Chapter 8 - An Introduction To Metabolism

Question 1

CHANGES IN FREE ENERGY IN A CELL

The reactions in a cell either release or consume energy. The illustration in the answer summarizes some of the components of the energy changes within a cell.

Answer 1

Question 2

Interactive question 8.1

Label boxes a-k

Answer 2

a. capacity to cause change
b. kinetic
c. motion
d. potential
e. position
f. conserved
g. created nor destroyed
h. first
i. transformed or transferred
j. entropy
k. second

Question 3

Interactive question 8.2

Complete the following table to show how the free energy of a system or reaction relates to its stability, tendency for spontaneous change, equilibrium, and capacity to do work.

Answer 3

Free Energy and Metabolism

An exergonic reaction (-ΔG) proceeds with a net release of free energy and is spontaneous. The magnitude of ΔG indicates the maximum amount of work the reaction can do.

Endergonic reactions (+ΔG) are nonspontaneous; they must absorb free energy from the surroundings.

The energy released by an exergonic reaction (-ΔG) is equal to the energy required by the reverse reaction (+ΔG).

Question 4

Interactive question 8.3

Develop a concept map on free energy and ΔG. The value in this exercise is for you to wrestle with and organize these concepts for yourself.

Remember that the concept map in the answer is only one way of structuring these ideas.

Answer 4

ATP powers cellular work by coupling exergonic reactions to endergonic reactions.

Central to a cell’s bioenergeticsis energy coupling, using exergonic processes to power endergonic ones. A cell ususally uses ATP as the immediate source of energy for its mechanical, transport, and chemical work.

Question 5

Interactive question 8.4

Label the three components (a through c) of the ATP molecule shown below.

a.

b.

c.

d. Indicate which bond is likely to break. By what chemical mechanism is the bond broken?
e. Explain why this reaction releases so much energy.

Answer 5

a. adenine
b. ribose
c. three phosphate groups
d. A hydrolysis reaction breaks the terminal phosphate bond and releases a molecule of inorganic phospate: ATP + H₂O → ADP + ℗_i
e. The negatively charged phosphate groups are crowded together, and their mutual repulsion makes this area instable. The chemical change to a more stable state of lower free energy accounts for the relatively high release of energy.

Question 6

Interactive question 8.5

In this graph of an exergonic reaction with and without an enzyme catalyst, label parts a through e.

Answer 6

a. free energy
b. transition state
c. E_A (free energy of activation) without enzyme
d. E_A with enzyme
e. ΔG of reaction

Question 7

Interactive question 8.6

Outline a catalytic cycle using the following diagrammatic enzyme. Sketch two appropriate substrate molecules and two products, identify the enzyme-substrate complex, and describe the key steps of the cycle.

Answer 7

Question 8

Interactive question 8.7

Draw a competitive and noncompetitive inhibitor, and indicate where each would bind to the enzyme molecule.

Answer 8

A competitive inhibitor would mimic the shape of the substrates and compete with them for the active site.

A noncompetitive inhibitor would be a shape that could bind to another site on the enzyme molecule and would change the shape of the active site such that the substrates could no longer fit.

Question 9

Interactive question 8.8

Both ATP and ADP serve as regulators of enzyme activity. In catabolic pathways, which of these molecules would you predict would act as an inhibitor?

Which molecule would you expect to act as an activator of anabolic pathways?

Answer 9

ATP would act as an inhibitor to catabolic pathways, slowing the breakdown of fuel molecules if sufficient energy is available in the cell.

ATP may act as an activator of anabolic pathways that store resources in more complex molecules.

Question 10

1. Catabolic and anabolic pathways are often coupled in a cell because
   a. the intermediates of a catabolic pathway are used in the anabolic pathway.
   b. both pathways use the same enzymes.
   c. the free energy released from one pathway is used to drive the other.
   d. the activation energy of the catabolic pathway can be used in the anabolic pathway.
   e. their enzymes are controlled by the same activators and inhibitors.

Answer 10

c. the free energy released from one pathway is used to drive the other.

