching makes a polysaccharide subject to hydrolytic attack by enzymes. 
The storage and release of glucose from liver cells is controlled by hormones. After we eat, the pancreas releases the hormone insulin, which promotes the storage of glucose as glycogen. 
Polysaccharides as Structural Molecules
Some types of polysaccharides function as structural components of cells. Cellulose is the most abundant of all the carbohydrates, which in turn are the most abundant of all the organic molecules on Earth. Plant cell walls contain cellulose and therefore cellulose is plentiful in the wood of tree trunks. The seeds of cotton plants have long fibers composed mostly of cellulose. Humans use wood for construction, and they use cotton fibers to make a cloth of the same name. 
The bonds joining the glucose subunits in cellulose are different from those found in starch and glycogen (Fig. 3.10c). As a result, the molecule is not helical; instead, the long glucose chains are held parallel to each other by hydrogen bonding to form strong microfibrils and then fibers. The fibers crisscross within plant cell walls for even more strength. 
The digestive juices of animals can’t hydrolyze cellulose, but some microorganisms can digest it. Cows and other ruminants (cud-chewing animals) have a special internal pouch where microorganisms break down cellulose to glucose. In humans, cellulose has the benefit of serving as dietary fiber, which maintains regular elimination.
Chitin, which is found in the exo-skeleton of crabs and related animals such as lobsters and insects, is also a polymer of glucose. However, each glucose subunit has an amino group (—NH2) attached to it. The linkage between the glucose molecules is like that found in cellulose; therefore, chitin is not digestible by humans but still has many good uses. Seeds are coated with chitin, and this protects them from attack by soil fungi. Because chitin also has antibacterial and antiviral properties, it is processed and used in medicine as a wound dressing and suture material. Chitin is even useful during the production of cosmetics and various foods.
Lipids
Although molecules classified as lipids are quite varied, they have one characteristic in common: They are all insoluble in water due to their nonpolar hydrocarbon chains. Salad dressings are rich in vegetable oils. Even after shaking, the vegetable oil will separate out from the water.
Fats (such as bacon fat, lard, and butter) and also oils (such as corn oil, olive oil, and coconut oil) are well-known lipids. In animals, fat is used for both insulation and long-term energy storage. Fat below the skin of marine mammals is called blubber; around the waist of humans, it is often referred to as a “spare tire.” Instead of fat, plants use oils for long-term energy storage. In animals, the secretions of oil glands help waterproof skin, hair, and feathers (Fig. 3.11). 
Several other types of molecules are lipids. Phospholipids, which are constructed similarly to fats and oils, are important components of the plasma membrane that surrounds cells. Steroids, with a structure entirely different from that of fats and oils, are an important type of lipid in the bodies of animals. Cholesterol, the molecule mentioned at the beginning of this chapter, and some hormones, such as the sex hormones, are examples of steroids. 
Fats and Oils: Long-term Energy Storage
Fats and oils contain two types of subunit molecules: glycerol and fatty acids (Fig. 3.12). Glycerol is a compound with three –OH groups. The –OH groups are polar; therefore, glycerol is soluble in water. A fatty acid has a long chain of carbon atoms bonded only to hydrogen, with a carboxyl group at one end. A fat or oil forms when the carboxyl portions of three fatty acids react with the –OH groups of glycerol. This is a dehydration reaction because in addition to a fat molecule, three molecules of water result. Fats and oils are degraded during a hydrolysis reaction, in which water is added to the molecule (see Fig. 3.12). Because three long fatty acids are attached to each glycerol molecule, fats and oils are called triglycerides and pack a lot of energy in one molecule. 
Fatty Acids
Most of the fatty acids in cells contain 16 or 18 carbon atoms per molecule, although smaller ones are also found. Fatty acids are either saturated or unsaturated. Saturated fatty acids have no double bonds between the carbon atoms. The carbon chain is saturated, so to speak, with all the hydrogens it can hold. Unsaturated fatty acids have double bonds in the carbon chain wherever the number of hydrogens is less than two per carbon atom.
In general, oils are liquids at room temperature because they contain unsaturated fatty acids. Notice in Figure 3.13a that the double bond creates a bend in the fatty acid chain. Such kinks prevent close packing between the hydrocarbon chains and account for the fluidity of oils. On the other hand, butter, which contains saturated fatty acids, is a solid at room temperature (Fig. 3.13b). 
