Biology Chapter 8 – CC – Practice Test

Which of the following correctly states the relationship between anabolic and catabolic pathways? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) Catabolic pathways produce usable cellular energy by synthesizing more complex organic molecules.
B) The flow of energy between catabolic and anabolic pathways is reversible.
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) Degradation of organic molecules by anabolic pathways provides the energy to drive catabolic pathways.

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

Which of the following situations does not represent an energy transformation? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) the production of sugars from carbon dioxide and water during photosynthesis
B) the coupling of ATP hydrolysis to the production of a proton gradient across a membrane by a proton pump
C) the release of carbon dioxide as glucose is broken down during cellular respiration
D) the production of ATP using phosphate from the breakdown of glucose
E) All of the listed responses represent energy transformations.

the coupling of ATP hydrolysis to the production of a proton gradient across a membrane by a proton pump Correct. The energy stored in ATP is used to create an electrochemical gradient of protons that contains energy.

Organisms are described as thermodynamically open systems. Which of the following statements is consistent with this description? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) Because energy must be conserved, organisms constantly recycle energy and thus need no input of energy.
B) Heat produced by the organism is conserved in the organism and not lost to the environment.
C) The metabolism of an organism is isolated from its surroundings.
D) Organisms acquire energy from, and lose energy to, their surroundings.
E) All of the listed responses are correct.

Organisms acquire energy from, and lose energy to, their surroundings. Correct. Open systems are those in which energy and materials can be exchanged between the system and its surroundings.

Consider the growth of a farmer’s crop over a season. Which of the following correctly states a limitation imposed by the first or second law of thermodynamics? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) To obey the first law, the crops must represent an open system.
B) Growth of the crops must occur spontaneously.
C) The process of photosynthesis produces energy that the plant uses to grow.
D) The entropy of the universe must decrease to account for the increased entropy associated with plant growth.
E) All of the listed responses are correct.

To obey the first law, the crops must represent an open system. Correct. 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.

Which of the following states the relevance of the first law of thermodynamics to biology? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

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

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

Which is the most abundant form of energy in a cell? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) kinetic energy
B) chemical energy
C) heat
D) electrochemical gradients
E) mechanical energy

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

Which of the following is an example of the second law of thermodynamics as it applies to biological reactions? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) The aerobic respiration of one molecule of glucose produces six molecules each of carbon dioxide and water.
B) All types of cellular respiration produce ATP.
C) Cellular respiration releases some energy as heat.
D) The first and second choices are correct.
E) The first, second, and third choices are correct.

The aerobic respiration of one molecule of glucose produces six molecules each of carbon dioxide and water. Correct. The second law of thermodynamics states that every energy transformation makes the universe more disordered—carbon dioxide and water are more disordered than glucose.

According to the second law of thermodynamics, which of the following is true? (see book section: Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics)

A) 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.
C) All reactions produce some heat.
D) The total amount of energy in the universe is constant.
E) Energy conversions increase the order in the univers

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

If the entropy of a living organism is decreasing, which of the following is most likely to be occurring simultaneously? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) In this situation, the second law of thermodynamics must not apply.
B) The first law of thermodynamics is being violated.
C) 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.
E) The entropy of the organism’s environment must also be decreasing.

Energy input into the organism must be occurring to drive the decrease in entropy. Correct. 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 of the following has the most free energy per molecule? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) a sugar molecule
B) a fatty acid molecule
C) a starch molecule
D) an amino acid molecule
E) a cholesterol molecule

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

The electronic arrangement in which of the following molecules means that this (these) molecule(s) has/have greater free energy than the others? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) methane
B) carbon dioxide
C) water
D) The first two molecules listed are roughly equal in the energy they contain.
E) The first three molecules listed are roughly equal in the energy they contain.

methane Correct. Bonds that contain electrons that are more equally shared between atoms release more energy when the electrons from those bonds are used to form lower energy bonds.