Question 11

2. When glucose and O₂ are converted to CO₂ and H₂O, changes in total energy, entropy , and free energy are as follows:
   a. -ΔH, -ΔS, -ΔG
   b. -ΔH, +ΔS, -ΔG
   c. -ΔH, +ΔS, +ΔG
   d. +ΔH, +ΔS, +ΔG
   e. +ΔH, -ΔS, +ΔG

Answer 11

b. -ΔH, +ΔS, -ΔG

Question 12

3. When amino acids join to form a protein, the following energy and entropy changes apply:
   a. +ΔH, -ΔS, +ΔG
   b. +ΔH, +ΔS, -ΔG
   c. +ΔH, +ΔS, +ΔG
   d. -ΔH, +ΔS, +ΔG
   e. -ΔH, -ΔS, +ΔG

Answer 12

a. +ΔH, -ΔS, +ΔG

Question 13

4. A negative ΔG means that
   a. the quantity G of energy is available to do work.
   b. the reaction is spontaneous.
   c. the reactants have more free energy than the products.
   d. the reaction is exergonic.
   e. all of the above are true.

Answer 13

e. all of the above are true.

Question 14

5. According to the first law of thermodynamics,
   a. for every action there is an equal and opposite reaction.
   b. every energy transfer results in an increase in disorder or entropy.
   c. the total amount of energy in the universe is conserved or constant.
   d. energy can be transferred or transformed, but disorder always increases.
   e. potential energy is converted to kinetic energy and kinetic enrgy is converted to heat.

Answer 14

c. the total amount of energy in the universe is conserved or constant.

Question 15

6. What is meant by an induced fit?
   a. The binding of the substrate is an energy-requiring process.
   b. A competitive inhibitor can outcompete the substrate for the active site.
   c. The binding of the substrate changes the shape of the active site, which can stress or bend substrate bonds.
   d. The active site creates a microenvironment ideal for the reaction.
   e. Substrates are held in the active site by hydrogen and ionic bonds.

Answer 15

c. The binding of the substrate changes the shape of the active site, which can stress or bend substrate bonds.

Question 16

7. One way in which a cell maintains metabolic disequilibrium is to
   a. siphon products of a reaction off to the next step in a metabolic pathway.
   b. provide a constant supply of enzymes for critical reactions.
   c. use feedback inhibition to turn off pathways.
   d. use allosteric enzymes that can bind to activators or inhibitors.
   e. use the energy from anabolic pathways to drive catabolic pathways.

Answer 16

a. siphon products of a reaction off to the next step in a metabolic pathway.

Question 17

8. In an experiment, changing the pH from 7 to 6 resulted in an increase in product formation. From this we could conclude that
   a. the enzyme became saturated at pH 6.
   b. the enzyme’s optimal pH is 6.
   c. this enzyme works best in a neutral pH.
   d. the temperature must have increased when the pH was changed to 6.
   e. the enzyme was in a more active shape at pH 6.

Answer 17

e. the enzyme was in a more active shape at pH 6.

Question 18

9. When substance A was added to an enzyme reaction, product formation decreased. The addition of more substrate did not increase product formation. From this we conclude that substance A could be
   a. product molecules.
   b. a cofactor.
   c. an allosteric enzyme.
   d. a competitive inhibitor.
   e. a noncompetitive inhibitor.

Answer 18

e. a noncompetitive inhibitor.

Question 19

10. The formation of ATP from ADP and inorganic phosphate
    a. is an exergonic process.
    b. transfers the phosphate to another intermediate that becomes mor reactive.
    c. produces an unstable energy compound that can drive cellular work.
    d. has a ΔG of -7.3 kcal/mol under standard conditions.
    e. involves the hydrolysis of a phosphate bond.

Answer 19

c. produces an unstable energy compound that can drive cellular work.

Question 20

11. At equilibrium
    a. no enzymes are functioning.
    b. free energy is at a minimum.
    c. the forward and backward reactions have stopped.
    d. the products and reactants have equal values of H.
    e. a reaction has a +ΔG.

Answer 20

b. free energy is at a minimum.

Question 21

12. In cooperativity,
    a. a cellular organelle contains all the enzymes needed for a metabolic pathway.
    b. a product of a pathway serves as a competitive inhibitor of an early enzyme in the pathway.
    c. a molecule bound to the active site of one subunit of an enzyme affects the active site of other subunits.
    d. the allosteric site is filled with an activator molecule.
    e. the product of one reaction serves as the substrate for the next in intricately ordered metabolic pathways.