Saturated fats in particular are associated with a cardiovascular disease called atherosclerosis, in which lipid material, called plaque, accumulates inside blood vessels. Plaque contributes to high blood pressure and heart attacks. Even more harmful than naturally occurring saturated fats are the so-called trans fats, which are vegetable oils hydrogenated commercially to make them solid. Trans fats are often found in processed foods—margarine, baked goods, and fried foods in particular. Unsaturated oils, particularly monounsaturated (one double bond) oils but also polyunsaturated (many double bonds) oils, have been found to be protective against atherosclerosis (Fig. 3.13c). 
Phospholipids: Membrane Components
Phospholipids, as implied by their name, contain a phosphate group. Essentially, a phospholipid is constructed like a fat, except that in place of the third fatty acid attached to glycerol, there is a charged phosphate group. The phosphate group is usually bonded to another polar group, indicated by R in Figure 3.14a. This portion of the molecule is the polar head, while the hydrocarbon chains of the fatty acids are the nonpolar tails. 
Because phospholipids have hydrophilic (polar) heads and hydrophobic  (nonpolar) tails, they tend to arrange themselves so that only the polar heads are adjacent to a watery medium. Between two compartments of water, such as the outside and inside of a cell, phospholipids become a bilayer in which the hydrophilic heads project outward and the hydrophobic tails project inward. The bulk of the plasma membrane that surrounds cells consists of a fairly fluid phospholipid bilayer, as do all the other membranes in the cell (Fig. 3.14b). A plasma membrane is absolutely essential to the structure and function of a cell, and thus phospholipids are vitally important to humans and other organisms. 
Steroids: Four Fused Rings
Steroids are lipids that have entirely different structures from those of fats. Steroid molecules have skeletons of four fused hydrocarbon rings, shown in red in Figure 3.15. Each type of steroid differs primarily in the types of functional groups attached to the carbon skeleton.
Cholesterol (Fig. 3.15a) is a component of an animal cell’s plasma membrane, and it is the precursor of other steroids, such as the sex hormones testosterone and estrogen. The male sex hormone, testosterone, is formed primarily in the testes, and the female sex hormone, estrogen, is formed primarily in the ovaries (Fig. 3.15b,c). Testosterone and estrogen differ only by the functional groups attached to the same carbon skeleton, and yet they have a profound effect on the body and the sexuality of humans and other animals.
Most likely you are aware that some people take anabolic steroids, synthetic testosterone, to increase muscle mass. The result is usually unfortunate. The presence of the steroid in the body upsets the normal hormonal balance: The testes atrophy, and males may develop breasts; females tend to grow facial hair and lose hair on their heads. Because the taking of steroids gives athletes an unfair advantage and destroys their health—heart, kidney, liver, and psychological disorders are common—they are banned by professional athletic associations.
Proteins
Proteins are of primary importance in the structure and function of cells. Here are some of their many functions:
SupportSome proteins are structural proteins. Examples include the protein in spider webs; keratin, the protein that makes up hair and fingernails; and collagen, the protein that lends support to skin, ligaments, and tendons (Fig. 3.16a). 
MetabolismSome proteins are enzymes. They bring reactants together and thereby speed chemical reactions in cells. They are specific for one particular type of reaction and can function at body temperature.
TransportChannel and carrier proteins in the plasma membrane allow substances to enter and exit cells. Other proteins transport molecules in the blood of animals—for example, hemoglobin is a complex protein that transports oxygen (Fig. 3.16b). 
DefenseSome proteins, called antibodies, combine with disease-causing agents to prevent them from destroying cells and upsetting homeostasis, the relative constancy of the internal environment.
RegulationHormones are regulatory proteins. They serve as intercellular messengers that influence the metabolism of cells. For example, the hormone insulin regulates the content of glucose in the blood and in cells, while growth hormone determines the height of an individual.
MotionThe contractile proteins actin and myosin allow parts of cells to move and cause muscles to contract (Fig. 3.16c). Muscle contraction enables animals to move from place to place.
The structures and functions of cells differ according to the type of protein they contain. Muscle cells contain actin and myosin; red blood cells contain hemoglobin; support cells contain collagen; and so forth.
Amino Acids: Subunits of Proteins
A significant carbon atom in an amino acid bonds to a hydrogen atom and also to three other groups of atoms (Fig. 3.17a). The name amino acid is appropriate because one of these groups is an –NH2 (amino group) and another is a –COOH (an acid group as discussed on page 31). The third group is the R group for an amino acid.