Which part of the equation ΔG = ΔH – TΔS tells you if a process is spontaneous? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) ΔG
B) ΔH
C) ΔS
D) All of these values reveal the direction in which a reaction will go.
E) TΔS

ΔG Correct. In any spontaneous process, the free energy of a system decreases. The change in free energy equals the change in total energy minus the change in entropy times the temperature.

If, during a process, the system becomes more ordered, then _____. (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) ΔG is positive
B) ΔS is negative
C) ΔG is negative
D)ΔH is positive
E) ΔH is negative

ΔS is negative Correct. In an endergonic reaction, in which order is increased, the change in entropy, symbolized by ΔS, is negative.

When one molecule is broken down into six component molecules, which of the following will always be true? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) ΔS is negative.
B) ΔH is negative.
C ) ΔG is positive.
D) An input of free energy is needed.
E) ΔS is positive.

ΔS is positive. Correct. The large increase in disorder associated with this reaction means that entropy (ΔS) increases

From the equation ΔG = ΔH – TΔS it is clear that _____. (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) a decrease in the system’s total energy will increase the probability of spontaneous change
B) increasing the entropy of a system will increase the probability of spontaneous change
C) increasing the temperature of a system will increase the probability of spontaneous change
D) The first and second choices are correct.
E) The first three choices are correct.

The first three choices are correct.

An exergonic (spontaneous) reaction is a chemical reaction that _____. (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) occurs only when an enzyme or other catalyst is present
B) releases energy when proceeding in the forward direction
C) is common in anabolic pathways
D) cannot occur outside of a living cell
E) leads to a decrease in the entropy of the universe

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

Which of the following reactions would be endergonic? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
B) HCl → H+ + Cl-
C) ATP → ADP + Pi
D) glucose + fructose → sucrose
E) All of the listed responses are correct.

glucose + fructose → sucrose Correct. 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.

Molecules A and B contain 110 kcal/mol of free energy and molecules B and C contain 150 kcal/mol of energy. A and B are converted to C and D. What can be concluded? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) A and B will be converted to C and D with a net release of energy.
B) The conversion of A and B to C and D is exergonic; the products are less organized than the reactants.
C) The conversion of A and B to C and D is spontaneous.
D) The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants.
E) The entropy in the products, C and D, is higher than in the reactants, A and B.

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

Which of the following determines the sign of ΔG for a reaction? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

A) the free energy of the products
B) the free energy of the reactants
C) the enzyme catalyzing the reaction having a low affinity for the products
D) the free energy of the reactants and the free energy of the products
E) the enzyme catalyzing the reaction having a high affinity (strength of binding) for the reactants

the free energy of the reactants and the free energy of the products Correct. By subtracting the free energy of the reactants from the free energy of the products, the ΔG can be calculated and the difference in these values determines the sign of the difference.

Metabolic pathways in cells are typically far from equilibrium. Which of the following processes tend(s) to keep these pathways away from equilibrium? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously)

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.

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

Which of the following 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? (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) proton movement against a gradient of protons
B) the beating of cilia
C) The chemical synthesis of ATP
D) chromosome movement on microtubules
facilitated diffusion

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

In general, the hydrolysis of ATP drives cellular work by _____. (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) acting as a catalyst
B) releasing heat
C) changing to ADP and phosphate
D) lowering the free energy of the reaction
E) releasing free energy that can be coupled to other reactions

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

Which of the following statements correctly describes some aspect of ATP hydrolysis being used to drive the active transport of an ion into the cell against the ion’s concentration gradient? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously and see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) The hydrolysis of ATP is endergonic, and the active transport is exergonic.
B) This is an example of energy coupling.
C) Both ATP hydrolysis and active transport are spontaneous because they result in an increase in entropy of the system.
D) Neither ATP hydrolysis nor active transport is spontaneous.
E) ATP is acting as a transport protein to facilitate the movement of the ion across the plasma membrane.

