Chapter 8 Practice Questions

What does this represent? Is it endergonic or exergonic? C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Breakdown of glucose; exergonic (increased entropy)

T/F: The orderliness of a system is related to the change in enthalpy (ΔH)

False

T/F: Because energy must be conserved, organisms constantly recycle energy and thus need no input of energy.

False Energy is conserved, but utilization of energy by organisms requires that some energy be lost as heat. This energy must be replaced by new energy from the environment

T/F: The process of photosynthesis produces energy that the plants use to grow.

False Energy is not produced by photosynthesis; it is converted (consumed) form energy in the form of sunlight into energy in the form of chemical bonds (sugars)

T/F: Growth of plants/crops must occur spontaneously.

False Growth of plants/crops requires an input of energy. Spontaneous processes require no input of energy

T/F: The active site allows the reaction to occur under the same environmental conditions as the reaction without the enzyme.

False The active site often changes the local environment to one that favors the reaction by lowering the activation energy.

T/F: The entropy of the universe must decrease to account for the increased entropy associated with plant growth

False The growth of plants represents a decrease in entropy, which by the second law requires a corresponding increase in entropy of the plant's surroundings (the universe)

T/F: The metabolism of an organism is isolated from its surroundings.

False This describes a closed system.

T/F: Heat produced by the organism is conserved in the organism and not lost to the environment.

False This would describe a closed system; organisms constantly lose heat to their environment as a consequence of being open systems.

Which of the following reactions could be coupled to an endergonic reaction with ΔG =+3.56 kJ/mol? A → B, ΔG = +6.08 kJ/mol C → D, ΔG = +3.56 kJ/mol E → F, ΔG = 0 kJ/mol G → H, ΔG = -1.22 kJ/mol I → J, ΔG = -5.91 kJ/mol

I → J, ΔG = -5.91 kJ/mol

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. What explains this phenomenon?

In the cell, the hydrolysis of ATP is coupled to other endergonic reactions = means that less of the free energy is released as heat

What do the sign and magnitude of the ΔG of a reaction tell us about the speed of the reaction?

Neither the sign nor the magnitude of ΔG has anything to do with the speed of a reaction.

Noncompetitive inhibition occurs when a molecule dissimilar to the substrate interacts with the enzyme at a site that is separate from the active site. That being said, would an increase in substrate increase the reaction rate? If no, why?

No; because the enzyme is being inhibited not at the active site, but at an allosteric site

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?

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

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?

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

What is meant by the "induced fit" of an enzyme?

The enzyme changes its shape slightly as the substrate binds to it.

What best characterizes the role of ATP in cellular metabolism?

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

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?

The reaction that proceeds to convert A and B to C and D is endergonic; the products are more organized than the reactants. (C and D contain more energy than do A and B; therefore they are more organized and their construction required an input of energy > the difference between the energies of the reactants and products is a positive number)

T/F: Facilitated diffusion is a process that does not involve work and occurs spontaneously.

True

T/F: The action of competitive inhibitors may be reversible or irreversible.

True

T/F: 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

True

T/F: When ATP is hydrolyzed without coupling, all of the energy is released as heat.

True (like in a test tube)

T/F: Organisms acquire energy from, and lose energy to, their surroundings.

True Open systems are those in which energy and materials can be exchanged between the system and its surroundings

T/F: In any spontaneous process, the free energy of a system decreases.

True The change in free energy equals the change in total energy minus the change in entropy times the temperature.

T/F: Anabolic pathways synthesize more complex organic molecules using energy derived from catabolic pathways

True The synthesis of complex molecules in anabolic pathways requires an input of energy from catabolic pathways

T/F: A substrate free in solution has no effect on the structure of the enzyme

True There must be a closer interaction for structural changes to occur.

T/F: Heat is released in the hydrolysis of ATP

True but this heat is not available to do cellular work

Does this statement describe the first law of thermodynamics: Energy can be freely transformed among different forms as long as the total energy is conserved.

Yes The first law of thermodynamics states that energy can be transformed but can be neither created nor destroyed.

Does this statement describe the second law of thermodynamics: The aerobic respiration of one molecule of glucose produces six molecules each of carbon dioxide and water.

Yes The second law of thermodynamics states that every energy transformation makes the universe more disordered—carbon dioxide and water are more disordered than glucose.

