Practice Test: Chapter 6- An Introduction to Metabolism

Which statement about enzymes is true? (Section 6.4) A. Enzymes speed up the rate of the reaction without changing the delta G for the reaction. B. The more heat that is added to a reaction, the faster the enzymes will function. C. Enzymes react with their substrate (form chemical bonds), forming an enzyme-substrate complex, which irreversibly alters the enzyme. D. Enzymes increase the rate of a reaction by raising the activation energy for reactions. E. All of the choices are correct.

A. Enzymes speed up the rate of the reaction without changing the delta G for the reaction. Feedback- Enzymes speed up reactions by lowering the activation energy barrier for the reaction. Enzymes cannot change the delta G for the reaction.

Organisms are described as thermodynamically open systems. Which statement is consistent with this description? (Section 6.1) A. Organisms acquire energy from one object to another. B. Because energy must be conserved, organisms constantly recycle energy and thus need no input of energy. C. Heat produced by the organism is conserved in the organism and not lost to the environment. D. The metabolism of an organism is isolated from its surroundings. E. All of the choices are correct.

A. Organisms acquire energy from one object to another. Feedback- Open systems are those in which energy and materials can be exchanged between the system and its surroundings.

Which statement about the active site of an enzyme is correct? (Section 6.4) A. The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits. B. The active site has a fixed structure (shape). C. Coenzymes are rarely found in the active site of an enzyme. D. The structure of the active site is not affected by changes in temperature. E. The active site allows the reaction to occur under the same environmental conditions as the reaction without the enzyme.

A. The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits. Feedback- Only a restricted region of an enzyme molecule (the active site) binds to the substrate. Usually, the active site is formed by only a few of the amino acids, with the rest of the protein molecule providing a framework that reinforces the configuration of the active site.

Molecules A and B contain 110 kcal/mol of free energy and molecules C and D contain 150 kcal/mol of energy. A and B are converted to C and D. What can be concluded? (Section 6.2) A. The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants. B. The entropy in the products, C and D, is higher than in the reactants, A and B. C. A and B will be converted to C and D with a net release of energy. D. The conversion of A and B to C and D is exergonic; the products are less organized than the reactants. E. The conversion of A and B to C and D is spontaneous.

A. The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants. Feedback- C and D contain more energy than do A and B; therefore they are more organized and their construction requires an input of energy.

Which statement correctly describes an aspect of ATP hydrolysis driving the active transport of an ion into the cell against its concentration gradient? (Section 6.2 and Section 6.3) A. This is an example of energy coupling. B. The hydrolysis of ATP is endergonic, and the active transport is exergonic. C. ATP is acting as a transport protein to facilitate the movement of the ion across the plasma membrane. D. Both ATP hydrolysis and active transport are spontaneous because they result in an increase in entropy of the system. E. Neither ATP hydrolysis nor active transport is spontaneous.

A. This is an example of energy coupling. Feedback- The free energy released from the hydrolysis of ATP is coupled to the energy-requiring active transport of the ion across a membrane.

Which is an example of the cellular work accomplished with the free energy derived from the hydrolysis of ATP, involved in the production of electrochemical gradients? (Section 6.3) A. proton movement against a gradient of protons B. facilitated diffusion C. the beating of cilia D. chromosome movement on microtubules E. the chemical synthesis of ATP Feedback- Protein pumps that hydrolyze ATP can generate electrochemical gradients.

A. proton movement against a gradient of protons Feedback- Protein pumps that hydrolyze ATP can generate electrochemical gradients.

The formation of glucose-6-phosphate from glucose is an endergonic reaction and is coupled to which reaction or pathway? (Section 6.3) A. the hydrolysis of ATP B. the active transport of a phosphate ion into the cell. C. the formation of ATP from ADP + Pi D. the contraction of a muscle cell E. the conversion of glucose + fructose to make sucrose

A. the hydrolysis of ATP Feedback- With the help of specific enzymes, the cell is able to couple the energy of ATP hydrolysis directly to endergonic processes by transferring a phosphate group from ATP to some other molecule, such as glucose.

Which does not represent an energy transformation? (Section 6.1) A. the release of carbon dioxide as glucose is broken down during cellular respiration B. the production of sugars from carbon dioxide and water during photosynthesis C. the production of ATP using phosphate from the breakdown of glucose D. the coupling of ATP hydrolysis to the production of a proton gradient across a membrane by a proton pump E. All of the listed responses represent energy transformations.

A. the release of carbon dioxide as glucose is broken down during cellular respiration Feedback- This deals with changes in forms of matter, not energy.

