Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
923
24
Homeostatic Imbalance 24.1
Studies of metabolic poisons support the chemiosmotic model
of oxidative phosphorylation. For example, cyanide (the gas used
in gas chambers) disrupts oxidative phosphorylation by binding
to cytochrome oxidase and blocking electron flow from com-
plex IV to oxygen (see Figure 24.9). Poisons called “uncouplers”
abolish the proton gradient by making the inner mitochondrial
membrane permeable to H
1
. Consequently, although the elec-
tron transport chain continues to deliver electrons to oxygen at
a furious pace and oxygen consumption rises, no ATP is made.
When ADP enters the mitochondrial matrix, it stimulates
the production of ATP. As ADP is transported in, ATP moves
out in a coupled transport process.
Summary of ATP Production
±e average person at rest uses
energy at the rate of roughly 100 kcal/hour, which is equal to
116 watts, or slightly more than that of a standard lightbulb.
±is may seem a tiny amount, but from a biochemical stand-
point it places a staggering power demand on our mitochon-
dria. Luckily, they are up to the task.
When O
2
is present, cellular respiration is remarkably ef-
ficient. Of the 686 kcal of energy present in 1 mole of glucose,
as much as 262 kcal can be captured in ATP bonds. (±e rest
is liberated as heat.) ±is corresponds to an energy capture of
about 38%, making cells far more efficient than any man-made
machines, which capture only 10–30% of the energy available
to them.
smallest rotary motors. As the protons take this “route” they cre-
ate an electrical current, and ATP synthase harnesses this electri-
cal energy to catalyze attachment of a phosphate group to ADP
to form ATP
(Figure 24.11)
. ±e enzyme’s subunits appear to
work together like gears. As the ATP synthase core rotates, ADP
and inorganic phosphate are pulled in and ATP is churned out,
completing the process of oxidative phosphorylation.
How exactly does ATP synthase work? Studies of its mo-
lecular structure are providing answers. ±e enzyme complex
consists of two major linked parts: (1) a
rotor
embedded in the
inner mitochondrial membrane (Figure 24.11), and (2) a
knob
extending into the mitochondrial matrix, which is stabilized by
a
stator
anchored in the membrane. A rod connects the rotor
and the knob. ±e current created by the downhill flow of H
1
causes the rotor and rod to rotate, just as flowing water turns a
water wheel. ±is rotation activates catalytic sites in the knob
where ADP and P
i
are combined to make ATP.
Notice something here: ±e ATP synthase works like an ion
pump running in reverse. Recall from Chapter 3 that ion pumps
use ATP as their energy source to transport ions against an elec-
trochemical gradient. Here we have ATP synthases using the
energy of a proton gradient to power ATP synthesis.
±e proton gradient also supplies energy to pump needed
metabolites (ADP, pyruvic acid, inorganic phosphate) and cal-
cium ions across the relatively impermeable inner mitochon-
drial membrane. ±e outer membrane is freely permeable to
these substances, so no “help” is needed there. ±e supply of
energy from oxidation is not limitless however, so when more
of the gradient energy is used to drive these transport processes,
less is available to make ATP.
Figure 24.10
Atomic force microscopy reveals the structure
of energy-converting ATP synthase rotor rings.
Mitochondrial matrix
Intermembrane space
ADP
+
H
+
H
+
H
+
H
+
H
+
+
+
+
+
+
+
+
+
H
+
H
+
ATP
P
i
A
stator
anchored in
the membrane holds
the knob stationary.
As the rotor spins, a
rod
connecting the
cylindrical rotor and
knob also spins.
The protruding,
stationary
knob
contains three
catalytic sites that
join inorganic
phosphate to ADP to
make ATP when the
rod is spinning.
A
rotor
in the
membrane spins
clockwise when H
+
flows through it down
the H
+
gradient.
Figure 24.11
Structure and function of ATP synthase.
previous page 957 Human Anatomy and Physiology (9th ed ) 2012 read online next page 959 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off