Chapter 3 The Molecules of Cells

Introduction Most of the world’s population cannot digest milk-based foods. These people are lactose intolerant, because they lack the enzyme lactase which is needed to digest the sugar in milk, lactose. This illustrates the importance of biological molecules, such as lactase, in the daily functions of living organisms. © 2012 Pearson Education, Inc. 2

Molecular structure of enzyme Lactase Figure 3.0_2 Molecular structure of enzyme Lactase Figure 3.0_2 Chapter 3: Ribbon model of lactase 3

Introduction to Organic Compounds Figure 3.0_1 Chapter 3: Big Ideas Introduction to Organic Compounds Carbohydrates Figure 3.0_1 Chapter 3: Big Ideas Lipids Proteins Nucleic Acids 4

INTRODUCTION TO ORGANIC COMPOUNDS © 2012 Pearson Education, Inc. 5

3.1 Life’s molecular diversity is based on the properties of carbon Diverse molecules found in cells are composed of carbon bonded to other carbons and atoms of other elements. Carbon-based molecules are called organic compounds. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Students might need to be reminded about the levels of biological organization. The relationship between atoms, monomers, and polymers can be confusing as each is discussed. Consider noting these relationships somewhere in the classroom (such as on the board) where students can quickly glance for reassurance. Teaching Tips One of the great advantages of carbon is its ability to form up to four bonds, permitting the assembly of diverse components and branching configurations. Challenge your students to find another element that might also permit this sort of adaptability. (Like carbon, silicon has four electrons in its outer shell.) Toothpicks and gumdrops (or any other pliable small candy) permit the quick construction of chemical models. Different candy colors can represent certain atoms. The model of the methane molecule in Figure 3.1 can thus easily be demonstrated (and consumed)! © 2012 Pearson Education, Inc. 6

3.1 Life’s molecular diversity is based on the properties of carbon By sharing electrons, carbon can bond to four other atoms and branch in up to four directions. Methane (CH4) is one of the simplest organic compounds. Four covalent bonds link four hydrogen atoms to the carbon atom. Each of the four lines in the formula for methane represents a pair of shared electrons. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Students might need to be reminded about the levels of biological organization. The relationship between atoms, monomers, and polymers can be confusing as each is discussed. Consider noting these relationships somewhere in the classroom (such as on the board) where students can quickly glance for reassurance. Teaching Tips One of the great advantages of carbon is its ability to form up to four bonds, permitting the assembly of diverse components and branching configurations. Challenge your students to find another element that might also permit this sort of adaptability. (Like carbon, silicon has four electrons in its outer shell.) Toothpicks and gumdrops (or any other pliable small candy) permit the quick construction of chemical models. Different candy colors can represent certain atoms. The model of the methane molecule in Figure 3.1 can thus easily be demonstrated (and consumed)! © 2012 Pearson Education, Inc. 7

3.1 Life’s molecular diversity is based on the properties of carbon Methane and other compounds composed of only carbon and hydrogen are called hydrocarbons. Carbon, with attached hydrogens, can bond together in chains of various lengths. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Students might need to be reminded about the levels of biological organization. The relationship between atoms, monomers, and polymers can be confusing as each is discussed. Consider noting these relationships somewhere in the classroom (such as on the board) where students can quickly glance for reassurance. Teaching Tips One of the great advantages of carbon is its ability to form up to four bonds, permitting the assembly of diverse components and branching configurations. Challenge your students to find another element that might also permit this sort of adaptability. (Like carbon, silicon has four electrons in its outer shell.) Toothpicks and gumdrops (or any other pliable small candy) permit the quick construction of chemical models. Different candy colors can represent certain atoms. The model of the methane molecule in Figure 3.1 can thus easily be demonstrated (and consumed)! © 2012 Pearson Education, Inc. 8

Structural formula Ball-and-stick model Space-filling model Figure 3.1A Structural formula Ball-and-stick model Space-filling model Figure 3.1A Three representations of methane (CH4) The four single bonds of carbon point to the corners of a tetrahedron. 9

3.1 Life’s molecular diversity is based on the properties of carbon A carbon skeleton is a chain of carbon atoms that can be branched or unbranched. Compounds with the same formula but different structural arrangements are call isomers. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Students might need to be reminded about the levels of biological organization. The relationship between atoms, monomers, and polymers can be confusing as each is discussed. Consider noting these relationships somewhere in the classroom (such as on the board) where students can quickly glance for reassurance. Teaching Tips One of the great advantages of carbon is its ability to form up to four bonds, permitting the assembly of diverse components and branching configurations. Challenge your students to find another element that might also permit this sort of adaptability. (Like carbon, silicon has four electrons in its outer shell.) Toothpicks and gumdrops (or any other pliable small candy) permit the quick construction of chemical models. Different candy colors can represent certain atoms. The model of the methane molecule in Figure 3.1 can thus easily be demonstrated (and consumed)! © 2012 Pearson Education, Inc. 10

Length. Carbon skeletons vary in length. Figure 3.1B Length. Carbon skeletons vary in length. Ethane Propane Branching. Skeletons may be unbranched or branched. Butane Isobutane Double bonds. Skeletons may have double bonds. Figure 3.1B Four ways that carbon skeletons can vary 1-Butene 2-Butene Rings. Skeletons may be arranged in rings. Cyclohexane Benzene 11

Length. Carbon skeletons vary in length. Figure 3.1B_1 Length. Carbon skeletons vary in length. Ethane Propane Figure 3.1B_1 Four ways that carbon skeletons can vary (part 1) 12

Skeletons may be unbranched or branched. Figure 3.1B_2 Branching. Skeletons may be unbranched or branched. Figure 3.1B_2 Four ways that carbon skeletons can vary (part 2) Butane Isobutane 13

Skeletons may have double bonds. Figure 3.1B_3 Double bonds. Skeletons may have double bonds. 1-Butene 2-Butene Figure 3.1B_3 Four ways that carbon skeletons can vary (part 3) 14

Rings. Skeletons may be arranged in rings. Figure 3.1B_4 Rings. Skeletons may be arranged in rings. Figure 3.1B_4 Four ways that carbon skeletons can vary (part 4) Cyclohexane Benzene 15

3.2 A few chemical groups are key to the functioning of biological molecules An organic compound has unique properties that depend upon the size and shape of the molecule and groups of atoms (functional groups) attached to it. A functional group affects a biological molecule’s function in a characteristic way. Compounds containing functional groups are hydrophilic (water-loving). Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips A drill with interchangeable drill bits is a nice analogy to carbon skeletons with different functional groups. The analogy relates the role of different functions to different structures. © 2012 Pearson Education, Inc. 16