Which of the following statements about enzymes is INCORRECT? a. An enzyme is consumed during the reaction it catalyzes. b. Enzymes can be used to accelerate both anabolic and catabolic reactions. c. An enzyme lowers the activation energy of a chemical reaction. d. Most enzymes are proteins. e. An enzyme is very specific in terms of the substrate to which it binds.

a. An enzyme is consumed during the reaction it catalyzes.

In general, the hydrolysis of ATP drives cellular work by __________. a. releasing free energy that can be coupled to other reactions b. lowering the activation energy of the reaction c. acting as a catalyst d. changing to ADP and phosphate e. releasing heat

a. releasing free energy that can be coupled to other reactions (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... a. the potential energy of the products is less than the potential energy of the reactants b. activation energy is required c. it absorbs more energy d. the products are less complex than the reactants e. activation energy exceeds net energy release

a. the potential energy of the products is less than the potential energy of the reactants (the formation of new bonds releases more energy than was invested in breaking the old bonds)

Which of the following statements about allosteric proteins is/are true? a. They are acted on by inhibitors. b. They are sensitive to environmental conditions. c. They exist in active and inactive conformations. d. All of the responses are correct e. None are correct

all are true

An enzyme could increase the rate of a reaction by... orienting substrates. inducing strain on the substrate. reacting chemically with the substrate. donating a proton to the substrate. All of the above

all of the above

The process of stabilizing the structure of an enzyme in its active form by the binding of a molecule outside the active site is an example of...

allosteric activation

Which of the following statements about enzymes is/are true? a. Enzymes increase the rate of a reaction by raising the activation energy for reactions. b. Enzymes speed up the rate of the reaction without changing the DG for the reaction. c. Enzymes react with their substrate (form chemical bonds), forming an enzyme-substrate complex, which irreversibly alters the enzyme. d. The more heat that is added to a reaction, the faster the enzymes will function. e. All of the listed responses are correct

b. Enzymes speed up the rate of the reaction without changing the DG for the reaction.

How do enzymes lower activation energy? a. by increasing reactivity of products b. by locally concentrating the reactants c. by harnessing heat energy to drive the breakage of bonds between atoms d. The first two responses are correct. e. The second and third choices are correct.

b. by locally concentrating the reactants

Which of the following statements about the active site of an enzyme is correct? a. Coenzymes are rarely found in the active site of an enzyme. b. The active site allows the reaction to occur under the same environmental conditions as the reaction without the enzyme. c. The active site may resemble a groove or pocket in the surface of a protein into which the substrate fits. d. The structure of the active site is not affected by changes in temperature. e. The active site has a fixed structure (shape).

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

A competitive inhibitor of the activity of an enzyme... a. binds to an allosteric site on the enzyme. b. permanently inhibits the activity of the enzyme. c. binds to the active site of the enzyme. d. is a feedback inhibitor. e. binds to the same extent at all substrate concentrations.

c. binds to the active site of the enzyme.

The active site of an enzyme: a. never changes shape. b. does not form chemical bonds with substrates. c. determines, by its structure, the specificity of the enzyme. d. is only good once. e. changes the equilibrium constant of the reaction

c. determines, by its structure, the specificity of the enzyme.

Organisms are described as thermodynamically open systems. This means that _____. a. the metabolism of an organism is isolated from its surroundings b. organisms can reverse the increase in entropy c. organisms acquire energy from their surroundings d. organisms are capable of circumventing the second law of thermodynamics e. organisms create their own energy

c. organisms acquire energy from their surroundings

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

negative

The free energy derived from the hydrolysis of ATP can be used to perform many kinds of cellular work. Which of the following is an example of the cellular work involved in the production of electrochemical gradients? a. the beating of cilia b. the chemical synthesis of ATP c. proton movement against a gradient of protons d. facilitated diffusion e. chromosome movement on microtubules

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

In an enzyme molecule with multiple subunits, the binding of one substrate molecule to the active site of one subunit causes all the subunits to assume their active conformation, via the mechanism of induced fit. This is called ____________. allosteric activation allosteric inhibition competitive inhibition cooperativity feedback inhibition

cooperativity

Which of the following statements correctly describe(s) the role or roles of heat in biological reactions? 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 always 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.

Metabolic pathways in cells are typically far from equilibrium. Which of the following processes tend(s) to keep these pathways away from equilibrium? 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.

Which of the following reactions would be endergonic? a. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O b. ATP → ADP + Pi c. HCl → H+ + Cl- d. glucose + fructose → sucrose e. All of the listed responses are correct.

d. glucose + fructose → sucrose (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.