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

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 _____. (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

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

the negatively charged phosphate groups vigorously repel one another and the terminal phosphate group is more stable in water than it is in ATP

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 of the following best explains these observations? (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) In cells, ATP is hydrolyzed to ADP and Pi, but in the test tube it is hydrolyzed to carbon dioxide and water.
B) Cells have the ability to store heat; this cannot happen in a test tube.
C) Cells are less efficient at energy metabolism than reactions that are optimized 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 endergonic reactions.

In the cell, the hydrolysis of ATP is coupled to other endergonic reactions. Correct. 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.

What best characterizes the role of ATP in cellular metabolism? (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) The DG associated with its hydrolysis is positive.
B) It is catabolized to carbon dioxide and water.
C) The free energy released by ATP hydrolysis may be coupled to an endergonic process via the formation of a phosphorylated intermediate.
D) The charge on the phosphate group of ATP tends to make the molecule very water-soluble.
E) The release of free energy during the hydrolysis of ATP heats the surrounding environment.

The free energy released by ATP hydrolysis may be coupled to an endergonic process via the formation of a phosphorylated intermediate.

The formation of glucose-6-phosphate from glucose is an endergonic reaction and is coupled to which of the following reactions or pathways? (see book section: Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions)

A) the hydrolysis of ATP
B) the conversion of glucose + fructose to make sucrose
C) the formation of ATP from ADP + Pi
D) the active transport of a phosphate ion into the cell
E) the contraction of a muscle cell

the hydrolysis of ATP Correct. 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.

A chemical reaction is designated as exergonic rather than endergonic when _____. (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously and see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) the products are less complex than the reactants
B) activation energy is required
C) the potential energy of the products is less than the potential energy of the reactants
D) it absorbs more energy
E) activation energy exceeds net energy release

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

Which of the following is changed by the presence of an enzyme in a reaction? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously and see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) the activation energy
B) the magnitude of ΔG.
C) the G value for the reactants
D) the G value for the products
E) The sign of ΔG

the activation energy Correct. An enzyme lowers the amount of energy required to get the reactants to the transition state.

What do the sign and magnitude of the ΔG of a reaction tell us about the speed of the reaction? (see book section: Concept 8.2: The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously and see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) The more negative the ΔG, the faster the reaction is.
B) The sign does not matter, but the larger the magnitude of ΔG, the faster the reaction.
C) Neither the sign nor the magnitude of ΔG have anything to do with the speed of a reaction.
D) The sign does not matter, but the smaller the magnitude of ΔG, the faster the reaction.
E) The sign determines whether the reaction is spontaneous, and the magnitude determines the speed.

Neither the sign nor the magnitude of ΔG have anything to do with the speed of a reaction. Correct. 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).

How do enzymes lower activation energy? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) by locally concentrating the reactants
B) The second and third choices above are correct.
C) by harnessing heat energy to drive the breakage of bonds between atoms
D) by increasing reactivity of products
E) The first two responses above are correct.

by locally concentrating the reactants Correct. One of the ways enzymes work is to increase the concentrations of reactants at a single place.

Which of the following statements about enzymes is true? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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

Enzymes speed up the rate of the reaction without changing the (change in)G for the reaction. Correct. Enzymes speed up reactions by lowering the activation energy barrier for the reaction. Enzymes cannot change the DG for the reaction.

Which of the following statements about enzyme function is correct? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) Enzymes can change the equilibrium point of reactions, but they cannot speed up reactions because they cannot change the net energy output.
B) Enzymes can lower the activation energy of reactions, but they cannot change the equilibrium point 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 greatly speed up reactions, but they cannot change the net energy output because they cannot change the activation energy.
E) None of the listed responses is correct

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

A plot of reaction rate (velocity) 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 _____. (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) the enzyme was denatured
B) the cofactors required by the enzyme system lack the thermal energy required to activate the enzyme
C) there is too little activation energy available
D) the substrate becomes a competitive inhibitor at lower temperature
E) the hydrogen bonds that define the structure of the enzyme’s active site are unstable

there is too little activation energy available Correct. 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 of the following statements about enzymes is incorrect? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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

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

Which of the following statements about the active site of an enzyme is correct? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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

The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits. Correct. 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.