3.2 A few chemical groups are key to the functioning of biological molecules The functional groups are hydroxyl group—consists of a hydrogen bonded to an oxygen, carbonyl group—a carbon linked by a double bond to an oxygen atom, carboxyl group—consists of a carbon double-bonded to both an oxygen and a hydroxyl group, amino group—composed of a nitrogen bonded to two hydrogen atoms and the carbon skeleton, and phosphate group—consists of a phosphorus atom bonded to four oxygen atoms. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips A drill with interchangeable drill bits is a nice analogy to carbon skeletons with different functional groups. The analogy relates the role of different functions to different structures. © 2012 Pearson Education, Inc. 17

Table 3.2_1 Table 3.2_1 Important chemical groups of organic compounds (part 1) 18

Table 3.2_2 Table 3.2_2 Important chemical groups of organic compounds (part 2) 19

3.2 A few chemical groups are key to the functioning of biological molecules An example of similar compounds that differ only in functional groups is sex hormones. Male and female sex hormones differ only in functional groups. The differences cause varied molecular actions. The result is distinguishable features of males and females. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips A drill with interchangeable drill bits is a nice analogy to carbon skeletons with different functional groups. The analogy relates the role of different functions to different structures. © 2012 Pearson Education, Inc. 20

Testosterone Estradiol Figure 3.2 Figure 3.2 Differences in the chemical groups of sex hormones 21

3.3 Cells make a huge number of large molecules from a limited set of small molecules There are four classes of organic molecules produced by organisms: carbohydrates, proteins, lipids, and nucleic acids. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips 1. Train cars linking together to form a train is a nice analogy to linking monomers to form polymers. Consider adding that as the train cars are joined, a puff of steam appears—a reference to water production and a dehydration reaction when linking molecular monomers. 2. The authors note that the great diversity of polymers mainly results from the arrangement of polymers, the different sequences made possible by combinations or permutations of the same monomers. Consider illustrating this by simply asking students how many different ways can we arrange the letters A, B, and C, using each letter, and only once, to form 3-lettered words. The answer is 6 permutations: ABC, ACB, BAC, BCA, CBA, CAB (the factorial of 3). And if letters can be repeated, the answer is 27 (= 33): AAA, BBB, CCC, ABB, ACC, etc. © 2012 Pearson Education, Inc. 22

3.3 Cells make a huge number of large molecules from a limited set of small molecules The four classes of biological molecules contain very large molecules. They are often called macromolecules because of their large size. They are also called polymers because they are made from identical building blocks strung together. The building blocks of polymers are called monomers. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips 1. Train cars linking together to form a train is a nice analogy to linking monomers to form polymers. Consider adding that as the train cars are joined, a puff of steam appears—a reference to water production and a dehydration reaction when linking molecular monomers. 2. The authors note that the great diversity of polymers mainly results from the arrangement of polymers, the different sequences made possible by combinations or permutations of the same monomers. Consider illustrating this by simply asking students how many different ways can we arrange the letters A, B, and C, using each letter, and only once, to form 3-lettered words. The answer is 6 permutations: ABC, ACB, BAC, BCA, CBA, CAB (the factorial of 3). And if letters can be repeated, the answer is 27 (= 33): AAA, BBB, CCC, ABB, ACC, etc. © 2012 Pearson Education, Inc. 23

3.3 Cells make a huge number of large molecules from a limited set of small molecules Monomers are linked together to form polymers through dehydration reactions, which remove water. Polymers are broken apart by hydrolysis, the addition of water. All biological reactions of this sort are mediated by enzymes, which speed up chemical reactions in cells. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips 1. Train cars linking together to form a train is a nice analogy to linking monomers to form polymers. Consider adding that as the train cars are joined, a puff of steam appears—a reference to water production and a dehydration reaction when linking molecular monomers. 2. The authors note that the great diversity of polymers mainly results from the arrangement of polymers, the different sequences made possible by combinations or permutations of the same monomers. Consider illustrating this by simply asking students how many different ways can we arrange the letters A, B, and C, using each letter, and only once, to form 3-lettered words. The answer is 6 permutations: ABC, ACB, BAC, BCA, CBA, CAB (the factorial of 3). And if letters can be repeated, the answer is 27 (= 33): AAA, BBB, CCC, ABB, ACC, etc. © 2012 Pearson Education, Inc. 24

3.3 Cells make a huge number of large molecules from a limited set of small molecules A cell makes a large number of polymers from a small group of monomers. For example, proteins are made from only 20 different amino acids and DNA is built from just four kinds of nucleotides. The monomers used to make polymers are universal. Student Misconceptions and Concerns General biology students might not have previously taken a chemistry course. The concept of molecular building blocks that cannot be seen can be abstract and difficult to comprehend for such students. Concrete examples from our diets and good images will increase comprehension. Teaching Tips 1. Train cars linking together to form a train is a nice analogy to linking monomers to form polymers. Consider adding that as the train cars are joined, a puff of steam appears—a reference to water production and a dehydration reaction when linking molecular monomers. 2. The authors note that the great diversity of polymers mainly results from the arrangement of polymers, the different sequences made possible by combinations or permutations of the same monomers. Consider illustrating this by simply asking students how many different ways can we arrange the letters A, B, and C, using each letter, and only once, to form 3-lettered words. The answer is 6 permutations: ABC, ACB, BAC, BCA, CBA, CAB (the factorial of 3). And if letters can be repeated, the answer is 27 (= 33): AAA, BBB, CCC, ABB, ACC, etc. © 2012 Pearson Education, Inc. 25

Building a polymer chain Figure 3.3A_s1 Building a polymer chain Unlinked monomer Short polymer Figure 3.3A_s1 Dehydration reaction building a polymer chain (step 1) 26

Dehydration reactions build a polymer chain Figure 3.3A_s2 Dehydration reactions build a polymer chain Unlinked monomer Short polymer Dehydration reaction forms a new bond Figure 3.3A_s2 Dehydration reaction building a polymer chain (step 2) Longer polymer 27

Breaking a polymer chain Figure 3.3B_s1 Breaking a polymer chain Figure 3.3B_s1 Hydrolysis breaking down a polymer (step 1) 28

Hydrolysis breaks a polymer chain Figure 3.3B_s2 Hydrolysis breaks a polymer chain Hydrolysis breaks a bond Figure 3.3B_s2 Hydrolysis breaking down a polymer (step 2) 29

Dehydration synthesis and Hydrolysis Short polymer Monomer Longer polymer

CARBOHYDRATES © 2012 Pearson Education, Inc. 31

3.4 Monosaccharides are the simplest carbohydrates Carbohydrates range from small sugar molecules (monomers) to large polysaccharides. Sugar monomers are monosaccharides, such as those found in honey, glucose, and fructose. Monosaccharides can be hooked together to form more complex sugars and polysaccharides. Student Misconceptions and Concerns 1. Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). 2. The abstract nature of chemistry can be discouraging to many students. Consider starting out this section of lecture by examining the chemical groups on a food nutrition label. Candy bars with peanuts are particularly useful, as they contain significant amounts of all three sources of calories (carbohydrates, proteins, and lipids). Teaching Tips If your lectures will eventually include details of glycolysis and aerobic respiration, this is a good point to introduce the basic concepts of glucose as fuel. Just introducing this conceptual formula might help: eating glucose and breathing oxygen produces water and usable energy (used to build ATP) plus heat and carbon dioxide exhaled in our breath. © 2012 Pearson Education, Inc. 32

