Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

64 Chapter 2: Cell Chemistry and Bioenergetics
food
molecule
energetically
favorable
reaction
ENERGY
ENERGY
ENERGY
molecule
needed by cell
energetically
unfavorable
reaction
Figure 2–31 Energy transfer and the
role of activated carriers in metabolism.
By serving as energy shuttles, activated
carrier molecules perform their function
as go-betweens that link the breakdown
of food molecules and the release of
energy (catabolism) to the energy-requiring
biosynthesis of small and large organic
molecules (anabolism).
oxidized food
molecule
activated carrier molecule
molecule
available in cell
CATABOLISM
ANABOLISM
are ATP and two molecules that are closely related to each other, NADH and
NADPH. Cells use such activated carrier molecules like money to pay for reactions
that otherwise could not take place.
MBoC6 m2.55/2.31
The Formation of an Activated Carrier Is Coupled to an
Energetically Favorable Reaction
Coupling mechanisms require enzymes and are fundamental to all the energy
transactions of the cell. The nature of a coupled reaction is illustrated by a
mechanical analogy in Figure 2–32, in which an energetically favorable chemical
reaction is represented by rocks falling from a cliff. The energy of falling rocks
would normally be entirely wasted in the form of heat generated by friction when
the rocks hit the ground (see the falling-brick diagram in Figure 2–17). By careful
design, however, part of this energy could be used instead to drive a paddle wheel
that lifts a bucket of water (Figure 2–32B). Because the rocks can now reach the
ground only after moving the paddle wheel, we say that the energetically favorable
reaction of rock falling has been directly coupled to the energetically unfavorable
reaction of lifting the bucket of water. Note that because part of the energy is
used to do work in Figure 2–32B, the rocks hit the ground with less velocity than in
Figure 2–32A, and correspondingly less energy is dissipated as heat.
Similar processes occur in cells, where enzymes play the role of the paddle
wheel. By mechanisms that we discuss later in this chapter, enzymes couple an
Figure 2–32 A mechanical model
illustrating the principle of coupled
chemical reactions. The spontaneous
reaction shown in (A) could serve as an
analogy for the direct oxidation of glucose
to CO 2 and H 2 O, which produces heat
only. In (B), the same reaction is coupled
to a second reaction; this second reaction
is analogous to the synthesis of activated
carrier molecules. 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 (C).
(A) (B) (C)
hydraulic
machines
USEFUL
WORK
heat
heat
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 kinetic energy stored in
the raised bucket of water can be
used to drive hydraulic machines that
carry out a variety of useful tasks