What is meant by the "induced fit" of an enzyme? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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

The enzyme changes its shape slightly as the substrate binds to it. Correct. The enzyme changes slightly to bind to the substrate and catalyze the reaction.

Which of the following statements correctly describe(s) the role(s) of heat in biological reactions? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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.

The first and second choices are correct. Correct. 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 of the following would lead to a faster conversion of substrate into product under these saturated conditions? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

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.

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

Which of the following environments or actions would not affect the rate of an enzyme reaction? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) cooling the enzyme
B) pH
C) heating the enzyme
D) substrate concentration
E) None of the listed responses is correct.

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

Enzyme activity is affected by pH because _____. (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the active site
B) changes in pH can cause loss of cofactors from the enzyme
C) low pH will denature all enzymes
D) most substrates don’t function well at high or low pH
E) the binding of hydrogen ions to the enzyme absorbs energy and thus there may not be enough energy to overcome the activation energy barrier

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

Which of these statements about enzyme inhibitors is true? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) When the product of an enzyme or an enzyme sequence acts as its inhibitor, this is known as positive feedback.
B) A competitive inhibitor binds to the enzyme at a place that is separate from the active site.
C) Inhibition of enzyme function by compounds that are not substrates is something that only occurs under controlled conditions in the laboratory.
D) A noncompetitive inhibitor does not change the shape of the active site.
E) The action of competitive inhibitors may be reversible or irreversible.

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

Succinylcholine is structurally almost identical to acetylcholine. 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 of the following correctly explains this observation? (see book section: Concept 8.4: Enzymes speed up metabolic reactions by lowering energy barriers)

A) Succinylcholine must be a noncompetitive inhibitor.
B) Succinylcholine must be a competitive inhibitor with acetylcholine.
C) Succinylcholine must be an allosteric regulator for this enzyme.
D) The activation energy barrier for succinylcholine hydrolysis is higher than for acetylcholine hydrolysis.
E) The active site must have the wrong configuration to permit succinylcholine binding.

Succinylcholine must be a competitive inhibitor with acetylcholine. Correct. Competitive inhibition occurs when a molecule mimics the substrate by competing with it at the active site.

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule is an example of _____. (see book section: Concept 8.5: Regulation of enzyme activity helps control metabolism)

A) allosteric regulation
B) cooperativity
C) feedback inhibition
D) noncompetitive inhibition
E) competitive inhibition

allosteric regulation Correct. The molecule in this example would be termed an allosteric activator.

Which of the following statements about allosteric proteins is/are true? (see book section: Concept 8.5: Regulation of enzyme activity helps control metabolism)

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 listed responses are correct.
E) None of the three listed responses is correct.

All of the first three listed responses are correct.

The binding of an allosteric inhibitor to an enzyme causes the rate of product formation by the enzyme to decrease. Which of the following best explains why this decrease occurs? (see book section: Concept 8.5: Regulation of enzyme activity helps control metabolism)

A) The allosteric inhibitor causes free energy change of the reaction to increase.
B) The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site.
C) The allosteric inhibitor binds to the substrate and prevents it from binding at the active site.
D) The allosteric inhibitor lowers the temperature of the active site.
E) The allosteric inhibitor binds to the active site, preventing the substrate from binding.

The allosteric inhibitor causes a structural change in the enzyme that prevents the substrate from binding at the active site. Correct. 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.

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 of anabolic and catabolic pathways, which of the following statements is most likely to be true? (see book section: Concept 8.5: Regulation of enzyme activity helps control metabolism)

A) High levels of ADP act as an allosteric activator of catabolic pathways
B) High levels of ADP act as an allosteric inhibitor of catabolic pathways.
C) High levels of ATP act as an allosteric activator of anabolic pathways.
D) High levels of ATP act as an allosteric activator of catabolic pathways.
E) High levels of ADP act as an allosteric inhibitor of anabolic pathways.

High levels of ADP act as an allosteric activator of catabolic pathways Correct. 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.