Bees with honey, a mixture of two monosaccharides Figure 3.4A Bees with honey, a mixture of two monosaccharides Figure 3.4A Bees with honey, a mixture of two monosaccharides 33

3.4 Monosaccharides are the simplest carbohydrates The carbon skeletons of monosaccharides vary in length. Glucose and fructose are six carbons long. Others have three to seven carbon atoms. Monosaccharides are the main fuels for cellular work and used as raw materials to manufacture other organic molecules. Student Misconceptions and Concerns 1. Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). 2. The abstract nature of chemistry can be discouraging to many students. Consider starting out this section of lecture by examining the chemical groups on a food nutrition label. Candy bars with peanuts are particularly useful, as they contain significant amounts of all three sources of calories (carbohydrates, proteins, and lipids). Teaching Tips If your lectures will eventually include details of glycolysis and aerobic respiration, this is a good point to introduce the basic concepts of glucose as fuel. Just introducing this conceptual formula might help: eating glucose and breathing oxygen produces water and usable energy (used to build ATP) plus heat and carbon dioxide exhaled in our breath. © 2012 Pearson Education, Inc. 34

Glucose (an aldose) Fructose (a ketose) Figure 3.4B Figure 3.4B Structures of glucose and fructose Glucose (an aldose) Fructose (a ketose) 35

3.4 Monosaccharides are the simplest carbohydrates Many monosaccharides form rings. The ring diagram may be abbreviated by not showing the carbon atoms at the corners of the ring and drawn with different thicknesses for the bonds, to indicate that the ring is a relatively flat structure with attached atoms extending above and below it. Student Misconceptions and Concerns 1. Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). 2. The abstract nature of chemistry can be discouraging to many students. Consider starting out this section of lecture by examining the chemical groups on a food nutrition label. Candy bars with peanuts are particularly useful, as they contain significant amounts of all three sources of calories (carbohydrates, proteins, and lipids). Teaching Tips If your lectures will eventually include details of glycolysis and aerobic respiration, this is a good point to introduce the basic concepts of glucose as fuel. Just introducing this conceptual formula might help: eating glucose and breathing oxygen produces water and usable energy (used to build ATP) plus heat and carbon dioxide exhaled in our breath. © 2012 Pearson Education, Inc. 36

Abbreviated structure Figure 3.4C 6 5 4 1 3 2 Figure 3.4C Three representations of the ring form of glucose Structural formula Abbreviated structure Simplified structure 37

3.5 Two monosaccharides are linked to form a disaccharide Two monosaccharides (monomers) can bond to form a disaccharide in a dehydration reaction. The disaccharide sucrose is formed by combining a glucose monomer and a fructose monomer. The disaccharide maltose is formed from two glucose monomers. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips Learning the definitions of word roots is invaluable when learning science. Learning the meaning of the prefix word roots “mono” (one), “di” (two), and “poly” (many) helps to distinguish the structures of various carbohydrates. © 2012 Pearson Education, Inc. 38

Glucose Fructose Sucrose

Glucose Glucose Maltose Figure 3.5_s2 Glucose Glucose Figure 3.5_s2 Disaccharide formation by a dehydration reaction (step 2) Maltose 40

Animation: Disaccharides Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips Learning the definitions of word roots is invaluable when learning science. Learning the meaning of the prefix word roots “mono” (one), “di” (two), and “poly” (many) helps to distinguish the structures of various carbohydrates. Animation: Disaccharides Right click on animation / Click play © 2012 Pearson Education, Inc. 41

3.6 CONNECTION: What is high-fructose corn syrup, and is it to blame for obesity? Sodas or fruit drinks probably contain high-fructose corn syrup (HFCS). Fructose is sweeter than glucose. To make HFCS, glucose atoms are rearranged to make the glucose isomer, fructose. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips 1. The widespread use of high-fructose corn syrup can be surprising to students. Consider asking each student to bring to class a product label that indicates the use of high-fructose corn syrup (HFCS) as an ingredient. 2. Consider an assignment for students to access the Internet and find reliable sources that discuss high rates of sugar consumption in the modern diet. The key, of course, is in the quality of the resource. Consider limiting their search to established nonprofit organizations (American Cancer Society, American Heart Association, etc.) and peer-reviewed journals. © 2012 Pearson Education, Inc. 42

3.6 CONNECTION: What is high-fructose corn syrup, and is it to blame for obesity? High-fructose corn syrup (HFCS) is used to sweeten many beverages and may be associated with weight gain. Good health is promoted by a diverse diet of proteins, fats, vitamins, minerals, and complex carbohydrates and exercise. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips 1. The widespread use of high-fructose corn syrup can be surprising to students. Consider asking each student to bring to class a product label that indicates the use of high-fructose corn syrup (HFCS) as an ingredient. 2. Consider an assignment for students to access the Internet and find reliable sources that discuss high rates of sugar consumption in the modern diet. The key, of course, is in the quality of the resource. Consider limiting their search to established nonprofit organizations (American Cancer Society, American Heart Association, etc.) and peer-reviewed journals. © 2012 Pearson Education, Inc. 43

3.7 Polysaccharides are long chains of sugar units macromolecules and polymers composed of thousands of monosaccharides. Polysaccharides may function as storage molecules or structural compounds. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips 1. A simple exercise demonstrates the enzymatic breakdown of starches into sugars. If students place an unsalted cracker in their mouths, holding it in their mouths while it mixes well with saliva, they might soon notice that a sweeter taste begins to emerge. The salivary enzyme amylase begins the digestion of starches into disaccharides, which may be degraded further by other enzymes. These disaccharides are the source of the sweet taste. 2. The text notes that cellulose is the most abundant organic molecule on Earth. Ask your students why this is true. 3. The cellophane wrap often used to package foods is a biodegradable material derived from cellulose. Consider challenging students to create a list of other cellulose-derived products (such as paper.) 4. An adult human may store about a half of a kilogram of glycogen in the liver and muscles of the body, depending up recent dietary habits. A person who begins dieting might soon notice an immediate weight loss of 2–4 pounds (1–2 kilograms) over several days, reflecting reductions in stored glycogen, water, and intestinal contents (among other factors). © 2012 Pearson Education, Inc. 44