A plot of reaction rate against temperature for an enzyme indicates little activity at 10°C and 45°C, with peak activity at 35°C. The most reasonable explanation for the low velocity at 10°C is that ___________. (Section 6.4) A. there is too little activation energy available B. the enzyme was denatured C. the hydrogen bonds that define the structure of the enzyme's active site are unstable D. the substrate becomes a competitive inhibitor at lower temperature E. the cofactors required by the enzyme system lack the thermal energy required to activate the enzyme

A. there is too little activation energy available Feedback- The environment usually supplies activation energy in the form of heat. The lower the temperature, the less energy that is available to overcome the activation energy barrier.

Which statement about enzymes is incorrect? (Section 6.4) A. Enzymes can be used to accelerate both anabolic and catabolic reactions. B. An enzyme is consumed during the reaction it catalyzes. C. An enzyme is very specific in terms of which substrate it binds to. D. An enzyme lowers the activation energy of a chemical reaction. E. Most enzymes are proteins.

B. An enzyme is consumed during the reaction it catalyzes. Feedback- If enzymes were consumed during the reaction they catalyzed, they would not be able to act as a catalyst. This statement is incorrect.

Succinylcholine is almost identical to acetylcholine structurally. If succinylcholine is added to a mixture that contains acetylcholine and the enzyme that hydrolyzes acetylcholine (but not succinylcholine), the rate of acetylcholine hydrolysis is decreased. Subsequent addition of more acetylcholine restores the original rate of acetylcholine hydrolysis. Which correctly explains this observation? (Section 6.4) A. Succinylcholine must be an allosteric regulator for this enzyme. B. Succinylcholine must be a competitive inhibitor for this reaction. C. The activation energy barrier for succinylcholine hydrolysis is higher than for acetylcholine hydrolysis. D. Succinylcholine must be a noncompetitive inhibitor. E. The active site must have the wrong configuration to permit succinylcholine binding.

B. Succinylcholine must be a competitive inhibitor for this reaction. Feedback- Competitive inhibition occurs when a molecule mimics the substrate by competing with it at the active site.

According to the second law of thermodynamics, which is true? (Section 6.1) A. All reactions produce some heat. B. The decrease in entropy associated with life must be compensated for by increased entropy in the environment in which life exists. C. The total amount of energy in the universe is constant. D. Energy conversions increase the order in the universe. E. The entropy of the universe is constantly decreasing.

B. The decrease in entropy associated with life must be compensated for by increased entropy in the environment in which life exists. Feedback- The second law of thermodynamics demands that total entropy must increase with any reaction.

What best characterizes the role of ATP in cellular metabolism? (Section 6.3) A. The delta G associated with its hydrolysis is positive. B. The free energy released by ATP hydrolysis may be coupled to an endergonic process via the formation of a phosphorylated intermediate. C. The release of free energy during the hydrolysis of ATP heats the surrounding environment. D. It is catabolized to carbon dioxide and water. E. The charge on the phosphate group of ATP tends to make the molecule very water-soluble.

B. The free energy released by ATP hydrolysis may be coupled to an endergonic process via the formation of a phosphorylated intermediate. Feedback:This is one way a cell can use an exergonic reaction to drive an endergonic reaction.

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule is an example of __________. (Section 6.5) A. noncompetitive inhibition B. allosteric regulation C. competitive inhibition D. feedback inhibition E. cooperativity Feedback- The molecule in this example is an allosteric activator.

B. allosteric regulation Feedback- The molecule in this example is an allosteric activator.

How do enzymes lower activation energy? (Section 6.4) A. by increasing reactivity of products B. by locally concentrating the reactants in a position in which they are more likely to react C. by harnessing heat energy to drive the breakage of bonds between atoms D. The first two responses above are correct. E. The second and third choices are correct.

B. by locally concentrating the reactants in a position in which they are more likely to react Feedback- One of the ways enzymes work is to increase the concentrations of reactants at a single place.

Which aspect of a reaction is changed by the presence of an enzyme? (Section 6.2 and Section 6.4) A. the sign of delta G B. the activation energy needed C. the magnitude of delta G D. the G value for the reactants E. the G value for the products

B. the activation energy needed Feedback- An enzyme lowers the amount of energy required to get the reactants to the transition state.

Which has the most free energy per molecule? (Section 6.2) A. a cholesterol molecule B. a sugar molecule C. a fatty acid molecule D. an amino acid molecule E. a starch molecule

E. a starch molecule Feedback- Starch is a large polymer of sugar and thus contains many covalent bonds.

Which state the relevance of the first law of thermodynamics to biology? (Section 6.1) A. Photosynthetic organisms produce energy in sugars from sunlight. B. The total energy taken in by an organism must be greater than the total energy stored or released by the organism. C. Energy can be freely transformed among different forms as long as the total energy is conserved. D. Energy is destroyed as glucose is broken down during cellular respiration. E. Living organisms must increase the entropy of their surroundings.

