Essential Cell Biology 5th edition

468 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
Figure 14–18 The electrochemical proton
gradient across the inner mitochondrial
membrane is used to drive some coupled
transport processes. The charge on each
of the transported molecules is indicated for
comparison with the membrane potential,
which is negative inside, as shown. Pyruvate
and inorganic phosphate (P i ) are moved
into the matrix along with protons, as the
protons move down their electrochemical
gradient. Both are negatively charged,
so their movement is opposed by the
negative membrane potential; however,
the H + concentration gradient—the pH
gradient—is harnessed in a way that
nevertheless drives their inward transport.
ADP is pumped into the matrix and ATP
is pumped out by an antiport process
that uses the voltage gradient across the
membrane to drive the exchange. The
outer mitochondrial membrane is freely
permeable to all of these compounds due
to the presence of porins in the membrane
(not shown). The active transport of
molecules across membranes by carrier
proteins and the generation of a membrane
potential are discussed in Chapter 12.
CYTOSOL
INTERMEMBRANE SPACE
3 –
ADP
voltage gradient + + + +
drives ADP–ATP
exchange
_ _ _ _
pH gradient
drives pyruvate
import
pyruvate –
pyruvate –
3 – 4 –
ADP ATP
ADP
4 –
ATP
+ + + +
_ _ _ _
outer membrane
inner membrane
MATRIX
pH gradient
drives phosphate
import
produced in mitochondria ECB5 are e14.18/14.18
exported into the cytosol, where they are
most needed. (A small amount of ATP is used within the mitochondrion
itself to power DNA replication, protein synthesis and translocation, and
other energy-consuming reactions that occur there.) With all of this backand-forth,
a typical ATP molecule in a human cell will shuttle out of a
mitochondrion, then back in as ADP, more than once every minute.
As discussed in Chapter 3, most biosynthetic enzymes drive energetically
unfavorable reactions by coupling them to the energetically favorable
hydrolysis of ATP (see Figure 3−32). The pool of ATP in a cell is thus used
to drive a huge variety of cell processes in much the same way that a
battery is used to drive an electric engine. To serve as a readily available
energy source, the concentration of ATP in the cytosol must be kept about
10 times higher than that of ADP. When the activity of mitochondria is
halted, ATP levels fall dramatically and the cell’s battery runs down.
Eventually, energetically unfavorable reactions can no longer take place
and the cell dies. The poison cyanide, which blocks electron transport in
the inner mitochondrial membrane, causes cell death in exactly this way.
3 –
ATP
–
P
H + pyruvate –
–
P
H+ H +
H +
H + H + H +
–
P
4 –
H +
Cell Respiration Is Amazingly Efficient
The oxidation of sugars to produce ATP may seem unnecessarily complex.
Surely the process could be accomplished more directly—perhaps
by eliminating the citric acid cycle or some of the steps in the respiratory
chain. Such simplification would certainly make the chemistry easier to
learn—but it would not be as helpful for the cell. As discussed in Chapter
13, the oxidative pathways that allow cells to extract energy from food
in a usable form involve many intermediates, each differing only slightly
from its predecessor. In this way, the huge amount of energy locked up in
food molecules can be parceled out into small packets that can be captured
in activated carriers such as NADH and FADH 2 (see Figure 13−1).
Much of the energy carried by NADH and FADH 2 is ultimately converted
into the bond energy of ATP. How much ATP each of these activated carriers
can produce depends on several factors, including where its electrons
enter the respiratory chain. The NADH molecules produced in the mitochondrial
matrix during the citric acid cycle pass their high-energy