3.7 Polysaccharides are long chains of sugar units Starch is a polysaccharide, composed of glucose monomers, and used by plants for energy storage. Glycogen is used by animals for energy storage. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips 1. A simple exercise demonstrates the enzymatic breakdown of starches into sugars. If students place an unsalted cracker in their mouths, holding it in their mouths while it mixes well with saliva, they might soon notice that a sweeter taste begins to emerge. The salivary enzyme amylase begins the digestion of starches into disaccharides, which may be degraded further by other enzymes. These disaccharides are the source of the sweet taste. 2. The text notes that cellulose is the most abundant organic molecule on Earth. Ask your students why this is true. 3. The cellophane wrap often used to package foods is a biodegradable material derived from cellulose. Consider challenging students to create a list of other cellulose-derived products (such as paper.) 4. An adult human may store about a half of a kilogram of glycogen in the liver and muscles of the body, depending up recent dietary habits. A person who begins dieting might soon notice an immediate weight loss of 2–4 pounds (1–2 kilograms) over several days, reflecting reductions in stored glycogen, water, and intestinal contents (among other factors). © 2012 Pearson Education, Inc. 45

3.7 Polysaccharides are long chains of sugar units Cellulose is a polymer of glucose and forms plant cell walls. Chitin is a polysaccharide and used by insects and crustaceans to build an exoskeleton. Student Misconceptions and Concerns Consider reinforcing the three main sources of calories with food items that clearly represent each group. Bring clear examples to class as visual references. For example, a can of Coke or a bag of sugar for carbohydrates, a tub of margarine for lipids, and some beef jerky for protein (although some fat and carbohydrates might also be included). Teaching Tips 1. A simple exercise demonstrates the enzymatic breakdown of starches into sugars. If students place an unsalted cracker in their mouths, holding it in their mouths while it mixes well with saliva, they might soon notice that a sweeter taste begins to emerge. The salivary enzyme amylase begins the digestion of starches into disaccharides, which may be degraded further by other enzymes. These disaccharides are the source of the sweet taste. 2. The text notes that cellulose is the most abundant organic molecule on Earth. Ask your students why this is true. 3. The cellophane wrap often used to package foods is a biodegradable material derived from cellulose. Consider challenging students to create a list of other cellulose-derived products (such as paper.) 4. An adult human may store about a half of a kilogram of glycogen in the liver and muscles of the body, depending up recent dietary habits. A person who begins dieting might soon notice an immediate weight loss of 2–4 pounds (1–2 kilograms) over several days, reflecting reductions in stored glycogen, water, and intestinal contents (among other factors). © 2012 Pearson Education, Inc. 46

Starch granules in potato tuber cells Starch Figure 3.7 Starch granules in potato tuber cells Starch Glucose monomer Glycogen granules in muscle tissue Glycogen Cellulose microfibrils in a plant cell wall Cellulose Figure 3.7 Polysaccharides Hydrogen bonds Cellulose molecules 47

Starch granules in potato tuber cells Starch Figure 3.7_1 Starch granules in potato tuber cells Starch Glucose monomer Figure 3.7_1 Polysaccharides (part 1) 48

Glycogen granules in muscle tissue Figure 3.7_2 Glycogen granules in muscle tissue Glycogen Figure 3.7_2 Polysaccharides (part 2) 49

Cellulose microfibrils in a plant cell wall Cellulose Figure 3.7_3 Cellulose microfibrils in a plant cell wall Cellulose Hydrogen bonds Cellulose molecules Figure 3.7_3 Polysaccharides (part 3) 50

Figure 3.7_4 Figure 3.7_4 Polysaccharides (part 4) 51

3.7 Polysaccharides are long chains of sugar units Polysaccharides are usually hydrophilic (water-loving). Bath towels are often made of cotton, which is mostly cellulose, and water absorbent. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 52

LIPIDS © 2012 Pearson Education, Inc. 53

3.8 Fats are lipids that are mostly energy-storage molecules are water insoluble (hydrophobic, or water-fearing) compounds, are important in long-term energy storage, contain twice as much energy as a polysaccharide, and consist mainly of carbon and hydrogen atoms linked by nonpolar covalent bonds. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 54

Water beading on the oily coating of feathers Figure 3.8A Water beading on the oily coating of feathers Figure 3.8A Water beading on the oily coating of feathers 55

3.8 Fats are lipids that are mostly energy-storage molecules Lipids differ from carbohydrates, proteins, and nucleic acids in that they are not huge molecules and not built from monomers. Lipids vary a great deal in structure and function. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 56

3.8 Fats are lipids that are mostly energy-storage molecules We will consider three types of lipids: fats, phospholipids, and steroids. A fat is a large lipid made from two kinds of smaller molecules, glycerol and fatty acids. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 57

3.8 Fats are lipids that are mostly energy-storage molecules A fatty acid can link to glycerol by a dehydration reaction. A fat contains one glycerol linked to three fatty acids. Fats are often called triglycerides because of their structure. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 58

Animation: Fats Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. Animation: Fats Right click on animation / Click play © 2012 Pearson Education, Inc. 59

A dehydration reaction linking a fatty acid to glycerol Figure 3.8B Fatty acid Glycerol A dehydration reaction linking a fatty acid to glycerol Figure 3.8B A dehydration reaction linking a fatty acid molecule to a glycerol molecule 60

(A fat molecule made from glycerol and three fatty acids) Figure 3.8C Fatty acids Glycerol A triglyceride (A fat molecule made from glycerol and three fatty acids) Figure 3.8C A fat molecule (triglyceride) consisting of three fatty acids linked to glycerol 61

3.8 Fats are lipids that are mostly energy-storage molecules Some fatty acids contain one or more double bonds, forming unsaturated fatty acids that have one fewer hydrogen atom on each carbon of the double bond, cause kinks or bends in the carbon chain, and prevent them from packing together tightly and solidifying at room temperature. Fats with the maximum number of hydrogens are called saturated fatty acids. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 62

3.8 Fats are lipids that are mostly energy-storage molecules Unsaturated fats include corn and olive oils. Most animal fats are saturated fats. Hydrogenated vegetable oils are unsaturated fats that have been converted to saturated fats by adding hydrogen. This hydrogenation creates trans fats associated with health risks. Homework: Compare butter, margarine, corn oil & lard. Student Misconceptions and Concerns 1. Students may struggle with the concept that a pound of fat contains more than twice the calories of a pound of sugar. It might seem that a pound of food would potentially add on a pound of weight. Other students may have never understood the concept of calories in the diet, simply following general guidelines of avoiding fatty foods. Furthermore, fiber and water have no caloric value but add to the weight of food. Consider class discussions that explore student misconceptions about calories, body weight, and healthy diets. 2. Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips 1. The text in Module 3.8 notes the common observation that vinegar and oil do not mix in this type of salad dressing. A simple demonstration can help make this point. In front of the class, mix together colored water and a yellow oil (corn or canola oil work well). Shake up the mixture and then watch as the two separate. (You may have a mixture already made ahead of time that remains separated; however, the dye may bleed between the oil and the water.) Placing the mixture on an overhead projector or other well-illuminated imaging device makes for a dramatic display of hydrophobic activity! 2. The text notes that a gram of fat stores more than twice the energy of a gram of polysaccharide, such as starch. You might elaborate with a simple calculation to demonstrate how a person’s body weight would vary if the energy stored in body fat were stored in carbohydrates instead. If a 100-kg man carried 25% body fat, he would have 25 kg of fat in his body. Fat stores about 2.25 times more energy per gram than carbohydrate. What would be the weight of the man if he stored the energy in the fat in the form of carbohydrate? (2.25 x 25 = 56.25 kg of carbohydrate + 75kg (nonfat body weight) = 131.25 kg, an increase of 31.25%) 3. Margarine in stores commonly comes in liquid squeeze containers, in tubs, and in sticks. These forms reflect increasing amounts of hydrogenation, gradually increasing the stiffness from a liquid, to a firmer spread, to a firm stick of margarine. As noted in the text, recent studies have suggested that unsaturated oils become increasingly unhealthy as they are hydrogenated. Students might therefore remember that as margarine products increase in stiffness, they generally become less healthy. Public attention to hydrogenation and the health risks of the resulting trans fats are causing changes in the use of products containing trans fats. © 2012 Pearson Education, Inc. 63