Enzymes can __________, but they cannot __________ because they cannot __________. a. greatly speed up reactions ... change the net energy output ... change the activation energy b. change the equilibrium point of reactions ... speed up reactions ... change the net energy output c. greatly speed up reactions ... change the activation energy ... change the net energy output d. lower the activation energy of reactions ... change the equilibrium point ... change the net energy output e. none of the above

d. lower the activation energy of reactions ... change the equilibrium point ... change the net energy output

An exergonic (spontaneous) reaction is a chemical reaction that a. occurs only when an enzyme or other catalyst is present b. cannot occur outside of a living cell c. leads to a decrease in the entropy of the universe d. releases energy when proceeding in the forward direction e. is common in anabolic pathways

d. releases energy when proceeding in the forward direction (exergonic reactions proceed with a net release of free energy, and they occur spontaneously)

If the entropy of a living organism is decreasing, which of the following is most likely to be occurring simultaneously? a. The entropy of the organism's environment must also be decreasing. b. Heat is being used by the organism as a source of energy. c. The first law of thermodynamics is being violated. d. In this situation, the second law of thermodynamics must not apply. e. Energy input into the organism must be occurring to drive the decrease in entropy

e. Energy input into the organism must be occurring to drive the decrease in entropy ~ 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 thermodynamics is INCORRECT? a. Free energy is given off in an exergonic reaction. b. Free energy can be used to do work. c. A spontaneous reaction is exergonic. d. Free energy tends always to a minimum. e. Entropy always tends towards a minimum.

e. Entropy always tends towards a minimum.

Consider this reaction: Glucose + 6 O2 → 6 CO2 + 6 H2O (ΔG = -686 kcal/mol). Which of the following statements about this reaction is NOT true? a. The reaction is spontaneous in a thermodynamic sense. b. A small amount of energy (activation energy) must be supplied to start the reaction, which then proceeds with a release of energy. c. The reaction is exergonic. d. The reaction results in an increase in entropy. e. The reaction must be coupled to an exergonic reaction.

e. The reaction must be coupled to an exergonic reaction.

Enzyme activity is affected by pH because __________.

high or low pH may disrupt hydrogen bonding or ionic interactions and thus change the shape of the active site

The decrease in entropy associated with life must be compensated for by ________ entropy in the environment in which life exists.

increase The second law of thermodynamics demands that total entropy increase with any reaction.

From the equation ΔG = ΔH - TΔS it is clear that... a decrease in the system's total energy will _________ the probability of spontaneous change, _________ the entropy of a system will increase the probability of spontaneous change, and increasing the temperature of a system will _________ the probability of spontaneous change

increase; increasing; increase

Which of the following environments or actions would NOT affect the rate of an enzyme reaction? substrate concentration pH heating the enzyme cooling the enzyme None of the listed responses is correct.

none of the listed responses

Which of the following is changed by the presence of an enzyme in a reaction? the G value for the products the magnitude of ΔG the activation energy the sign of ΔG the G value for the reactants

the activation energy (an enzyme lowers the amount of energy required to get the reactants to the transition state)

The speed of the reaction is determined by what two factors?

the activation energy barrier of the reaction and the temperature (which determines how many reactants have the energy to overcome the barrier)

What determines the sign of ΔG for a reaction?

the free energy of the reactants and the free energy of the products (Products - Reactants)

The formation of glucose-6-phosphate from glucose is an endergonic reaction and is coupled to which of the following reactions or pathways?

the hydrolysis of ATP (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)

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 __________.

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

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 __________.

there is too little activation energy available (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)

As the kinetic energy of the reactants increases, the reactants are more likely to interact with what two things?

with each other directly, or with the active site of an enzyme (this would increase the reaction rate)

Which part of the equation ΔG = ΔH - TΔS tells you if a process is spontaneous?

ΔG

If, during a process, the system becomes more ordered, then ΔG is positive ΔS is negative ΔG is negative ΔH is negative ΔH is positive

ΔS is negative

When one molecule is broken down into six component molecules, which of the following will always be true? ΔG is positive. ΔH is negative. ΔS is positive. ΔS is negative. An input of free energy is needed.

ΔS is positive (the large increase in disorder associated with this reaction means that entropy (ΔS) increases) also ΔG would be negative (no energy input is needed)