Amino acids differ according to their particular R group. The R groups range in complexity from a single hydrogen atom to a complicated ring compound. The unique chemical properties of an amino acid depend on those of the R group. For example, some R groups are polar and some are not. Also, the amino acid cysteine has an R group that ends with a sulfhydryl (–SH) group, which often serves to connect one chain of amino acids to another by a disulfide bond, –S–S–. Four amino acids commonly found in cells are shown in Figure 3.17b.
Peptides
Figure 3.18 shows how two amino acids join by a dehydration reaction between the carboxyl group of one and the amino group of another. The resulting covalent bond between two amino acids is called a peptide bond. The atoms associated with the peptide bond share the electrons unevenly because oxygen is more electronegative than nitrogen. Therefore, the hydrogen attached to the nitrogen has a slightly positive charge (), while the oxygen has a slightly negative charge ():
A peptide is two or more amino acids covalently bonded together, and a polypeptide is a chain of many amino acids joined by peptide bonds. A protein may contain more than one polypeptide chain; therefore, you can see why a protein could be composed of a very large number of amino acids. Each polypeptide has its own normal sequence. The amino acid sequence determines the final three-dimensional shape of the protein. Proteins that have an abnormal sequence of amino acids often have the wrong shape and cannot function properly.
Shape of Proteins
All proteins have levels of structure. These levels are called the primary, secondary, tertiary, and quaternary structures (Fig. 3.19). A protein’s sequence of amino acids is called its primary structure. Consider that an almost infinite number of words can be constructed by changing the number and sequence of the 26 letters in our alphabet. In the same way, many different proteins can result by varying the number and sequence of just 20 amino acids. After a chain of amino acids forms, it coils or folds in a particular way. This shape is called the protein’s secondary structure. The polypeptide can have the spiral shape of a helix, or it can turn back on itself, forming a pleated sheet. Regardless, hydrogen bonding between peptide bonds maintains the secondary structure of a protein. Fibrous proteins are structural proteins that have only a secondary structure. Keratin, the fibrous protein in hair, fingernails, horns, and feathers, has many helical regions. The silk threads spun by spiders are largely pleated sheets, as is collagen, the protein that gives shape to the skin, tendons, ligaments, cartilage, and bones of animals.
Globular proteins have a tertiary structure. Their rounded, three-dimensional shape results from the folding and twisting of the secondary structure. Various types of bonding between R groups maintain the tertiary structure. Enzymes are globular proteins. An enzyme has an optimum temperature and pH at which it works best and maintains its normal shape. A rise in temperature or a change in pH can disrupt the interactions between the R groups and change the shape of the enzyme. When an enzyme loses its shape, it is denatured and can’t function any more. 
Some proteins, such as hemoglobin and certain enzymes, have a quaternary structure, which means they consist of more than one polypeptide, each with its own primary, secondary, and tertiary structure. 
Nucleic Acids
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are the nucleic acids in cells. Early investigators called them nucleic acids because they were first detected in the nucleus. As already stated, DNA stores genetic information. Each DNA molecule contains many genes, and genes specify the sequence of the amino acids in proteins. RNA is the helper that takes this information to the site of protein synthesis. 
Nucleic acids are polymers in which the monomer is called a nucleotide. All nucleotides are composed of three parts: a phosphate, a 5-carbon sugar, and a nitrogen-containing base (Fig. 3.20a). The phosphate is simply —PO4. The sugar is deoxyribose in DNA and ribose in RNA, which accounts for their names. (Deoxyribose has one less oxygen than does ribose.) 
As you can see in Figure 3.20b, DNA is a double helix, meaning that two strands spiral around one another. In each strand, the backbone of the molecule is composed of phosphates bonded to sugars, and the bases project to the side. Interestingly, the base guanine (G) is always paired with cytosine (C), and the base adenine (A) is always paired with thymine (T). This is called complementary base pairing. Complementary base pairing is very important when DNA makes a copy of itself, a process called replication. 
RNA differs from DNA not only by its sugar but also because it uses the base uracil instead of thymine. Whereas DNA is double-stranded, RNA is single-stranded (Fig. 3.20c).Complementary base pairing also allows DNA to pass genetic information to RNA. The information is in the sequence of bases. DNA has a triplet code, and every three bases stands for one of the 20 amino acids in cells. Once you know the sequence of bases in a gene, you know the sequence of amino acids in a polypeptide. Many scientists around the world are involved in the Human Genome Project. Not long ago, members of this project determined the sequence of all the base pairs in all of our 25,000 genes. They hope this information will lead to cures for many human illnesses that are caused by proteins that don’t function as they should. 