3.9 Phospholipids and steroids are important lipids with a variety of functions Phospholipids are structurally similar to fats and the major component of all cells. Phospholipids are structurally similar to fats. Fats contain three fatty acids attached to glycerol. Phospholipids contain two fatty acids attached to glycerol. Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips Before explaining the properties of a polar molecule such as a phospholipid, have students predict the consequences of adding phospholipids to water. See if the class can generate the two most common configurations: (1) a lipid bilayer encircling water (water surrounding the bilayer and water contained internally) and (2) a micelle (polar heads in contact with water and hydrophobic tails clustered centrally). © 2012 Pearson Education, Inc. 64

Q: How is a phospholipid different from a triglyceride? Figure 3.9A-B Phosphate group Glycerol Water Hydrophilic heads Hydrophobic tails Symbol for phospholipid Water Figure 3.9A-B Detail of a phospholipid membrane Q: How is a phospholipid different from a triglyceride? 65

3.9 Phospholipids and steroids are important lipids with a variety of functions Phospholipids cluster into a bilayer of phospholipids. The hydrophilic heads are in contact with the water of the environment and the internal part of the cell. The hydrophobic tails band in the center of the bilayer. Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips Before explaining the properties of a polar molecule such as a phospholipid, have students predict the consequences of adding phospholipids to water. See if the class can generate the two most common configurations: (1) a lipid bilayer encircling water (water surrounding the bilayer and water contained internally) and (2) a micelle (polar heads in contact with water and hydrophobic tails clustered centrally). © 2012 Pearson Education, Inc. 66

Section of a phospholipid bilayer in cell membrane Figure 3.9B Section of a phospholipid bilayer in cell membrane Water Hydrophilic head Hydrophobic tail Symbol for phospholipid Figure 3.9B Section of a phospholipid membrane Water 67

3.9 Phospholipids and steroids are important lipids with a variety of functions Steroids are lipids in which the carbon skeleton contains four fused rings. Cholesterol is a common component in animal cell membranes and starting material for making steroids, including sex hormones. Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips Before explaining the properties of a polar molecule such as a phospholipid, have students predict the consequences of adding phospholipids to water. See if the class can generate the two most common configurations: (1) a lipid bilayer encircling water (water surrounding the bilayer and water contained internally) and (2) a micelle (polar heads in contact with water and hydrophobic tails clustered centrally). © 2012 Pearson Education, Inc. 68

Cholesterol, a steroid molecule Figure 3.9C Cholesterol, a steroid molecule Figure 3.9C Cholesterol, a steroid 69

Compare cholesterol & sex hormones Figure 3.9B & Fig. 3.2 Cholesterol compared to sex hormones, estradiol and testosterone

3.10 CONNECTION: Anabolic steroids pose health risks (self-study) are synthetic variants of testosterone, can cause a buildup of muscle and bone mass, and are often prescribed to treat general anemia and some diseases that destroy body muscle. Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips The consequences of steroid abuse will likely be of great interest to your students. However, the reasons for the damaging consequences might not be immediately clear. As time permits, consider noting the diverse homeostatic mechanisms that normally regulate the traits affected by steroid abuse. © 2012 Pearson Education, Inc. 71

3.10 CONNECTION: Anabolic steroids pose health risks Anabolic steroids are abused by some athletes with serious consequences, including violent mood swings, depression, liver damage, cancer, high cholesterol, and high blood pressure. Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips The consequences of steroid abuse will likely be of great interest to your students. However, the reasons for the damaging consequences might not be immediately clear. As time permits, consider noting the diverse homeostatic mechanisms that normally regulate the traits affected by steroid abuse. © 2012 Pearson Education, Inc. 72

Pay special attention to the chemical structures in figure in page 70. Figure 3.10 Bodybuilder Read article in Scientific American Current issues in Biology V2 titled “The next generation cheating ” p 64-72. Pay special attention to the chemical structures in figure in page 70. These issues came with the Campbell textbook. 73

3.10 CONNECTION: Anabolic steroids pose health risks When anabolic steroids are used, the body produces less of natural male sex hormones that can cause shrunken testicles, reduced sex drive, infertility and breast enlargement Student Misconceptions and Concerns Students might struggle to extrapolate the properties of lipids to their roles in an organism. Ducks float because their feathers repel water instead of attracting it. Hair on our heads remains flexible because of oils produced in our scalp. Examples such as these help connect the abstract properties of lipids to concrete examples in our world. Teaching Tips The consequences of steroid abuse will likely be of great interest to your students. However, the reasons for the damaging consequences might not be immediately clear. As time permits, consider noting the diverse homeostatic mechanisms that normally regulate the traits affected by steroid abuse. © 2012 Pearson Education, Inc. 74

PROTEINS © 2012 Pearson Education, Inc. 75

3.11 Proteins are made from amino acids linked by peptide bonds involved in nearly every dynamic function in your body and very diverse, with tens of thousands of different proteins, each with a specific structure and function, in the human body. Proteins are composed of differing arrangements of a common set of just 20 amino acid monomers. Teaching Tips 1. Many analogies help students appreciate the diversity of proteins that can be made from just 20 amino acids. The authors note that our language uses combinations of 26 letters to form words. Proteins are much longer “words,” creating even more diversity. Another analogy is to trains. This builds upon the earlier analogy when polymers were introduced. Imagine making different trains about 100 cars long, using any combination of 20 types of railroad cars. Mathematically, the number of possible trains is 20100, a number beyond imagination. 2. The authors note that the difference between a polypeptide and a protein is analogous to the relationship between a long strand of yarn and a sweater knitted from yarn. Proteins are clearly more complex! © 2012 Pearson Education, Inc. 76

