Raven/Johnson Biology 8e Chapter 03– Answers 1. How is a polymer formed from multiple monomers? a. From the growth of the chain of carbon atoms b. By the removal of an –OH group and a hydrogen atom c. By the addition of an –OH group and a hydrogen atom d. Through hydrogen bonding The correct answer is b— A. Answer a is incorrect. The addition of carbon atoms to a molecule is a consequence of the formation of a polymer, but it does not describe how a polymer is formed. The correct answer is b—By the removal of an –OH group and a hydrogen atom B. Answer b is correct. The growth of complex biological molecules occurs through condensation or dehydration reactions that form covalent bonds between monomers by the removal of a molecule of water (H2O, or OH + H). The correct answer is b— C. Answer c is incorrect. The addition of –OH and H, in other words, H2O to a covalent bond is a process called hydrolysis. Hydrolysis reactions break covalent bonds and therefore break up polymers. The correct answer is b— D. Answer d is incorrect. Hydrogen bonds are important for stabilizing molecules, but covalent bonds are what are required to build polymers from monomers. 2. Why are carbohydrates an important molecule for energy storage? a. The C–H bonds found in carbohydrates store energy. b. The double bond between carbon and oxygen are very strong. c. The electronegativity of the oxygen atoms means that a carbohydrate is made up of many polar bonds. d. They can form ring structures in the aqueous environment of a cell. The correct answer is a—The C–H bonds found in carbohydrates store energy. A. The C–H bonds are nonpolar (review chapter 2) and therefore have high potential energy The correct answer is a— B. Answer b is incorrect. Double bonds are strong covalent bonds, however C=O bonds only exist in the linear forms of carbohydrate molecules. The ring form of carbohydrates is more common in biological systems. The correct answer is a— C. Answer c is incorrect. Oxygen is an electronegative atom and the bonds between oxygen and hydrogen are polar (allowing carbohydrates to interact with water). However, polar bonds are not good at storing energy. The correct answer is a— D. Answer d is incorrect. The ability to form ring structures does not influence the ability of carbohydrates to store energy. The energy is stored in the C–H bond. Raven/Johnson Biology 8e Chapter 03– Answers 3. Plant cells store energy in the form of _________, and animal cells store energy in the form of ___________ a. fructose; glucose b. disaccharides; monosaccharides c. cellulose; chitin d. starch; glycogen The correct answer is d— A. Answer a is incorrect. Fructose and glucose are structural isomers of each other. These monomers can be used as energy by the cell, but they do not store energy inside cells. The correct answer is d— B. Answer b is incorrect. Monosaccharide is the name given to the monomer unit of a carbohydrate. A disaccharide is formed when two monosaccharides become linked through a covalent bond. Disaccharides are used for transporting carbohydrates, not storage. The correct answer is d— C. Answer c is not correct. Cellulose forms the cell walls of plant cells, and chitin forms the cell walls of animal cells. Cell walls provide structural support to cells, but do not store energy. The correct answer is d—starch; glycogen D. Answer d is correct. Starches are large, branched polymers of glucose that store energy for plants. Likewise, glycogen is a glucose polymer that is stored for energy use by animal cells. 4. Which carbohydrate would you find as part of a molecule of RNA? a. Galactose b. Deoxyribose c. Ribose d. Glucose The correct answer is c— A. Answer a is incorrect. Galactose is a six-carbon sugar that is a stereoisomer of glucose. The correct answer is c— B. Answer b is incorrect. Deoxyribose is found in DNA, not RNA. The correct answer is c—Ribose C. Answer c is correct. Ribose is the five-carbon sugar that is part of the nucleotides that make up RNA. The correct answer is c— D. Answer d is incorrect. Glucose is a general name for any five-carbon sugar. This term is too general to correctly answer this question. 5. What makes cellulose different from starch? a. Starch is produced by plant cells, and cellulose is produced by animal cells. Raven/Johnson Biology 8e Chapter 03– Answers b. Cellulose forms long filaments, and starch is highly branched. c. Starch is insoluble, and cellulose is soluble. d. All of the above. The correct answer is b— A. Answer a is incorrect. Both starch and cellulose are the products of plant cells. The correct answer is b— Cellulose forms long filaments, and starch is highly branched. B. Answer b is correct. Cellulose filaments are long, unbranched polymers of glucose. These straight fibers assemble to give strength to the cell walls of plants. Starch is a highly branched glucose polymer that functions to store energy for plant cells. The correct answer is b— C. Answer c is incorrect. If cellulose was soluble, then plant cell walls would not exist. The correct answer is b— D. Answer d is incorrect. Answers a and c are also incorrect. 