1. A covalent bond between two atoms represents what kind of energy? a. Kinetic energy b. Potential energy c. Mechanical energy d.

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1 1. A covalent bond between two atoms represents what kind of energy? a. Kinetic energy b. Potential energy c. Mechanical energy d. Solar energy A. Answer a is incorrect. Kinetic energy is the energy of moving things (refer to Figure 6.1b). A covalent bond is not moving. Potential energy B. Answer b is correct. Energy is stored within covalent bonds. This energy can be used by the cell for any number of activities. C. Answer c is incorrect. Mechanical energy is a form of kinetic energy. Covalent bonds do not contain kinetic energy D. Answer d is incorrect. Solar energy is a specific type of energy derived from the Sun. As you will learn later, solar energy can be used to create covalent bonds; however, it is not the energy found linking two atoms. 2. During a redox reactions the molecule that gains an electron has been a. reduced and now has a higher energy level b. oxidized and now has a lower energy level c. reduced and now has a lower energy level d. oxidized and now has a higher energy level reduced and now has a higher energy level A. Answer a is correct. The addition of an electron to a molecule is called reduction because of the increase in the negative charge due to the presence of the electron. B. Answer b is incorrect. Oxidation occurs when an electron is lost from a molecule, not when it is gained. C. Answer c is incorrect. The reduced molecule as a greater potential energy level due to the presence of the electron. D. Answer d is incorrect. Molecules that lose electrons have a lower potential energy.

2 3. An endergonic reaction has the following properties a. + G and the reaction is spontaneous b. + G and the reaction is not spontaneous c. G and the reaction is spontaneous d. G and the reaction is not spontaneous A. Answer a is incorrect. An endergonic reaction is not spontaneous. The change in free energy is positive, meaning the reaction will absorb energy from the surroundings. + G and not spontaneous B. Answer b is correct. The free energy change is positive and the reaction will not occur spontaneously. C. Answer c is incorrect. The free energy change is positive, not negative, and endergonic reactions are not spontaneous. D. Answer d is incorrect. The free energy change is positive not negative, but it is true that endergonic reactions are not spontaneous. 4. A spontaneous reaction is one in which a. the reactants have a higher free energy than the products b. the products have a higher free energy than the reactants c. energy is required d. entropy is decreased the reactants have a higher free energy than the products A. Answer a is correct. A spontaneous reaction is like a rock rolling down hill. The beginning material (the reactants) must have greater free energy than the ending material (products). B. Answer b is incorrect. To be spontaneous, a reaction must move from high free energy to low free energy. C. Answer c is incorrect. A spontaneous reaction does not require energy. The energy for the reaction is found in the potential energy of the reactants. D. Answer d is incorrect. Entropy is increased as a result of a spontaneous reaction.

3 5. What is activation energy? a. The thermal energy associated with random movements of molecules b. The energy released through the active breaking of chemical bonds c. The difference in free energy between reactants and products d. The energy required to initiate a chemical reaction A. Answer a is incorrect. Activation energy is specific to chemical reactions. Thermal energy is a very general property of molecules. B. Answer b is incorrect. Enthalpy is the term associated with the energy stored within chemical bonds. C. Answer c is incorrect. Free energy is a measure of the overall likelihood that a reaction will occur. The energy required to start a chemical reaction D. Answer d is correct. Activation energy represents the initial investment of energy required to start even the most spontaneous reaction. 6. Which of the following is NOT a property of a catalyst? a. A catalyst reduces the activation energy of a reaction. b. A catalyst lowers the free energy of the reactants. c. A catalyst does not change as a result of the reaction. d. A catalyst works in both the forward and reverse directions of a reaction. A. Answer a is incorrect. Enzymes reduce the activation energy for a reaction, making the reaction more likely to occur. A catalyst lowers the free energy of the reactants. B. Answer b is correct. The free energy of the reactants is a property of the molecules and cannot be altered by the presence of an enzyme. The enzyme can only affect the activation energy for a reaction. C. Answer c is incorrect. The definition of a catalyst requires that it does not change as a result of the reaction. D. Answer d is incorrect. A catalyst only facilitates a reaction. It cannot determine the direction of the reaction.