C. Energy can be freely transformed among different forms as long as the total energy is conserved. Feedback- The first law of thermodynamics states that energy can be transformed but can be neither created nor destroyed.

What is meant by the induced fit of an enzyme? (Section 6.4) A. The substrate can be altered so that it is induced to fit into the enzyme's active site. B. The shape of the active site is nearly perfect for specifically binding the enzyme's substrate(s). C. The enzyme changes its shape slightly as the substrate snugly binds to it. D. The presence of the substrate in solution induces the enzyme to slightly change its structure. E. The enzyme structure is altered so that it can be induced to fit many different types of substrate.

C. The enzyme changes its shape slightly as the substrate snugly binds to it. Feedback- The enzyme changes slightly to bind to the substrate and catalyze the reaction.

Consider the growth of a farmer's crop over a season. Which correctly states a limitation imposed by the first or second law of thermodynamics? (Section 6.1) A. The process of photosynthesis produces energy that the plant uses to grow. B. The entropy of the universe must decrease to account for the increased entropy associated with plant growth. C. To obey the first law, the crops must represent an open system. D. Growth of the crops must occur spontaneously. E. All of the choices are correct.

C. To obey the first law, the crops must represent an open system. Feedback- The first law states that energy cannot be created. The growth of plants stores much energy in the body of the plant. That energy must have been obtained from the plant's environment; thus, the plant must be an open system.

The ability to do work or the capacity to cause change is an example of __________. A. pinocytosis B. phagocytosis C. energy D. All of the choices are examples. E. None of the choices are correct. Feedback- Energy is the capacity to cause change. In everyday life, energy is important because some forms of energy can be used to do work.

C. energy Feedback- Energy is the capacity to cause change. In everyday life, energy is important because some forms of energy can be used to do work.

Which reaction would be endergonic? (Section 6.2) A. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O B. ATP → ADP + Pi C. glucose + fructose → sucrose D. HCl → H++ Cl− E. All of the listed responses are correct.

C. glucose + fructose → sucrose Feedback- In this case, the product is more complex (lower entropy) than the reactants, and like many anabolic reactions, this one requires an input of energy.

Which determines the sign of delta G for a reaction? (Section 6.2) A. the free energy of the products B. the free energy of the reactants C. the free energy of the reactants and the free energy of the products D. the enzyme catalyzing the reaction having a high affinity (strength of binding) for the reactants E. the enzyme catalyzing the reaction having a low affinity for the products

C. the free energy of the reactants and the free energy of the products Feedback- By subtracting the free energy of the reactants from the free energy of the products, the delta G can be calculated and the difference in these values determines the sign of the difference.

Much of the suitability of ATP as an energy intermediary is related to the instability of the bonds between the phosphate groups. These bonds are unstable because __________. (Section 6.3) A. the phosphate groups are polar and are attracted to the water in the cell's interior B. the valence electrons in the phosphorus atom have less energy on average than those of other atoms C. the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP D. the bonds between the phosphate groups are unusually strong and breaking them releases free energy E. they are hydrogen bonds, which are only about 10% as strong as covalent bonds

C. the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP Feedback- Negative charges repel each other. Loss of the terminal phosphate removes some of the repulsion.

Which statement about allosteric proteins is true? (Section 6.5) A. They exist in active and inactive conformations. B. They are acted on by inhibitors. C. They are sensitive to environmental conditions. D. All of the first three choices are correct. E. None of the three choices is correct.

D. All of the first three choices are correct.

Which correctly states the relationship between anabolic and catabolic pathways? (Section 6.1) A. The flow of energy between catabolic and anabolic pathways is reversible. B. Degradation of organic molecules by anabolic pathways provides the energy to drive catabolic pathways. C. Energy derived from catabolic pathways is used to drive the breakdown of organic molecules in anabolic pathways. D. Anabolic pathways synthesize more complex organic molecules using the energy derived from catabolic pathways. E. Catabolic pathways produce usable cellular energy by synthesizing more complex organic molecules.

D. Anabolic pathways synthesize more complex organic molecules using the energy derived from catabolic pathways. Feedback- The synthesis of complex molecules in anabolic pathways requires an input of energy from catabolic pathways.