6. A molecule of DNA or RNA is a polymer of— a. monosaccharides b. nucleotides c. amino acids d. fatty acids The correct answer is b— A. Answer a is incorrect. Monosaccharides are the monomer form of a carbohydrate. The correct answer is b—nucleotides B. Answer b is correct. Nucleotides are the monomers of nucleic acids. They are composed of a ribose sugar, nitrogenous base, and a phosphate group. The correct answer is b— C. Answer c is incorrect. Amino acids are the monomers that make up proteins The correct answer is b— D. Answer d is incorrect. Fatty acids are lipid molecules. 7. What chemical bond is responsible for linking amino acids together to form a protein? a. Phosphodiester b. ß-1,4 linkage c. Peptide d. Hydrogen The correct answer is c— A. Answer a is incorrect. A phosphodiester bond is found linking the nucleotides that form nucleic acids. The correct answer is c— Raven/Johnson Biology 8e Chapter 03– Answers B. Answer b is incorrect. A ß-1,4 linkage is found in carbohydrate polymers such as cellulose. The correct answer is c—Peptide C. Answer c is correct. Peptide bonds are the covalent bonds between amino acids. The correct answer is c— D. Answer d is incorrect. Hydrogen bonds contribute to the structure of proteins, but are not involved in holding the individual amino acids together. 8. The double helix structure of a molecule of DNA is stabilized b— a. phosphodiester bonds b. peptide bonds c. an α helix d. hydrogen bonds The correct answer is d— A. Answer a is incorrect. Phosphodiester bonds are responsible for maintaining the sugar– phosphate backbone of the DNA molecule, but they are not involved in stabilizing the double helix. The correct answer is d— B. Answer b is incorrect. Peptide bonds are the covalent bonds responsible for linking amino acids together to form proteins. The correct answer is d— C. Answer c is incorrect. Don’t be fooled by the word “helix”! An α helix is a form of protein structure and has nothing to do with DNA. Answer c is not correct. The correct answer is d—hydrogen bonds D. Answer d is correct. Hydrogen bonds are formed between the nitrogenous bases of the two strands of DNA. It is the hydrogen bond formation between A:T and G:C that hold the DNA together. 9. Which of the following is NOT a difference between DNA and RNA? a. Deoxyribose sugar versus ribose sugar b. Thymine versus uracil c. Double stranded versus single stranded d. Phosphodiester versus hydrogen bonds The correct answer is d— A. Answer a is incorrect. The “D” in DNA stands for deoxyribose and the “R” in RNA stands for ribose. The correct answer is d— B. Answer b is incorrect. DNA and RNA differ in one nucleotide. DNA uses thymine, but RNA uses uracil. The correct answer is d— Raven/Johnson Biology 8e Chapter 03– Answers C. Answer c is incorrect. DNA is made up of two strands of nucleic acid polymer while RNA is only made of one strand. The correct answer is d—Phosphodiester versus hydrogen bond D. Answer d is correct. Both DNA and RNA rely on the presence of phosphodiester bonds to hold the sugar–phosphate backbone of the polymer together. The double strands that make up a molecule of DNA are held together by hydrogen bonds. However, even though RNA is a single-stranded molecule, it can fold back onto itself—a property that is stabilized by hydrogen bonds. 10. What monomers make up a protein? a. Monosaccharides b. Nucleotides c. Amino acids d. Fatty acids The correct answer is c— A. Answer a is incorrect. Monosaccharides are the monomer units of carbohydrates. The correct answer is c— B. Answer b is incorrect. Nucleotides are the monomer units of nucleic acids like DNA and RNA. The correct answer is c—Amino acids C. Amino acids with their variable R-groups are the monomers used to form proteins. The correct answer is c— D. Answer d is incorrect. Fatty acids are found in lipids. 11. Which part of an amino acid has the greatest influence on the overall structure of a protein? a. The (–NH2) amino group b. The R-group c. The (–COOH) carboxyl group d. Both a and c The correct answer is b— A. Answer a is incorrect. The amino group of the amino acid participates in the formation of the peptide bond, but does not influence the overall structure of the protein. The correct answer is b—The R-group B. Answer b is correct. The R-group is the variable part of an amino acid. Some R-groups are polar and hydrophilic. Other R-groups are nonpolar and hydrophobic. Some R-groups are large and others are small. The combination of these factors leads to differences in the shape of proteins. The correct answer is b— C. Answer c is incorrect. The carboxyl group of the amino acid participates in the formation of the covalent peptide bond, but is not involved in protein structure beyond this level. Raven/Johnson Biology 8e Chapter 03– Answers The correct answer is b— D. Answer d is incorrect. The amino and carboxyl groups of an amino acid are the key functional groups involved in peptide bond formation, but are less influential than Rgroups in determining protein shape. 