Relationship Between Proteins and Nucleic Acids
We have learned that the functional group of an amino acid determines its behavior, and that the order of the amino acids within a polypeptide determines its shape. The shape of a protein determines its function. The structure and function of cells are determined by the types of proteins they contain. The same is true for organisms—that is, they differ with regard to their proteins. Next, we learned that DNA, the genetic material, bears instructions for the sequence of amino acids in polypeptides. The proteins of organisms differ because their genes differ. 
The relationship between a gene, a protein, and illness is illustrated by sickle cell disease. In sickle cell disease, the individual’s red blood cells are sickle-shaped because in one particular spot, the amino acid valine (val) appears where the amino acid glutamate (glu) should be (Fig. 3.21). This substitution makes red blood cells lose their normally round, flexible shape and become hard and rigid. When these abnormal red blood cells go through the small blood vessels, they clog the flow of blood and break apart. This condition can cause pain, organ damage, and a low red blood cell count (called anemia), and is often fatal.
What’s the root of the problem? The individual inherited a faulty code for an amino acid in one of the hemoglobin’s polypeptides.
The Chapter In Review
Summary
3.1		Organic Molecules
In order to be organic, a molecule must contain carbon and hydrogen. Organic molecules have different chemical properties depending on the carbon skeleton and attached functional groups. Some functional groups are nonpolar (hydrophobic), and others are polar (hydrophilic).
3.2		The Organic Molecules of Cells
Polymers (many monomers bonded together) are synthesized by a dehydration reaction and degraded by a hydrolysis reaction.
Living things are composed of four types of molecules: carbohydrates, lipids, proteins, and nucleotides. The characteristics of these molecules are summarized in the following chart:
Carbohydrates
	•	Glucose is a monosaccharide that serves as blood sugar and as a monomer of starch, glycogen, and cellulose. Its isomers are fructose and galactose.
	•	Sucrose is a disaccharide (composed of glucose and fructose) that we know as table sugar.
	•	Starch, a polymer of glucose, stores energy in plants.
	•	Glycogen, a polymer of glucose, stores energy in animals.
	•	Maltose is a disaccharide that results from the digestion of starch and glycogen.
	•	Cellulose is a polymer of glucose that makes up the structure of plant cell walls.
Lipids
Fats and oils, which are composed of glycerol and fatty acids, are called triglycerides. Triglycerides made of saturated fatty acids (having no double bonds) are solids and are called fats. Triglycerides composed of unsaturated fatty acids (having double bonds) are liquids and are called oils. 
Phospholipids have the same structure as triglycerides, except that a group containing phosphate takes the place of one fatty acid. Phospholipids make up the plasma membrane as well as other cellular membranes.
Steroids are composed of four fused hydrocarbon rings. Cholesterol, a steroid, is a component of the plasma membrane. The sex hormones testosterone and estrogen are steroids.
Proteins
Proteins are polymers of amino acids. A peptide is composed of two amino acids; a polypeptide contains many amino acids. A peptide bond is a polar covalent bond.
A protein has levels of structure:
	•	Primary structure is the primary sequence of amino acids.
	•	Secondary structure is a helix or pleated sheet.
	•	Tertiary structure forms due to bending and twisting of the secondary structure, as seen in globular proteins.
	•	Quaternary structure occurs when a protein has more than one polypeptide.
Nucleic Acids
Cells and organisms differ because of their proteins, which are coded for by genes composed of nucleic acids. Nucleic acids are polymers of nucleotides.
DNA (deoxyribonucleic acid) contains deoxyribose sugar and is the chemical that makes up our genes. DNA is double-stranded and shaped like a double helix. The two strands of DNA are joined by complementary base pairing: Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
RNA (ribonucleic acid) serves as a helper to DNA during protein synthesis. Its sugar is ribose, and it contains the base uracil in place of thymine. RNA is single-stranded and thus does not form a double helix. Table 3.1 compares the structure of DNA and RNA.
Thinking Scientifically
	1.	Nutritionists advise us to avoid consuming oils from tropical plants because, compared to oils from temperate plants, they contain high levels of saturated fatty acids. Let’s try to determine a physiological reason for this difference in fatty acid composition.