3.11 Proteins are made from amino acids linked by peptide bonds Amino acids have an amino group and a carboxyl group (which makes it an acid). Also bonded to the central carbon is a hydrogen atom and a chemical group symbolized by R, which determines the specific properties of each of the 20 amino acids used to make proteins. Teaching Tips 1. Many analogies help students appreciate the diversity of proteins that can be made from just 20 amino acids. The authors note that our language uses combinations of 26 letters to form words. Proteins are much longer “words,” creating even more diversity. Another analogy is to trains. This builds upon the earlier analogy when polymers were introduced. Imagine making different trains about 100 cars long, using any combination of 20 types of railroad cars. Mathematically, the number of possible trains is 20100, a number beyond imagination. 2. The authors note that the difference between a polypeptide and a protein is analogous to the relationship between a long strand of yarn and a sweater knitted from yarn. Proteins are clearly more complex! © 2012 Pearson Education, Inc. 77

General structure of an amino acid Figure 3.11A General structure of an amino acid Amino group Carboxyl group Figure 3.11A General structure of an amino acid 78

3.11 Proteins are made from amino acids linked by peptide bonds Amino acids are classified as either hydrophobic or hydrophilic. Teaching Tips 1. Many analogies help students appreciate the diversity of proteins that can be made from just 20 amino acids. The authors note that our language uses combinations of 26 letters to form words. Proteins are much longer “words,” creating even more diversity. Another analogy is to trains. This builds upon the earlier analogy when polymers were introduced. Imagine making different trains about 100 cars long, using any combination of 20 types of railroad cars. Mathematically, the number of possible trains is 20100, a number beyond imagination. 2. The authors note that the difference between a polypeptide and a protein is analogous to the relationship between a long strand of yarn and a sweater knitted from yarn. Proteins are clearly more complex! © 2012 Pearson Education, Inc. 79

Hydrophobic Hydrophilic Leucine (Leu) Serine (Ser) Aspartic acid (Asp) Figure 3.11B Hydrophobic Hydrophilic Figure 3.11B Examples of amino acids with hydrophobic and hydrophilic R groups Leucine (Leu) Serine (Ser) Aspartic acid (Asp) 80

3.11 Proteins are made from amino acids linked by peptide bonds Amino acid monomers are linked together in a dehydration reaction, joining carboxyl group of one amino acid to the amino group of the next amino acid, and creating a peptide bond. Additional amino acids can be added by the same process to create a chain of amino acids called a polypeptide. Teaching Tips 1. Many analogies help students appreciate the diversity of proteins that can be made from just 20 amino acids. The authors note that our language uses combinations of 26 letters to form words. Proteins are much longer “words,” creating even more diversity. Another analogy is to trains. This builds upon the earlier analogy when polymers were introduced. Imagine making different trains about 100 cars long, using any combination of 20 types of railroad cars. Mathematically, the number of possible trains is 20100, a number beyond imagination. 2. The authors note that the difference between a polypeptide and a protein is analogous to the relationship between a long strand of yarn and a sweater knitted from yarn. Proteins are clearly more complex! © 2012 Pearson Education, Inc. 81

Peptide bond formation (step 1) Figure 3.11C_s1 Peptide bond formation (step 1) Carboxyl group Amino group Amino acid Amino acid Figure 3.11C_s1 Peptide bond formation (step 1) 82

Peptide bond formation (step 2) Figure 3.11C_s2 Peptide bond formation (step 2) Peptide bond Carboxyl group Amino group Dehydration reaction Amino acid Amino acid Dipeptide Figure 3.11C_s2 Peptide bond formation (step 2) 83

3.12 A protein’s specific shape determines its function Probably the most important role for proteins is as enzymes, proteins that serve as metabolic catalysts and regulate the chemical reactions within cells. Student Misconceptions and Concerns The functional significance of protein shape is an abstract molecular example of form and function relationships, which might be new to some students. The binding of an enzyme to its substrate is a type of molecular handshake, which permits specific interactions. To help students think about form and function relationships, share some concrete analogies in their lives—perhaps flathead and Phillips screwdrivers that match the proper type of screws or the fit of a hand into a glove. Teaching Tips Most cooking results in changes in the texture and color of food. The brown color of a cooked steak is the product of the denaturation of proteins. Fixatives such as formalin also denature proteins and cause color changes. Students who have dissected vertebrates will realize that the brown color of the muscles makes it look as if the animal has been cooked. © 2012 Pearson Education, Inc. 84

3.12 A protein’s specific shape determines its function Other proteins are also important. Structural proteins provide associations between body parts. Eg: hair, tendons, ligaments Contractile proteins are found within muscle. Defensive proteins include antibodies of the immune system. Signal proteins are best exemplified by hormones and other chemical messengers. Receptor proteins transmit signals into cells. Transport proteins carry oxygen. Storage proteins serve as a source of amino acids for developing embryos. Student Misconceptions and Concerns The functional significance of protein shape is an abstract molecular example of form and function relationships, which might be new to some students. The binding of an enzyme to its substrate is a type of molecular handshake, which permits specific interactions. To help students think about form and function relationships, share some concrete analogies in their lives—perhaps flathead and Phillips screwdrivers that match the proper type of screws or the fit of a hand into a glove. Teaching Tips Most cooking results in changes in the texture and color of food. The brown color of a cooked steak is the product of the denaturation of proteins. Fixatives such as formalin also denature proteins and cause color changes. Students who have dissected vertebrates will realize that the brown color of the muscles makes it look as if the animal has been cooked. © 2012 Pearson Education, Inc. 85

Spider silk is a protein Figure 3.13_1 Spider silk is a protein Figure 3.13_1 Spider web 86

Collagen, a protein in the connective tissue of skin, bone, tendons & Figure 3.13_2 Polypeptide chain Figure 3.13_2 Collagen Collagen, a protein in the connective tissue of skin, bone, tendons & ligaments. It accounts for 40% of proteins in the body. 87

Figure 3.12A Figure 3.12A Structural proteins make up hair, tendons, and ligaments; contractile proteins are found in muscles. 88

3.12 A protein’s specific shape determines its function A polypeptide chain contains hundreds or thousands of amino acids linked by peptide bonds. The amino acid sequence causes the polypeptide to assume a particular shape. The shape of a protein determines its specific function. Student Misconceptions and Concerns The functional significance of protein shape is an abstract molecular example of form and function relationships, which might be new to some students. The binding of an enzyme to its substrate is a type of molecular handshake, which permits specific interactions. To help students think about form and function relationships, share some concrete analogies in their lives—perhaps flathead and Phillips screwdrivers that match the proper type of screws or the fit of a hand into a glove. Teaching Tips Most cooking results in changes in the texture and color of food. The brown color of a cooked steak is the product of the denaturation of proteins. Fixatives such as formalin also denature proteins and cause color changes. Students who have dissected vertebrates will realize that the brown color of the muscles makes it look as if the animal has been cooked. © 2012 Pearson Education, Inc. 89