If the entropy of a living organism is decreasing, which is most likely to be occurring at the same time? (Section 6.1) A. In this situation, the second law of thermodynamics must not apply. B. The first law of thermodynamics is being violated. C. The entropy of the organism's environment must also be decreasing. D. Energy input into the organism must be occurring to drive the decrease in entropy. E. Heat is being used by the organism as a source of energy.

D. Energy input into the organism must be occurring to drive the decrease in entropy. Feedback- If the entropy is decreasing, this would tend to make the free energy change associated with this positive. Thus, an input of energy would be required to make this decrease in entropy occur.

Which statement about enzyme function is correct? (Section 6.4) A. Enzymes can greatly speed up reactions, but they cannot change the net energy output because they cannot change the activation energy. B. Enzymes can change the equilibrium point of reactions, but they cannot speed up reactions because they cannot change the net energy output. C. Enzymes can greatly speed up reactions, but they cannot change the activation energy because they cannot change the net energy output. D. Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point because they cannot change the net energy output. E. None of the listed responses is correct.

D. Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point because they cannot change the net energy output. Feedback- Enzymes lower the activation energy barrier of a reaction but do not change the free energy of the products.

What do the sign and magnitude of the delta G of a reaction tell us about the speed of the reaction? (Section 6.2 and Section 6.4) A. The sign determines whether the reaction is spontaneous, and the magnitude determines the speed. B. The sign does not matter, but the larger the magnitude of delta G, the faster the reaction. C. The more negative the delta G, the faster the reaction is. D. Neither the sign nor the magnitude of delta G have anything to do with the speed of a reaction. E. The sign does not matter, but the smaller the magnitude of delta G, the faster the reaction.

D. Neither the sign nor the magnitude of delta G have anything to do with the speed of a reaction. Feedback- The speed of a reaction is determined by the activation energy barrier of the reaction and the temperature (which determines how many reactants have the energy to overcome the barrier).

Which correctly describe(s) the role(s) of heat in biological reactions? (Section 6.4) A. Heat from the environment is necessary for substrates to get over the activation energy barrier. B. The kinetic energy of the substrates is increased as the amount of heat in the system is increased. C. Increasing the amount of heat in a system will increase the rate of enzyme-catalyzed reactions. D. The first and second choices are correct. E. The second and third choices are correct.

D. The first and second choices are correct. Feedback- As the heat energy in a system increases, so does the kinetic energy of the reactants. As the kinetic energy of the reactants increases, the reactants are more likely to interact (with each other directly, or with the active site of an enzyme). Subsequently, the reaction rate would increase.

Above a certain substrate concentration, the rate of an enzyme-catalyzed reaction drops as the enzymes become saturated. Which would lead to a faster conversion of substrate into product under these saturated conditions? (Section 6.4) A. an increase in concentration of enzyme B. increasing the temperature by a few degrees C. increasing the substrate concentration D. The first and second listed responses are correct. E. The first, second, and third listed responses are correct.

D. The first and second listed responses are correct. Feedback- Either increasing the enzyme concentration or slightly increasing the temperature will increase the rate of product formation.

Metabolic pathways in cells are typically far from equilibrium. Which process tends to keep these pathways away from equilibrium? (Section 6.2) A. the continuous removal of the products of a pathway to be used in other reactions B. an input of free energy from outside the pathway C. an input of heat from the environment D. The first and second listed responses are correct. E. The first, second, and third listed responses are correct.

D. The first and second listed responses are correct. Feedback- Pathways can be displaced from equilibrium either by adding free energy or by removing the products of the pathway by other reactions.

Which statement about enzyme inhibitors is true? (Section 6.4) A. Inhibition of enzyme function by compounds that are not substrates is something that only occurs under controlled conditions in the laboratory. B. A noncompetitive inhibitor does not change the shape of the active site. C. A competitive inhibitor binds to the enzyme at a place that is separate from the active site. D. The mimicking action of competitive inhibitors may be reversible or irreversible. E. When the product of an enzyme or an enzyme sequence acts as its inhibitor, this is known as positive feedback. Feedback- Competitive inhibitors that bind covalently to the enzyme would be irreversible, and those that bind weakly would be reversible.

D. The mimicking action of competitive inhibitors may be reversible or irreversible. Feedback- Competitive inhibitors that bind covalently to the enzyme would be irreversible, and those that bind weakly would be reversible.

Which functional group is associated with a release of energy when removed from a carbon skeleton? (Section 6.3) A. sulfhydryl B. hydroxyl C. carboxyl D. phosphate E. amino

D. phosphate Feedback- Cleaving a phosphate group from ATP releases energy that is used to perform many cellular functions.