			Look at the following graph and answer questions a–f.
		a.	What plant cell structure are we considering here? 
		b.	Linolenic acid is an unsaturated fatty acid. What happens to its level following exposure of plants to cold temperatures?
		c.	If you put a product with a high level of unsaturated fats (such as canola oil) and a product with a high level of saturated fats (such as butter) in your refrigerator (chilling temperature), how would they differ in consistency? 
		d.	Does chilling alter protein structure and function? In other words, does chilling cause denaturation? (Think of whether meat or eggs are damaged by chilling.) 
		e.	Therefore, which of the two membrane components—fatty acids or proteins—will be most dramatically affected by chilling temperatures? 
		f.	Finally, how does fatty acid composition relate to chilling tolerance? 
	2.	In order to understand the relationship between enzyme structure and function, researchers often study mutations that result in altered enzyme structure. In one bacterial enzyme, function is retained if a substituted amino acid has a nonpolar R group, but function is lost if the substituted amino acid has a polar R group. Why might that be? 
Testing Yourself
Choose the best answer for each question.
	1.	Which of the following is an organic molecule?
	a. CO2	d. O2
	b. H2O	e. More than one of these is correct.
	c. C6H12O6
	2.	Carbon requires how many electrons to complete its outer shell?
	a. 2	c. 4
	b. 3	d. Any one of these is correct.
	3.	A hydrocarbon chain
	a. contains carbon and hydrogen atoms only.
	b. is hydrophobic.
	c. is hydrophilic.
	d. Both a and b are correct.
	4.	Carbon chains can vary in 
	a. length.	c. branching pattern.
	b. number of double bonds.	d. All of these are correct.
	5.	Organic molecules containing carboxyl groups are
	a. nonpolar.	c. basic.
	b. acidic.	d.	
More than one of these is correct.
	6.	Monomers are attached together to create polymers when a hydroxyl group and a hydrogen atom are _______ in a _______ reaction.
	a. added, dehydration	c. added, hydrolysis
	b. removed, dehydration	d. removed, hydrolysis
	7.	Which of the following is a disaccharide?
	a. glucose	c. fructose
	b. ribose	d. sucrose
For questions 8–15, match the items to those in the key. Answers can be used more than once.
Key:
	a. carbohydrate
	b. lipid
	c. protein
	d. nucleic acid
	8.	Cellulose, the major component of plant cell walls.
	9.	Keratin, found in hair, fingernails, horns, and feathers.
	10.	Steroids such as cholesterol and sex hormones.
	11.	Composed of nucleotides.
	12.	Insoluble in water due to hydrocarbon chains.
	13.	Sometimes undergoes complementary base pairing.
	14.	May contain pleated sheets and helices.
	15.	May be a ring of six carbon atoms attached to hydroxyl groups.
	16.	A triglyceride contains
	a. glycerol and three fatty acids.
	b. glycerol and three sugars.
	c. protein and three fatty acids.
	d. protein and three sugars.
	 17.	Variations in three-dimensional shapes among proteins are due to bonding between the
	a. amino groups.	c. R groups.
	b. ion groups.	d. H atoms.
	18.	The following picture illustrates a(n)
	a. saturated fatty acid.	c. polyunsaturated fatty acid.
	b. monounsaturated fatty acid.	d. All of these are correct.
	 19.	The three-dimensional structure of a protein that contains two or more polypeptides is the 
	a. primary structure.	c. tertiary structure.
	b. secondary structure.	d. quaternary structure.
For questions 20–24, match the structure in the following diagram with the name of the functional group.
	20.	Phosphate
	21.	Carboxyl
	22.	Hydroxyl
	23.	Sulfhydral
	24.	Amino
	25.	Carbon and oxygen can be found in 
	a. amino acids.
	b. glucose.
	c. starch.
	d. All of these are correct.
	26.	Plants store glucose as
	a. maltose.
	b. glycogen.
	c. starch.
	d. None of these are correct.
	27.	The polysaccharide found in plant cell walls is
	a. glucose.	c. maltose.
	b. starch.	d. cellulose.
	28.	Phosphates can be found in
	a. DNA.	c. glucose.
	b. RNA.	d. Both a and b are correct. 
	29.	All _______________ are _______________.
	a. proteins, enzymes
	b. sugars, monosaccharides
	c. enzymes, proteins
	d. sugars, polysaccharides
	30.	_______________ is the precursor of _______________.
	a. Estrogen, cholesterol
	b. Cholesterol, glucose
	c. Testosterone, cholesterol
	d. Cholesterol, testosterone and estrogen
	31.	An example of a hydrocarbon would be
	a. heptane.
	b. glucose.
	c. maltose.
	d. None of these are correct.
