Essential Cell Biology 5th edition

104 CHAPTER 3 Energy, Catalysis, and Biosynthesis
(A) (B) (C)
heat
heat
hydraulic
machine
USEFUL
WORK
kinetic energy of falling rocks is
transformed into heat energy only
part of the kinetic energy is used to lift
a bucket of water, and a correspondingly
smaller amount is transformed into heat
the potential energy stored in the
raised bucket of water can be used to
drive hydraulic machines that carry out
a variety of useful tasks
Figure 3−29 A mechanical model
illustrates the principle of coupled
chemical reactions. (A) The spontaneous
reaction shown could serve as an analogy
for the direct oxidation of glucose to CO 2
and H 2 O, which produces only heat.
(B) The same reaction is coupled to a
second reaction, which could serve as
an analogy for the synthesis of activated
carriers. (C) The energy produced in (B) is
in a more useful form than in (A) and can
be used to drive a variety of otherwise
energetically unfavorable reactions.
QUESTION 3–7
Use Figure 3−29B to illustrate the
following reaction driven by the
hydrolysis of ATP:
X + ATP → Y + ADP + P i
A. In this case, which molecule or
molecules would be analogous to
(i) rocks at the top of the cliff,
(ii) broken debris at the bottom of
the cliff, (iii) the bucket at its highest
point, and (iv) the bucket on the
ground?
B. What would be analogous to
(i) the rocks hitting the ground in
the absence of the paddle wheel in
Figure 3−29A and (ii) the hydraulic
machine in Figure 3−29C?
To provide an everyday representation of how coupled reactions work,
let’s consider a mechanical analogy in which an energetically favorable
chemical reaction is represented by rocks falling from a cliff. The kinetic
energy of falling rocks would normally be entirely wasted in the form of
heat generated by friction when the rocks hit the ground (Figure 3−29A).
By careful design, however, part of this energy could be used to drive a
paddle wheel that lifts a bucket of water (Figure 3−29B). Because the
rocks can now reach the ground only after moving the paddle wheel,
we say that the energetically favorable reaction of rocks falling has been
directly coupled to the energetically unfavorable reaction of lifting the
bucket of water. Because part of the energy is used to do work in (B), the
ECB5 e3.30/3.29
rocks hit the ground with less velocity than in (A), and correspondingly
less energy is wasted as heat. The energy saved in the elevated bucket of
water can then be used to do useful work (Figure 3−29C).
Analogous processes occur in cells, where enzymes play the role of the
paddle wheel in Figure 3−29B. By mechanisms that we discuss in Chapter
13, enzymes couple an energetically favorable reaction, such as the oxidation
of food molecules, to an energetically unfavorable reaction, such
as the generation of activated carriers. As a result, the amount of heat
released by the oxidation reaction is reduced by exactly the amount of
energy that is stored in the energy-rich covalent bonds of the activated
carrier. That saved energy can then be used to power a chemical reaction
elsewhere in the cell.
ATP Is the Most Widely Used Activated Carrier
The most important and versatile of the activated carriers in cells is ATP
(adenosine 5ʹ-triphosphate). Just as the energy stored in the raised bucket
of water in Figure 3−29B can be used to drive a wide variety of hydraulic
machines, ATP serves as a convenient and versatile store, or currency, of
energy that can be used to drive a variety of chemical reactions in cells.
As shown in Figure 3−30, ATP is synthesized in an energetically unfavorable
phosphorylation reaction, in which a phosphate group is added
to ADP (adenosine 5ʹ-diphosphate). When required, ATP gives up this
energy packet in an energetically favorable hydrolysis to ADP and inorganic
phosphate (P i ). The regenerated ADP is then available to be used
for another round of the phosphorylation reaction that forms ATP, creating
an ATP cycle in the cell.