12. A mutation that alters a single amino acid within a protein can alter— a. the primary level of protein structure b. the secondary level of protein structure c. the tertiary level of protein structure d. all of the above The correct answer is d— A. Answer a is incorrect. Although changing an amino acid does change the primary sequence, this is not the only effect. The correct answer is d— B. Answer b is incorrect. Although changing an amino acid could alter a region of secondary structure, this is not the only effect. The correct answer is d— C. Answer c is incorrect. The tertiary level of protein structure is dependent on the secondary and primary levels of protein structure. Changing one level influences all levels. The correct answer is d—all of the above D. Answer d is correct. The levels of protein structure are all interrelated. The alteration of a single amino acid represents a change in the primary level of protein structure. This one change can alter the ability of the protein to form α helices or β-pleated sheets. The change in secondary structure is then reflected in the three-dimensional, or tertiary, level of the protein. 13. Which of these factors contributes to the diversity of protein form and function in the cell? a. Quaternary interactions between peptides b. Formation of helices and ß-pleated sheets c. The linear sequence of amino acids that makes up the polymer d. All of the above The correct answer is d— A. Answer a is incorrect. Quaternary interactions contribute to the diversity of protein forms by allowing for the formation of functional aggregates of distinct proteins. However, this is not the only factor that is important. The correct answer is d— B. Answer b is incorrect. The formation of helices and ß-pleated sheets contributes to the overall shape of the protein and in that way contributes to both form and function of the protein, but this is not the only factor. The correct answer is d— Raven/Johnson Biology 8e Chapter 03– Answers C. Answer c is incorrect. The linear sequence of the amino acids that makes up a protein is a critical property that determines the folding of the protein through the relative position of the various R-groups. Shape is a critical property that determines protein function, but it is not the only factor. The correct answer is d—All of the above D. Answer d is correct. All of the properties listed in answers a through d are important factors that will determine the size and shape, and therefore the form and function, of a protein. 14. What chemical property of lipids accounts for their insolubility in water? a. The length of the carbon chain b. The large number of nonpolar C–H bonds c. The branching of saturated fatty acids d. The C=C bonds found in unsaturated fatty acids The correct answer is b— A. Answer a is incorrect. The overall length of the carbon chain is not a factor in the ability of any molecule to interact with water. The ability to dissolve in water is a consequence of the ability of a molecule to form hydrogen bonds with water molecules. Carbon is not involved in hydrogen bond formation. The correct answer is b—The large number of nonpolar C–H bonds B. Answer b is correct. Lipids are made up mostly of C–H bonds, which are nonpolar. Hydrogen bonds can only form between two polar molecules. Water is polar, but lipids are not. The correct answer is b— C. Answer c is incorrect. Saturated fatty acids are not branched. The correct answer is b— D. Answer d is incorrect. The ability to interact with water molecules (to be soluble) depends on the presence of polar bonds within a molecule. The C=C bond is not polar. 15. The spontaneous formation of a lipid bilayer in an aqueous environment occurs because— a. the polar head groups of the phospholipids can interact with water b. the long fatty acid tails of the phospholipids can interact with water c. the fatty acid tails of the phospholipids are hydrophobic d. both a and c The correct answer is d— A. Answer a is incorrect. Since the head groups of the phospholipids are polar they are able to interact with water; however, this is not the only property that contributes to the formation of bilayers. The correct answer is d— B. Answer b is incorrect. The long-chain fatty acids are hydrophobic and as such do not interact with polar water molecules. Raven/Johnson Biology 8e Chapter 03– Answers The correct answer is d— C. Answer c is incorrect. The hydrophobicity of the fatty acid tails will cause them to orient themselves away from water; however, this is not the only property of a phospholipids that contributes to the formation of a bilayer. The correct answer is d—both a and c D. Answer d is correct. Phospholipid molecules have both polar and nonpolar or hydrophilic