	32.	A 5-carbon sugar is associated with
	a. glucose.
	b. a protein.
	c. DNA.
	d. lipids.
	33.	Nucleotides
	a.	
contain sugar, a nitrogen-containing base, and a phosphate molecule.
	b. are the monomers for fats and polysaccharides.
	c. join together by covalent bonding between the bases.
	d. are found in DNA, RNA, and proteins.
	34.	The joining of two adjacent amino acids is called a
	a. peptide bond.
	b. dehydration reaction.
	c. covalent bond.
	d. All of these are correct.
	35.	Which of the following pertains to an RNA nucleotide and not to a DNA nucleotide?
	a. contains the sugar ribose
	b. contains a nitrogen-containing base
	c. contains a phosphate molecule
	d. becomes bonded to other nucleotides by condensation
	36.	Saturated fatty acids and unsaturated fatty acids differ in
	a. the number of carbon-to-carbon bonds.
	b. their consistency at room temperature.
	c. the number of hydrogen atoms present.
	d. All of these are correct.
	37.	Which of the following is an example of a polysaccharide used for energy storage?
	a. cellulose
	b. glycogen
	c. cholesterol
	d. glucose
Go to www.mhhe.com/maderessentials for more quiz questions.
Bioethical Issue
Almost everything we buy is wrapped in plastic. Plastic is an ideal packaging material because it protects the product and is inexpensive. However, as our uses for plastic grow, problems associated with its manufacture and disposal also grow. Plastics are made from fossil fuels, a nonrenewable resource. In addition, the polymers used to produce plastic cannot be broken down by microorganisms, and therefore plastics are not biodegradable. Thus, researchers have been trying to create biodegradable plastics from a renewable resource—plants. 
Starch can be made into a bioplastic (biodegradable plastic), but it absorbs water, so it becomes deformed when wet. Alternatively, starch can be converted to lactic acid, and then the lactic acid molecules can be linked together to form a more stable bioplastic called polylactide. As you might expect, this extra processing adds to the cost of the plastic. Biodegradable plastics are 2–10 times more expensive than conventional plastic. In addition, in order to get them to decompose, they must be composted instead of sealed in a landfill. 
Would you pay more for products wrapped in bioplastic to benefit the environment? How much more? Would you make the effort to separate bioplastics from the remainder of your garbage so they could be sent to a composting site? Who should pay for more bioplastics research—the federal government or companies that manufacture plastic? 
Understanding the Terms
amino acid39
carbohydrate33
cellulose34
chitin34
cholesterol37
dehydration reaction33
denatured40
deoxyribose33
disaccharide33
DNA 
(deoxyribonucleic acid)41
enzyme38
fat35
fatty acid35
fibrous protein40
functional group31
globular protein40
glucose33
glycerol35
glycogen34
hemoglobin38
hydrolysis reaction33
hydrophilic31
hydrophobic31
inorganic chemistry30
isomer31
lipid35
monomer32
monosaccharide33
nucleic acid41
nucleotide41
oil35
organic30
organic chemistry30
peptide40
peptide bond39
phospholipid36
polymer32
polypeptide40
polysaccharide34
protein38
ribose33
RNA (ribonucleic acid)41
saturated fatty acid36
starch34
steroid37
unsaturated fatty acid36
Match the terms to these definitions:
a.	_______________	Molecules with identical molecular formulas but different arrangements of atoms. 
b.	_______________	Macromolecule created by linking together many monomers.
c.	_______________	Four fused hydrocarbon rings. 
d.	_______________	A molecule that speeds a reaction by bringing reactants together. 
e.	_______________	A glycerol molecule attached to two fatty acids and a phosphate group.
f.	_______________	The condition of an enzyme when it has lost its shape and can no longer function.
Figure 3.21Sickle cell disease.
One of the amino acid chains in hemoglobin is 146 amino acids long. Sickle cell disease, characterized by sickled red blood cells, results when valine occurs instead of glutamate at the sixth amino acid. 
Cholesterol found in foods such as egg yolks has many important functions, including the production of sex hormones.
Some human cells can produce up to 10,000 different types of proteins.
Dietary fiber found in supplements, green vegetables, and grains has no nutritional value.
Figure 3.1Organic molecules as structural materials.