Groove Figure 3.12B Figure 3.12B Ribbon model of the protein lysozyme 90

3.12 A protein’s specific shape determines its function If a protein’s shape is altered, it can no longer function. In the process of denaturation, a polypeptide chain unravels, loses its shape, and loses its function. Proteins can be denatured by changes in salt concentration, pH, or by high heat. Student Misconceptions and Concerns The functional significance of protein shape is an abstract molecular example of form and function relationships, which might be new to some students. The binding of an enzyme to its substrate is a type of molecular handshake, which permits specific interactions. To help students think about form and function relationships, share some concrete analogies in their lives—perhaps flathead and Phillips screwdrivers that match the proper type of screws or the fit of a hand into a glove. Teaching Tips Most cooking results in changes in the texture and color of food. The brown color of a cooked steak is the product of the denaturation of proteins. Fixatives such as formalin also denature proteins and cause color changes. Students who have dissected vertebrates will realize that the brown color of the muscles makes it look as if the animal has been cooked. © 2012 Pearson Education, Inc. 91

3.13 A protein’s shape depends on four levels of structure A protein can have four levels of structure: primary structure secondary structure tertiary structure quaternary structure Teaching Tips An examination of the fabrics and weave of a sweater might help students understand the levels of protein structure. Although not a perfect analogy, levels of organization can be better appreciated. Teasing apart a single thread reveals a simpler organization of smaller fibers woven together. In turn, threads are interlaced into a connected fabric, which may be further twisted and organized into a pattern or structural component of a sleeve. Challenge students to identify the limits of this analogy and identify aspects of protein structure not included (such as the primary structure of a protein, its sequence of amino acids). © 2012 Pearson Education, Inc. 92

3.13 A protein’s shape depends on four levels of structure The primary structure of a protein is its unique amino acid sequence. The correct amino acid sequence is determined by the cell’s genetic information. The slightest change in this sequence may affect the protein’s ability to function. Teaching Tips An examination of the fabrics and weave of a sweater might help students understand the levels of protein structure. Although not a perfect analogy, levels of organization can be better appreciated. Teasing apart a single thread reveals a simpler organization of smaller fibers woven together. In turn, threads are interlaced into a connected fabric, which may be further twisted and organized into a pattern or structural component of a sleeve. Challenge students to identify the limits of this analogy and identify aspects of protein structure not included (such as the primary structure of a protein, its sequence of amino acids). © 2012 Pearson Education, Inc. 93

3.13 A protein’s shape depends on four levels of structure Protein secondary structure results from coiling or folding of the polypeptide. Coiling results in a helical structure called an alpha helix. A certain kind of folding leads to a structure called a pleated sheet, which dominates some fibrous proteins such as those used in spider webs. Coiling and folding are maintained by regularly spaced hydrogen bonds between hydrogen atoms and oxygen atoms along the backbone of the polypeptide chain. Teaching Tips An examination of the fabrics and weave of a sweater might help students understand the levels of protein structure. Although not a perfect analogy, levels of organization can be better appreciated. Teasing apart a single thread reveals a simpler organization of smaller fibers woven together. In turn, threads are interlaced into a connected fabric, which may be further twisted and organized into a pattern or structural component of a sleeve. Challenge students to identify the limits of this analogy and identify aspects of protein structure not included (such as the primary structure of a protein, its sequence of amino acids). © 2012 Pearson Education, Inc. 94

3.13 A protein’s shape depends on four levels of structure The overall three-dimensional shape of a polypeptide is called its tertiary structure. Tertiary structure generally results from interactions between the R groups of the various amino acids. Disulfide bridges may further strengthen the protein’s shape. Teaching Tips An examination of the fabrics and weave of a sweater might help students understand the levels of protein structure. Although not a perfect analogy, levels of organization can be better appreciated. Teasing apart a single thread reveals a simpler organization of smaller fibers woven together. In turn, threads are interlaced into a connected fabric, which may be further twisted and organized into a pattern or structural component of a sleeve. Challenge students to identify the limits of this analogy and identify aspects of protein structure not included (such as the primary structure of a protein, its sequence of amino acids). © 2012 Pearson Education, Inc. 95

3.13 A protein’s shape depends on four levels of structure Two or more polypeptide chains (subunits) associate providing quaternary structure. Collagen is an example of a protein with quaternary structure. Collagen’s triple helix gives great strength to connective tissue, bone, tendons, and ligaments. Teaching Tips An examination of the fabrics and weave of a sweater might help students understand the levels of protein structure. Although not a perfect analogy, levels of organization can be better appreciated. Teasing apart a single thread reveals a simpler organization of smaller fibers woven together. In turn, threads are interlaced into a connected fabric, which may be further twisted and organized into a pattern or structural component of a sleeve. Challenge students to identify the limits of this analogy and identify aspects of protein structure not included (such as the primary structure of a protein, its sequence of amino acids). © 2012 Pearson Education, Inc. 96

Four Levels of Protein Structure Figure 3.13A_s1 Four Levels of Protein Structure Primary structure Amino acids Amino acids Figure 3.13A_s1 Four Levels of Protein Structure (step 1) 97

Beta pleated sheet Alpha helix Figure 3.13A-B_s2 Four Levels of Protein Structure Primary structure Amino acids Amino acids Secondary structure Hydrogen bond Beta pleated sheet Alpha helix Figure 3.13A-B_s2 Four Levels of Protein Structure (step 2) 98

Beta pleated sheet Alpha helix Figure 3.13A-C_s3 Four Levels of Protein Structure Primary structure Amino acids Amino acids Secondary structure Hydrogen bond Beta pleated sheet Alpha helix Tertiary structure Transthyretin polypeptide Figure 3.13A-C_s3 Four Levels of Protein Structure (step 3) 99

Beta pleated sheet Alpha helix Figure 3.13A-D_s4 Four Levels of Protein Structure Primary structure Amino acids Amino acids Secondary structure Hydrogen bond Beta pleated sheet Alpha helix Tertiary structure Transthyretin polypeptide Figure 3.13A-D_s4 Four Levels of Protein Structure (step 4) Quaternary structure Transthyretin, with four identical polypeptides 100

NUCLEIC ACIDS © 2012 Pearson Education, Inc. 101

3.14 DNA and RNA are the two types of nucleic acids The amino acid sequence of a polypeptide is programmed by a discrete unit of inheritance known as a gene. Genes consist of DNA(deoxyribonucleic acid), a type of nucleic acid. DNA is inherited from an organism’s parents. DNA provides directions for its own replication. DNA programs a cell’s activities by directing the synthesis of proteins. Student Misconceptions and Concerns Module 3.14 is the first time the authors present the concept of transcription and translation, discussed extensively in later chapters. The basic conceptual flow of information from DNA to RNA to proteins is essential to these later discussions. Teaching Tips The “NA” in the acronyms DNA and RNA stands for “Nucleic acid.” Students often do not make this association without assistance. © 2012 Pearson Education, Inc. 102