4 7. Where is the energy stored in a molecule of ATP? a. Within the bonds between nitrogen and carbon b. In the carbon bonds found in the ribose c. In the oxygen double bond d. In the bonds connecting the two terminal phosphate groups A. Answer a is incorrect. Energy is stored in all chemical bonds; however, the energy used by the cell is contained in the bonds that link the phosphate groups together in a molecule of ATP. B. Answer b is incorrect. Energy is stored within the covalent bonds that make up a sugar molecule; however, this does not provide an immediate source of energy. C. Answer c is incorrect. There is energy stored in a double bond; however, this energy is not easily accessible for the cell. In the bonds connecting the two terminal phosphate groups D. Answer d is correct. The covalent bonds connecting the terminal phosphate groups have a very high potential energy due to electrostatic repulsion of the negatively charged phosphates. This makes these bonds less stable, and breaking these bonds releases energy. 8. Why is ATP capable of driving endergonic reactions? a. Because ATP is a catalyst b. Energy released by ATP makes the G for coupled reactions more negative c. Energy released by ATP makes the G for coupled reactions more positive d. Because the conversion of ATP to ADP is also endergonic A. Answer a is incorrect. ATP is changed as a consequence of chemical reactions. A catalyst cannot be changed. Energy released by ATP makes the G for coupled reactions more negative B. Answer b is correct. The energy released by ATP hydrolysis can be coupled to an endergonic reaction. If the amount of energy released is greater than the amount of energy absorbed by the endergonic reaction, then the G for both reactions will be negative, and the coupled reactions will be spontaneous. C. Answer c is incorrect. Energy is released by ATP; however, a positive change in free energy makes a reaction less likely to happen, not more. D. Answer d is incorrect. Energy is released when ATP is converted to ADP. This is an exergonic reaction.

5 9. Which of the following statements is NOT true about enzymes? a. Enzymes use the three-dimensional shape of their active site to bind reactants. b. Enzymes lower the activation energy for a reaction. c. The process of the reactions alters the enzyme. d. Enzymes can catalyze the forward and reverse directions of a reaction. Answer c is correct A. Answer a is incorrect. The shape of the active site is critically important for determining the specificity of the enzyme. Answer c is correct B. Answer b is incorrect. The ability of enzymes to catalyze reactions depends on their ability to lower the activation energy of the reaction. Answer c is correct The process of the reactions alters the enzyme. C. Answer c is correct. A catalyst cannot be changed as a consequence of a reaction. Answer c is correct D. Answer d is incorrect. Enzymes, as catalysts, do not determine the direction of a chemical reaction. 10. What is the function of the active site of an enzyme? a. It binds to the substrate, forming an enzyme substrate complex. b. Side groups within the active site interact with the substrate. c. It binds to the product, triggering induced fit of the protein. d. Both a and b. A. Answer a is incorrect. Although this is a correct statement, it is not the only possibility. B. Answer b is incorrect. Although this is a correct statement, it is not the only possibility. C. Answer c is incorrect. After an enzyme catalyzes the substrate (reactants) into product, the product is released from the active site. Induced fit occurs when the substrate molecules first bind to the enzyme. Both a and b. D. Answer d is correct. Both a and b describe the function of the active site. 11. A multienzyme complex is capable of a. catalyzing a single reaction at a much greater rate b. catalyzing a series of reactions using multiple different enzymes c. lowering the activation energy for a reaction through the activity of multiple enzymes d. both a and c

6 A. Answer a is incorrect. A multienzyme complex contains multiple distinct enzymes, catalyzing multiple, related reactions. catalyzing a series of reactions using multiple different enzymes B. Answer b is correct. The enzymes within a multienzyme complex all work on complementary reactions in which the product of one reaction supplies the substrate for the next. C. Answer c is incorrect. Enzymes catalyze reactions by lowering the activation energy; however, this property is not unique to multienzyme complexes. D. Answer d is incorrect. Multienzyme complexes contain more than a single enzyme and although they do lower the activation energy, that property is not unique to these complexes. 12. What is the common factor that influences enzyme function at extreme temperatures or ph? a. The rate of movement of the substrate molecules b. The strength of the chemical bonds within the substrate c. The three-dimensional shape of the enzyme d. The rate of movement of the enzyme A. Answer a is incorrect. The rate of movement of substrate is important; however, this only describes the influence of temperature, not ph. B. Answer b is incorrect. Chemical bonds can be stressed by temperature and ph; however, this description does not address enzyme function. The three-dimensional shape of the enzyme C. Answer c is correct. Variables such as temperature and ph can alter the shape of an enzyme, thereby altering or destroying its ability to function. D. Answer d is incorrect. The movement of the enzyme is only influenced by temperature, not ph. 13. The discovery of ribozymes meant that a. only proteins have catalytic function b. only nucleic acids have catalytic function c. some RNAs have enzymatic activity d. RNA could be destroyed by enzymes