Some organic molecules are purely structural in nature. Related organic molecules (a) help hold a plant stem erect, (b) make the shell of a crab hard, and (c) support the wall of a bacterium. 
Figure 3.2Hydrocarbons are highly versatile.
Hydrocarbons contain only hydrogen and carbon. a. Even so, they can be quite varied according to the number of carbons, the placement of any double bonds, possible branching, and possible ring formation. b. Carbon-to-carbon bonds are a source of energy. Gasoline contains heptane and isooctane—the higher the octane rating (percentage), the smoother your car engine will run.
Figure 3.3Functional groups. 
Molecules with the same carbon skeleton can still differ according to the type of functional group attached. Many of these functional groups are polar, helping to make the molecule soluble in water. In this illustration, the remainder of the molecule, the hydrocarbon chain, is represented by an R.
Figure 3.4Carbohydrates.
The familiar carbohydrates, sugar and starch, are present in the foods shown. A diet loaded with these carbohydrates may make you prone to diabetes type 2 and perhaps to other illnesses as well. In contrast, moderate amounts of multigrain foods are better for you and provide fiber to keep you regular. 
Figure 3.7Synthesis and breakdown of polymers. 
a. In cells, synthesis often occurs when monomers bond during a dehydration reaction (removal of H2O). b. Breakdown occurs when the monomers in a polymer separate because of a hydrolysis reaction (the addition of H2O).
Figure 3.6Protein foods.
It’s surprising how little protein is needed in the diet. Just 6 ounces a day will provide you with all the amino acids you need. Beef, but not chicken, can be loaded with animal fat. Fish, however, contains beneficial oils that lower the incidence of cardiovascular disease. Also, the diet can include an egg a day, usually with no ill effects.
Figure 3.5Lipid foods.
A diet rich in fat can indeed make you fat. However, you do need some fat in your diet. Choose vegetable oils over the animal fat in lard, butter, and other dairy products. The fats in vegetable oils even lower your risk of cardiovascular disease if they haven’t been hydrogenated, as they are in processed foods. 
Figure 3.8Glucose. 
Each of these structural formulas represents glucose. a. The carbon skeleton and all attached groups are shown. b. The carbon skeleton is omitted. c. The carbon skeleton and attached groups are omitted. d. Only the ring shape of the molecule remains.
Figure 3.9Breakdown of maltose, a disaccharide. 
Maltose is the energy source for yeast during the production of beer. Yeasts differ as to the amount of maltose they convert to alcohol, so selection of the type of yeast is important.
Figure 3.10Starch and glycogen structure  and function.
a. Glucose is stored in plants as starch. The electron micrograph shows the location of starch in plant cells. Starch is a chain of glucose molecules that can be nonbranched or branched.  b. Glucose is stored in animals as glycogen. The electron micrograph shows glycogen deposits in a portion of a liver cell. Glycogen is a highly branched polymer of glucose molecules.  c. In plant cell walls, each cellulose fiber contains several microfibrils. Each microfibril contains many polymers of glucose hydrogen-bonded together. Three such polymers are shown.
Figure 3.11Preening in birds.
As a bird preens, it transfers oil to its feathers from a gland at the base of its tail. Because the oil repels water, the feathers become waterproof.
Figure 3.12Synthesis and breakdown of fat. 
Following a dehydration reaction, glycerol is bonded to three fatty acid molecules, and water is given off. Following a hydrolysis reaction, the bonds are broken due to the addition of water. R represents the remainder of the molecule, which in this case is a continuation of the hydrocarbon chain, composed of 16 or 18 carbons. 
Figure 3.13Fatty acids.
A fatty acid has a carboxyl group attached to a long hydrocarbon chain. a. If there are double bonds between some of the carbons in the chain, the fatty acid is monounsaturated. b. If there are no double bonds, the fatty acid is saturated. A diet high in saturated fats appears to contribute to diseases of the heart and blood vessels. c. Which fat or oil do you judge to be the most healthy to use? 
Figure 3.14Phospholipids form membranes.
a. Phospholipids are constructed like fats, except that in place of the third fatty acid, they have a charged phosphate group. The hydrophilic (polar) head group is soluble in water, whereas the two hydrophobic (nonpolar) tail groups are not. b. This causes the molecules to arrange themselves as a bilayer in the plasma membrane that surrounds a cell.