and hydrophobic properties. The chemical differences between the polar head groups and the nonpolar fatty acid tails drive the formation of bilayers when these molecules are exposed to an aqueous environment. Challenge Questions 1. Spider webs are made of “silk,” which is a long, fibrous protein. The threads you see in a web are actually composed of many individual proteins. One important structural motif within spider’s silk protein is a “β crystal.” β crystals are regions where the β-pleated sheets from the multiple individual protein fibers stack one upon the other. What chemical bonds are required for the formation of the β crystals? What level of protein structure is responsible for the formation of the silk you see in the web? Predict how the presence of the β-crystal motif would influence the physical properties of the silk protein. Answer—β-pleated sheets are regions of secondary protein structure that are held together by hydrogen bonds. The hydrogen bonds are formed between regions of the amino acid backbone of the protein (not between the R-groups). A β crystal would therefore be a region of β-pleated sheets from many different silk proteins, all held together by hydrogen bonds. The formation of a silk fiber is an example of quaternary protein structure because it involves many individual proteins. The stacking and stabilization by hydrogen bonding of multiple β-pleated sheets would create a stiff element within the protein. The β-crystal motif is associated with the strength of the silk—the more regions of β crystal, the stronger the silk. 2. How do the four biological macromolecules differ from one another? Refer to the diagram of the monomer structure in Figure 3.3 and summarize what “clues” you use to distinguish between these important molecules. Answer—Carbohydrates are characterized by the ratio of (CH2O)n. Carbohydrates will typically form ring structures with lots of –OH and –H groups extending from the carbon ring. Nucleic Acids are formed from nucleotides. A nucleotide is easy to recognize because it is made up of three distinct chemical groups: a ribose (five-carbon) sugar, a phosphate group (PO4), and a large nitrogenous base. Proteins are composed of amino acids. An amino acid is easy to spot because of the distinct carboxyl and amino groups at either end of the molecule. Also look for the variable R-groups that extend from the central carbon. Lipids might get confused with carbohydrates, but look closely and you will find that these molecules have a lot less oxygen. Lots of carbons and hydrogens all lined up is the tip off that you are looking at a lipid. Raven/Johnson Biology 8e Chapter 03– Answers 3. Hydrogen bonds play an important role in stabilizing and organizing biological macromolecules. Consider the four macromolecules discussed in this chapter. Describe three examples where hydrogen bond formation affects the form or function of the macromolecule. Answer—Nucleic Acids—Hydrogen bonds are important for complementary base-pairing between the two strands of nucleic acid that make up a molecule of DNA. Complementary basepairing can also occur within the single nucleic acid strand of a RNA molecule. Proteins—Hydrogen bonds are involved in both the secondary and tertiary levels of protein structure. The α helices and β-pleated sheets of secondary structure are stabilized by hydrogen bond formation between the amino and carboxyl groups of the amino acid backbone. Hydrogen bond formation between R-groups helps stabilize the three-dimensional folding of the protein at the tertiary level of structure. Carbohydrates—Hydrogen bonds are less important for carbohydrates; however, these bonds are responsible for the formation of the fibers of cellulose that make up the cell walls of plants. Lipids—Hydrogen bonds are not involved in the structure of lipid molecules. The inability of fatty acids to form hydrogen bonds with water is key to their hydrophobic nature. 4. The cells of your body are distinct even though they all contain the same genetic information. A brain cell has a different shape and function than a muscle cell, and a skin cell is very different from a blood cell. Use the information in Table 3.2 to develop an explanation for the diversity of specialized cellular structure and function found within your body. Answer—The diversity of protein function underlies the diversity of cellular function. Table 3.2 is just a short list of examples of different protein functions. All cells possess the same genetic “library,” however, different cells express different proteins. Muscle cells are specialized by their expression of actin and myosin, two proteins associated with movement. Blood cells are specialized to transport CO2 and O2 because of the presence of the protein hemoglobin in their cytoplasm. The concepts of protein structure and function are critical to a good understanding of biology because of this fundamental relationship between proteins and cells.