Under most conditions, the supply of energy by catabolic pathways is regulated by the demand for energy by anabolic pathways. Considering the role of ATP formation and hydrolysis in energy coupling, which is most likely to be true? (Section 6.5) A. High levels of ATP act as an allosteric activator of anabolic pathways. B. High levels of ATP act as an allosteric activator of catabolic pathways. C. High levels of ADP act as an allosteric inhibitor of catabolic pathways. D. High levels of ADP act as an allosteric inhibitor of anabolic pathways. E. High levels of ADP act as an allosteric activator of catabolic pathways

E. High levels of ADP act as an allosteric activator of catabolic pathways Feedback- In this case, high demand for energy by anabolic pathways uses ATP, increasing the level of ADP, which in turn increases the supply of ATP from catabolic pathways.

When 1 mole of ATP is hydrolyzed in a test tube without an enzyme, about twice as much heat is given off as when 1 mole of ATP is hydrolyzed in a cell. Which best explains these observations? (Section 6.3) A. Cells are less efficient at energy metabolism than reactions that are optimized in a test tube. B. In cells, ATP is hydrolyzed to ADP and Pi, but in the test tube it is hydrolyzed to carbon dioxide and water. C. Cells have the ability to store heat; this cannot happen in a test tube. D. The amount of heat released by a reaction has nothing to do with the free energy change of the reaction. E. In the cell, the hydrolysis of ATP is coupled to other reactions which are endergonic.

E. In the cell, the hydrolysis of ATP is coupled to other reactions which are endergonic. Feedback- The coupling of ATP to other endergonic processes in cells means that less of the free energy is released as heat. When ATP is hydrolyzed without this coupling, all of the energy is released as heat.

Which environment or action would not affect the rate of an enzymatic reaction? (Section 6.4) A. pH B. cooling the enzyme C. heating the enzyme D. substrate concentration E. None of the listed responses is correct.

E. None of the listed responses is correct. Feedback- Changes in temperature, substrate concentration, and pH are all likely to affect enzyme activity.

The binding of an allosteric inhibitor to an enzyme causes the rate of product formation by the enzyme to decrease. Which best explains why this decrease occurs? (Section 6.5) A. The allosteric inhibitor binds to the substrate and prevents it from binding at the active site. B. The allosteric inhibitor lowers the temperature of the active site. C. The allosteric inhibitor causes free energy change of the reaction to increase. D. The allosteric inhibitor binds to the active site, preventing the substrate from binding. E. The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site.

E. The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site. Feedback- In general, any allosteric regulator functions by changing the structure of the enzyme to either change the ability of the active site to bind the substrate or to facilitate the chemical reaction.

Which is the most abundant form of energy in a cell? (Section 6.1) A. mechanical energy B. heat C. kinetic energy D. electrochemical gradients E. chemical energy

E. chemical energy Feedback- All of the macromolecules that make up a cell contain large amounts of chemical (potential) energy, stored when they were made in anabolic reactions.

Enzyme activity is affected by pH because __________. (Section 6.4) A. most substrates don't function well at high or low pH B. changes in pH can cause loss of cofactors from the enzyme C. low pH will denature all enzymes D. the binding of hydrogen ions to the enzyme absorbs energy and thus, there may not be enough energy to overcome the activation energy barrier E. high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the active site

E. high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the active site Feedback- Each enzyme has an optimal pH at which it is most active, and variations in pH can alter the enzyme's structure, changing activity.

An exergonic (spontaneous) reaction is a chemical reaction that __________. (Section 6.2) A. cannot occur outside of a living cell B. is common in anabolic pathways C. occurs only when an enzyme or other catalyst is present D. leads to a decrease in the entropy of the universe E. releases free energy when proceeding in the forward direction

E. releases free energy when proceeding in the forward direction Feedback- Exergonic reactions proceed with a net release of free energy, and they occur spontaneously.

In general, the hydrolysis of ATP drives cellular work by __________. (Section 6.3) A. changing to ADP and phosphate B. releasing heat C. lowering the free energy of the reaction D. acting as a catalyst E. releasing free energy that can be coupled to other reactions

E. releasing free energy that can be coupled to other reactions Feedback- With the help of specific enzymes, the cell can couple the energy of ATP hydrolysis directly to endergonic processes.

A chemical reaction is designated as exergonic rather than endergonic when __________. (Section 6.2 and Section 6.4) A. activation energy is required B. it absorbs more energy C. activation energy exceeds net energy release D. the products are less complex than the reactants E. the potential energy of the products is less than the potential energy of the reactants

E. the potential energy of the products is less than the potential energy of the reactants Feedback- If a reaction is exergonic, the formation of new bonds releases more energy than was invested in breaking the old bonds.