Ch 6: An Introduction to Metabolism

Question 1

What is a catabolic pathway?

Answer 1

A metabolic pathway which releases energy by breaking down complex molecules to simpler compounds.

Question 2

What type of metabolic pathway consumes energy to build complicated molecules from simpler ones?

Answer 2

Anabolic pathway

Question 3

What is bioenergetics?

Answer 3

The study of how energy flows through living organisms.

Question 4

Gibbs free energy is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system, as in a living cell. The expression ΔG is given by:

Answer 4

ΔG = G_{final state} - G_{initial state}

Question 5

Much research has shown that only reactions with a negative ΔG can occur with no input of energy, so the value of ΔG tells us whether a particular reaction is a ___________ one.

Answer 5

spontaneous

Question 6

For a reaction to have a negative ΔG, the system must lose free energy during the change from initial state to final state. Because it has less free energy, the system in its final state is less likely to change and is therefore ____ ______ than it was previously. We can think of free energy as a measure of a system’s ___________—its tendency to change to a more stable state. Unstable systems (higher G) tend to change in such a way that they become more stable (lower G)

Answer 6

more stable

instability

Question 7

For a system at equilibrium, G is at its ______ ________ value in that system. We can think of the equilibrium state as a free-energy valley. Any change from the equilibrium position will have a positive ΔG and will not be __________.

Answer 7

lowest possible

spontaneous

Question 8

Because a system at equilibrium cannot spontaneously change, it can do __ ____. A process is spontaneous and can perform work only when it is moving toward equilibrium.

Answer 8

no work

Question 9

Based on their free-energy changes, chemical reactions can be classified as either exergonic (“energy outward”) or endergonic (“energy inward”). An exergonic reaction proceeds with a net release of free energy. Because the chemical mixture loses free energy (G decreases), ΔG is ________ for an exergonic reaction. Using ΔG as a standard for spontaneity, exergonic reactions are those that occur spontaneously. The magnitude of ΔG for an exergonic reaction represents the maximum amount of work the reaction can perform (some of the free energy is released as heat and cannot do work). The _______ the decrease in free energy, the greater the amount of work that can be done.

Answer 9

negative

greater

Question 10

An endergonic reaction requires energy input, and thus is, by definition, ___________.

Answer 10

nonspontaneous

Question 11

The overall reaction for cellular respiration is:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

686 kcal (2,870 kJ) of energy are made available for work for each mole (180 g) of glucose broken down by respiration under “standard conditions” (1 M of each reactant and product, 25°C, pH 7). Because energy must be conserved, the products of respiration store 686 kcal less ____ ______ per mole than the reactants. The products are the “exhaust” of a process that tapped the free energy stored in the bonds of the sugar molecules.

Answer 11

free energy

Question 12

It is important to realize that the breaking of bonds does ___ _______ ______; on the contrary, as you will soon see, it requires energy. The phrase “energy stored in bonds” is shorthand for the potential energy that can be released when new bonds are formed after the original bonds break, as long as the products are of lower free energy than the reactants.

Answer 12

not release energy

Question 13

An endergonic reaction is one that absorbs free energy from its surroundings. Because this kind of reaction essentially ______ free energy in molecules (G increases), ΔG is positive. Such reactions are nonspontaneous, and the magnitude of ΔG is the quantity of energy required to drive the reaction. If a chemical process is exergonic (downhill), releasing energy in one direction, then the reverse process must be endergonic (uphill), using energy.

Answer 13

stores

Question 14

If ΔG = −686  kcal/mol for respiration, which converts glucose and oxygen to carbon dioxide and water, then the reverse process—the conversion of carbon dioxide and water to glucose and oxygen—must be strongly endergonic, with ΔG = +686  kcal/mol. Such a reaction would never happen by itself.

How, then, do plants make sugar? They get the required energy (686 kcal to make a mole of glucose) by capturing light from the sun and converting its energy to ________ energy. Next, in a long series of exergonic steps, they gradually spend that chemical energy to ________ glucose molecules.

Answer 14

chemical

assemble

Question 15

Because systems at equilibrium are at a minimum of G and can do no work, a cell that has reached metabolic equilibrium is dead! The fact that metabolism as a whole is never at equilibrium is one of the defining features of ____.

Answer 15

life

Question 16

What are the 3 main kinds of work done by a cell?

Answer 16

chemical work

transport work

mechanical work

Question 17

A key feature in the way cells manage their energy resources to do this work is energy coupling, the use of an _________ _______ to drive an __________ one. ATP is responsible for mediating most energy coupling in cells, and in most cases it acts as the immediate source of energy that powers cellular work.

Answer 17

exergonic process

endergonic

Question 18

The bonds between the phosphate groups of ATP can be broken by hydrolysis. When the terminal phosphate bond is broken by the addition of a water molecule, a molecule of inorganic phosphate (HOPO₃²⁻, abbreviated Pi throughout this book) leaves the ATP. In this way, adenosine triphosphate becomes adenosine diphosphate, or ADP.

Question 19

Because their hydrolysis releases energy, the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, but the term is misleading. The phosphate bonds of ATP are not unusually ______ bonds, as “high-energy” may imply; rather, the reactants (ATP and water) themselves have high energy relative to the energy of the products (ADP and P_i). The release of energy during the hydrolysis of ATP comes from the chemical change to a state of _____ free energy, not from the phosphate bonds themselves.