Figure 3.15Steroid diversity.
a. All the steroids are derived from cholesterol, an important component of the plasma membrane. Cholesterol and all steroid molecules have four adjacent rings, but their effects on the body largely depend on the attached groups indicated in red. The different effects of (b) testosterone and (c) estrogen on the body are due to different functional groups attached to the same carbon skeleton. 
Figure 3.16Types of proteins.
a. The protein in hair, fingernails, and spider webs (keratin) is a structural protein, as is collagen. Collagen injections can reduce wrinkles and scarring by providing extra support. b. Hemoglobin, a major protein in red blood cells, is involved in transporting oxygen. c. Contractile proteins, actin and myosin, cause muscles to move.
Figure 3.17Amino acids.
a. Structure of an amino acid. b. Proteins contain 20 different kinds of amino acids, four of which are shown here. Amino acids differ by the particular R group (blue) attached to the central carbon. Some R groups are nonpolar and hydrophobic, some are polar and hydrophilic, and some are ionized and hydrophilic. 
Figure 3.18Synthesis and degradation of a peptide.
Following a dehydration reaction, a peptide bond joins two amino acids, and water is given off. Following a hydrolysis reaction, the bond is broken due to the addition of water.
Figure 3.19Levels of protein organization.
All proteins have a primary structure. Fibrous proteins have a secondary structure; they are either helices (e.g., keratin, collagen) or pleated sheets (e.g., silk). Globular proteins always have a tertiary structure, and most have a quaternary structure (e.g., hemoglobin, enzymes).
Figure 3.20DNA and RNA structure.
a. Structure of a nucleotide. b. Structure of DNA and its bases.  c. Structure of RNA in which uracil replaces thymine.
Check Your Progress
	1.	Describe the properties of a carbon atom that make it an ideal foundation for life.
	2.	List the four classes of organic molecules.
Answers:1. The carbon atom can bond with up to four different elements. Carbon-to-carbon bonds are stable, so long chains can be built. These chains can be variable in length and branching patterns. Some carbon molecules can create isomers.2. Carbohydrates, lipids, proteins, and nucleic acids.
Check Your Progress
	1.	Describe the significance of glucose in living systems.
	2.	 Explain why humans cannot utilize the glucose in cellulose.
	3.	Compare and contrast cellulose with chitin.
Answers:1. Most organisms use glucose as the energy source of choice. 2. Humans do not produce digestive juices capable of breaking the bonds attaching glucose subunits together in cellulose.3. Both cellulose and chitin are composed of glucose subunits linked together in the same way. In contrast to cellulose, chitin has an amino group attached to each glucose molecule. Cellulose is found in plant cell walls, while chitin is in the exoskeleton of some animals.
Check Your Progress
	1.	Compare and contrast a saturated fatty acid with an unsaturated fatty acid.
	2.	 Explain why phospholipids form a bilayer in a watery medium.
Answers:1. A saturated fatty acid contains no double bonds between carbon atoms, while an unsaturated fatty acid contains one or more double bonds. 2. Phospholipids arrange themselves so that their hydrophilic heads are adjacent to water, while the hydrophobic tails point inward toward each other.
Check Your Progress
List six functions of proteins.
Answer:Support, metabolism, transport, defense, regulation (hormones), and motion.
Check Your Progress
	1.	What is the primary structure of a protein?
	2.	 a. What does the peptide bond have to do with the secondary structure of a protein? b. What type of bonding maintains the tertiary structure of a protein?
	3.	List the three components of a nucleotide.
	4.	Explain why complementary base pairing is important for nucleic acids.
Answers:1. The protein’s sequence of amino acids is its primary structure. 2. a. The peptide bond includes a partially negative oxygen and a partially negative hydrogen. Therefore, hydrogen bonding between peptide bonds accounts for a protein’s secondary structure. b. Bonding between R groups, such as disulfide linkages.3. Phosphate, 5-carbon sugar, and nitrogen-containing base.4. Base pairing is important when DNA replicates and also for producing RNA molecules that are copies of genes.
a. Oleic acid, a monounsaturated fatty acid (one double bond) found in canola oil.
b. Stearic acid, a saturated fatty acid (no double bonds) found in butter.
c. Percentages of saturated and unsaturated fatty acids in fats and oils.
a. Structural proteins
b. Transport proteins
Table 3.1
DNA Structure Compared to RNA Structure
	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	Adenine, guanine, thymine, cytosine	Adenine, guanine, uracil, cytosine
Strands	Double-stranded with base pairing	Single-stranded
Helix	Yes	No