3.14 DNA and RNA are the two types of nucleic acids DNA does not build proteins directly. DNA works through an intermediary, ribonucleic acid (RNA). DNA is transcribed into RNA. RNA is translated into proteins. Student Misconceptions and Concerns Module 3.14 is the first time the authors present the concept of transcription and translation, discussed extensively in later chapters. The basic conceptual flow of information from DNA to RNA to proteins is essential to these later discussions. Teaching Tips The “NA” in the acronyms DNA and RNA stands for “Nucleic acid.” Students often do not make this association without assistance. © 2012 Pearson Education, Inc. 103

Figure 3.14_s1 Gene DNA Figure 3.14_s1 The flow of genetic information in the building of a protein (step 1) 104

Gene DNA Transcription Nucleic acids RNA Figure 3.14_s2 Figure 3.14_s2 The flow of genetic information in the building of a protein (step 2) 105

Gene DNA Transcription Nucleic acids RNA Translation Protein Figure 3.14_s3 Gene DNA Transcription Nucleic acids RNA Figure 3.14_s3 The flow of genetic information in the building of a protein (step 3) Translation Protein Amino acid 106

3.15 Nucleic acids are polymers of nucleotides DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are composed of monomers called nucleotides. Nucleotides have three parts: a five-carbon sugar called ribose in RNA and deoxyribose in DNA, a phosphate group, and a nitrogenous base. Teaching Tips When discussing the sequence of nucleotides in DNA and RNA, consider challenging your students with the following questions based upon prior analogies. If the 20 possible amino acids in a polypeptide represent “words” in a long polypeptide sentence, how many possible words are in the language of a DNA molecule? (Answer: Four nucleotides, GCAT, are possible). Are these the same “words” used in RNA? (Answer: No. Uracil substitutes for thymine.) © 2012 Pearson Education, Inc. 107

Nitrogenous base (adenine) Figure 3.15A Nitrogenous base (adenine) Figure 3.15A A nucleotide, consisting of a phosphate group, a sugar, and a nitrogenous base Phosphate group Sugar 108

3.15 Nucleic acids are polymers of nucleotides DNA nitrogenous bases are adenine (A), thymine (T), cytosine (C), and guanine (G). RNA also has A, C, and G, but instead of T, it has uracil (U). Teaching Tips When discussing the sequence of nucleotides in DNA and RNA, consider challenging your students with the following questions based upon prior analogies. If the 20 possible amino acids in a polypeptide represent “words” in a long polypeptide sentence, how many possible words are in the language of a DNA molecule? (Answer: Four nucleotides, GCAT, are possible). Are these the same “words” used in RNA? (Answer: No. Uracil substitutes for thymine.) © 2012 Pearson Education, Inc. 109

3.15 Nucleic acids are polymers of nucleotides A nucleic acid polymer, a polynucleotide, forms from the nucleotide monomers, when the phosphate of one nucleotide bonds to the sugar of the next nucleotide, by dehydration reactions, and by producing a repeating sugar-phosphate backbone with protruding nitrogenous bases. Teaching Tips When discussing the sequence of nucleotides in DNA and RNA, consider challenging your students with the following questions based upon prior analogies. If the 20 possible amino acids in a polypeptide represent “words” in a long polypeptide sentence, how many possible words are in the language of a DNA molecule? (Answer: Four nucleotides, GCAT, are possible). Are these the same “words” used in RNA? (Answer: No. Uracil substitutes for thymine.) © 2012 Pearson Education, Inc. 110

Sugar-phosphate backbone Figure 3.15B A Nucleotide T C G Figure 3.15B Part of a polynucleotide T Sugar-phosphate backbone 111

3.15 Nucleic acids are polymers of nucleotides Two polynucleotide strands wrap around each other to form a DNA double helix. The two strands are associated because particular bases always hydrogen bond to one another. A pairs with T, and C pairs with G, producing base pairs. RNA is usually a single polynucleotide strand. Teaching Tips When discussing the sequence of nucleotides in DNA and RNA, consider challenging your students with the following questions based upon prior analogies. If the 20 possible amino acids in a polypeptide represent “words” in a long polypeptide sentence, how many possible words are in the language of a DNA molecule? (Answer: Four nucleotides, GCAT, are possible). Are these the same “words” used in RNA? (Answer: No. Uracil substitutes for thymine.) © 2012 Pearson Education, Inc. 112

Base pair C A T C G C G T A C G A T A T G C A T A T T A Figure 3.15C Figure 3.15C DNA double helix G C A T A T T A 113

3.16 EVOLUTION CONNECTION: Lactose tolerance is a recent event in human evolution The majority of people stop producing the enzyme lactase in early childhood and do not easily digest the milk sugar lactose. Lactose tolerance represents a relatively recent mutation in the human genome and survival advantage for human cultures with milk and dairy products available year-round. Student Misconceptions and Concerns The evolution of lactose tolerance within human groups in East Africa does not represent a deliberate decision, yet this evolutionary change appears logical. Many students perceive adaptations as deliberate events with purpose. As students develop a better understanding of the mechanisms of evolution, it will be important to point out that mutations arise by chance, with the culling hand of natural selection favoring traits that convey an advantage. Organisms cannot plan evolutionary change. Teaching Tips The research revealing the separate evolution of lactose tolerance into human adulthood in several parts of the world provide another opportunity to help students understand the process of natural selection. Consider using this example to walk students through the steps of this evolutionary change. Help students to understand that people did not choose to be lactose tolerant as adults. Instead, the environment of nutritious dairy products created an adaptive advantage for those people fortunate enough to possess the lactose tolerant mutation. © 2012 Pearson Education, Inc. 114

3.16 EVOLUTION CONNECTION: Lactose tolerance is a recent event in human evolution Researchers identified three mutations that keep the lactase gene permanently turned on. The mutations appear to have occurred about 7,000 years ago and at the same time as the domestication of cattle in these regions. Student Misconceptions and Concerns The evolution of lactose tolerance within human groups in East Africa does not represent a deliberate decision, yet this evolutionary change appears logical. Many students perceive adaptations as deliberate events with purpose. As students develop a better understanding of the mechanisms of evolution, it will be important to point out that mutations arise by chance, with the culling hand of natural selection favoring traits that convey an advantage. Organisms cannot plan evolutionary change. Teaching Tips The research revealing the separate evolution of lactose tolerance into human adulthood in several parts of the world provide another opportunity to help students understand the process of natural selection. Consider using this example to walk students through the steps of this evolutionary change. Help students to understand that people did not choose to be lactose tolerant as adults. Instead, the environment of nutritious dairy products created an adaptive advantage for those people fortunate enough to possess the lactose tolerant mutation. © 2012 Pearson Education, Inc. 115