7 A. Answer a is incorrect. Ribozymes are catalytic RNAs. These may be complexed with protein, but the active element is RNA not protein. B. Answer b is incorrect. We have long known that proteins can be enzymes; the change was learning that RNAs could be enzymes as well. some RNAs have enzymatic activity C. Answer c is correct. A ribozyme is an RNA molecule that can act as an enzyme. The first ribozymes found were in self-splicing ribosomal RNA. Since then, other RNAs have been shown to be ribozymes. Current evidence is that the ribosome itself is a ribozyme. D. Answer d is incorrect. A ribozyme is an enzyme; however, it is not specific to degrading RNA it is RNA! 14. Molecules that bind within the active site of an enzyme are, whereas molecules that bind at a site distant from the active site are. a. cofactors; products b. competitive inhibitors; allosteric inhibitors c. noncompetitive inhibitors; competitive inhibitors d. products; coenzymes A. Answer a is incorrect. Cofactors are nonprotein components that contribute to enzyme function. Cofactors may be present within the active site, but do not bind in the same way as a substrate molecule would. competitive inhibitors; allosteric inhibitors B. Answer b is correct. Competitive inhibitors compete with substrate molecules for the chance to bind in the active site. Allosteric inhibitors bind at a location other than the active site (an allosteric site). C. Answer c is incorrect. Inhibitors that bind to the active site are said to be competitive. They are competing with substrate for the chance to bind to the active site. Binding sites outside of the active site are called allosteric sites. Since substrate does not bind here, there is no competition. D. Answer d is incorrect. Products are molecules that are released from an active site. They do not bind to this site in the same way as substrate molecules. Coenzymes are nonprotein organic molecules that help enzymes function.

8 Challenge Questions 1. Examine the graph showing the rate of reaction versus temperature for an enzymecatalyzed reaction in a human. a. Describe what is happening to the enzyme at around 40 o C. b. Explain why the line touches the x-axis at approximately 20 o C and 45 o C. c. Average body temperature for humans is 37 o C. Suggest a reason why the temperature optimum of this enzyme is greater than 37 o C. Answer a. At 40 C the enzyme is at it optimum. The rate of the reaction is at its highest level. b. Temperature is a factor that influences enzyme function. This enzyme does not appear to function at either very cold or very hot temperatures. The shape of the enzyme is affected by temperature, and the enzyme s structure is altered enough at extreme temperatures that it no longer binds substrate. Alternatively, the enzyme may be denatured that is a complete loss of normal three-dimensional shape at extreme temperatures. Think about frying an egg: What happens to the proteins in the egg? c. Everyone s body is slightly different. If the temperature optimum was very narrow, then the cells that make up a body would be vulnerable. Having a broad range of temperature optimums keeps the enzyme functioning. 2. Phosphofructokinase, an important enzyme found in the cytoplasm of all cells, functions to add a phosphate group to a molecule of fructose-6-phosphate. This enzyme functions early in glycolysis, an energy-yielding biochemical pathway discussed in chapter 7. The enzyme has an active site that binds fructose and ATP. An allosteric regulator site also binds ATP when cellular levels of ATP are very high. a. Predict the rate of the reaction if the levels of cellular ATP were low. b. Predict the rate of the reaction if levels of cellular ATP are very high. c. Describe what is happening to the enzyme when levels of ATP are very high. Answer a. The reaction rate would be slow because of the low concentration of the substrate ATP. The rate of reaction depends on substrate concentration. b. ATP acts like a noncompetitive, allosteric inhibitor when ATP levels are very high. If ATP binds to the allosteric site, then the reaction should slow down. c. When ATP levels are high, the excess ATP molecules bind to the allosteric site and inhibit the enzyme. The allosteric inhibitor functions by causing a change in the shape of the active site in the enzyme. This reaction is an example of feedback regulation because ATP is a final product of the overall series of reactions associated with glycolysis. The cell regulates glycolysis by regulating this early step catalyzed by phosphofructokinase; the allosteric inhibitor is the product of glycolysis (and later stages) ATP.

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