Answer 21

c. a molecule bound to the active site of one subunit of an enzyme affects the active site of other subunits.

Question 22

13. When a cell breaks down glucose, only about 40% of the energy is captured in ATP molecules. The remaining 60% of the energy is
    a. used to increase the order necessary for life to exist.
    b. lost as heat, in accordance with the second law of thermodynamics.
    c. used to increase the entropy of the system by converting kinetic energy into potential energy.
    d. stored in starch or glycogen for used later by the cell.
    e. released when the ATP molecules are hydrolyzed.

Answer 22

b. lost as heat, in accordance with the second law of thermodynamics.

Question 23

14. An endergonic reaction could be described as one that will
    a. proceed spontaneously with the addition of activation energy.
    b. produce products with more free energy than the reactants.
    c. not be able to be catalyzed by enzymes.
    d. release energy.
    e. produce ATP for energy coupling.

Answer 23

b. produce products with more free energy than the reactants.

Question 24

15. What is most directly responsible for the specificity of a protein enzyme?
    a. its primary structure
    b. its secondary and tertiary structure
    c. the shape and characteristics of its allosteric site
    d. its cofactors
    e. the R groups of the amino acids in its active site

Answer 24

e. the R groups of the amino acids in its active site

Question 25

16. Which of the following parameters does an enzyme raise?
    a. ΔG
    b. ΔH
    c. equilibrium of a reaction
    d. speed of a reaction
    e. free energy of activation

Answer 25

d. speed of a reaction

Question 26

17. Zinc, an essential trace element, may be found bound to the active site of some enzymes. What would be the most likely function of such zinc ions?
    a. coenzyme derived from a vitamin
    b. a cofactor necessary for catalysis
    c. a substrate of the enzyme
    d. a competitive inhibitor of the enzyme
    e. an allosteric activator of the enzyme

Answer 26

b. a cofactor necessary for catalysis

Question 27

18. Which line in the diagram indicates the ΔG of the enzyme-catalyzed reaction of L → M + N?

Answer 27

line d

Question 28

19. Which line in the diagram indicates the activation energy of the noncatalyzed reaction?

Answer 28

line c

Question 29

20. Which of the following terms would best describe this reaction?
    a. nonspontaneous
    b. -ΔG
    c. endergonic
    d. coupled reaction
    e. anabolic reaction

Answer 29

b. -ΔG

Question 30

21. A reaction that is spontaneous
    a. has a +ΔG.
    b. occurs very rapidly.
    c. does not require enzyme catalysis in a cell.
    d. will decrease the entropy of a system.
    e. is exergonic.

Answer 30

e. is exergonic.

Question 31

22. In the metabolic pathway, A → B → C → D → E, what effect would molecule E likely have on the enzyme that catalyzes A → B?
    a. allosteric inhibitor
    b. allosteric activator
    c. competitive inhibitor
    d. feedback activator
    e. coenzyme

Answer 31

a. allosteric inhibitor

Question 32

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_____ 1. the totality of an organism’s chemical processes

Answer 32

metabolism

Question 33

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_____ 2. pathways that require energy to combine molecules together

Answer 33

anabolic

Question 34

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_____ 3. the energy of motion

Answer 34

kinetic

Question 35

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_____ 4. enzymes that change between two shapes, depending on whether an activator or inhibitor is bound to them

Answer 35

allosteric

Question 36

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_____ 5. term for the measure of disorder or randomness

Answer 36

entropy

Question 37

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_____ 6. the energy that must be absorbed by molecules to reach the transition state

Answer 37

free energy of activation

Question 38

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_____ 7. inhibitors that decrease an enzyme’s activity by binding to the active site

Answer 38

competitive inhibitors

Question 39

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_____ 8. organic molecules that bind to enzymes and are necessary for their functioning

Answer 39

coenzymes

Question 40

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_____ 9. regulatory device in which the product of a pathway binds to an enzyme early in the pathway

Answer 40

feedback inhibition

Question 41

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_____ 10. more reactive molecules created by the transfer of a phosphate group from ATP

Answer 41

phosphorylated compound