Answer 19

strong

lower

Question 20

But why does the hydrolysis of ATP release so much energy? If we reexamine the ATP molecule, we can see that all three phosphate groups are negatively charged. These like charges are crowded together, and their mutual __________ contributes to the instability of this region of the ATP molecule. The triphosphate tail of ATP is the chemical equivalent of a __________ ______.

Answer 20

repulsion

compressed spring

Question 21

Because both directions of a reversible process cannot be downhill, the generation of ATP from ADP and P_i is necessarily __________.

Answer 21

endergonic

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Question 22

What type of reaction breaks the bonds that join the phosphate groups in an ATP molecule?

Answer 22

hydrolysis

Question 23

Which of the following statements about the role of ATP in cell metabolism is true?

The phosphate bonds of ATP are unusually strong bonds.

The free energy released by ATP hydrolysis has a much more negative ΔGΔG value than the hydrolysis of phosphate groups from other phosphorylated molecules.

The energy from the hydrolysis of ATP may be directly coupled to endergonic processes by the transfer of the phosphate group to another molecule.

Answer 23

The energy from the hydrolysis of ATP may be directly coupled to endergonic processes by the transfer of the phosphate group to another molecule.

Question 24

Enzymes work by…

Answer 24

…reducing the energy of activation.

Question 25

True or False?

Substrate molecules fit into the active site of an enzyme like a key fits into a lock.

Answer 25

False. As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate’s chemical groups and chemical groups on the side chains of the amino acids that form the active site. This shape change makes the active site fit even more snugly around the substrate. This induced fit is like a clasping handshake.

Question 26

Because they bind directly to the active site by covalent bonds, irreversible inhibitors __________ render an enzyme inactive. Some drugs are irreversible inhibitors, including the antibiotic penicillin (which inhibits an enzyme involved in bacterial cell-wall synthesis) and aspirin (which inhibits cyclooxygenase-2, the enzyme involved in the inflammatory reaction).

Answer 26

permanently

Question 27

Which of the following is true of energy coupling?

a. ) is the hydrolysis of ATP to ADP + P
b. ) involves transfer of a phosphate group from ATP to a reactant in an endergonic reaction
c. ) can be a barrier to the initiation of a reaction
d. ) is the use of an enzyme to reduce activation energy

Answer 27

b.) involves transfer of a phosphate group from ATP to a reactant in an endergonic reaction

Question 28

The active site itself is also not a _____ receptacle for the substrate. As the substrate enters the active site, the enzyme changes shape slightly due to interactions between the substrate’s chemical groups and chemical groups on the side chains of the amino acids that form the active site. This shape change makes the ______ site fit even more snugly around the substrate.

Answer 28

rigid

active

Question 29

What is this tightening of the binding (of the substrate to the active site) after initial contact called? This is like a clasping handshake, and it brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction.

Answer 29

induced fit

Question 30

What is a cofactor?

Answer 30

Any nonprotein molecule or ion that is required for the proper functioning of an enzyme. Cofactors can be permanently bound to the active site or may bind loosely and reversibly, along with the substrate, during catalysis.

Question 31

Where does a noncompetitive inhibitor bind?

Answer 31

To a site other than the active site (which then changes the shape of the enzyme’s active site)

Question 32

Questions from bio lab experiment on enzymes:

Starch is a _______ of glucose that is a carbohydrate reserve in plants, and is hydrolyzed by the enzyme amylase. Thus, starch is a _________ for amylase.

Answer 32

polymer

substrate

Question 33

Molecules naturally present in the cell regulate enzyme activity by acting as inhibitors. Such regulation – _________ __________ – is essential to the control of cellular metabolism.

Answer 33

selective inhibition

Question 34

__________ ___________ is the term used to describe any case in which a protein’s function at one site is affected by the binding of a regulatory molecule to a separate site. It may result in either inhibition or stimulation of an enzyme’s activity.

Answer 34

Allosteric regulation

Question 35

In the simplest kind of allosteric regulation, an activating or inhibiting regulatory molecule binds to a __________ site (sometimes called an allosteric site), often located where subunits join. The binding of an activator to a regulatory site stabilizes the shape that has functional active sites, whereas the binding of an inhibitor stabilizes the ________ ____ of the enzyme.

Answer 35

regulatory

inactive form

Question 36

[In regards to allosteric regulation]

Through this interaction of subunits, a single activator or inhibitor molecule that binds to one regulatory site will affect the active sites of ___ ________.

Answer 36

all subunits

Question 37

Cooperativity is another kind of allosteric activation. This happens when the binding of one substrate molecule to the active site of one subunit _____ ALL subunits in ______ conformation.

Answer 37

locks

active

Question 38

Cooperativity is considered allosteric regulation because even though substrate is binding to an active site, its binding affects catalysis in _______ active site.

Answer 38

another

Question 39

There is a common mode of metabolic control, called feedback inhibition, in which a metabolic pathway is halted by the inhibitory binding of its ___ _______ to an enzyme that acts early in the pathway.

Answer 39

end product

Question 40

The overall reaction for cellular respiration shows that this is an exergonic/endergonic reaction:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

ΔG = -686 kcal/mol (-2,870 kJ/mol)

Answer 40

exergonic reaction, bc ΔG is negative

Question 41

What can’t most cells harness heat to perform work?

Answer 41

Because a system can put thermal energy to work only when there is a temperature difference that results in the thermal energy flowing as heat from a warmer location to a cooler one. If temperature is uniform, as it is in a living cell, then the heat generated during a chemical reaction will simply warm a body of matter, such as the organism.

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