i
Figure 24.8
Oxidative phosphorylation has two phases:
Phase 1
: The electron transport chain creates a proton (H
+
)
gradient across the inner mitochondrial membrane using
high-energy electrons removed from food fuels.
Phase 2
: Chemiosmosis uses the energy of the proton
gradient to synthesize ATP.
Krebs
cycle
Glycolysis
Electron transport
chain and oxidative
phosphorylation
ATP
ATP
ATP
ADP +
Cyt c
Q
2 H
+
+
H
+
O
2
H
2
O
NAD
+
FAD
P
i
ATP
I
IV
V
II
III
ATP
synthase
Electron transport creates the proton gradient.
Phase 2
:
Chemiosmosis uses
the proton gradient
to synthesize ATP.
Intermembrane
space
Inner
mitochondrial
membrane
(crista)
Outer mitochondrial
membrane
+
+
+
+
+ +
+
+
+
+
+
+
+
+ +
+
+
+
+
+
+
+
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+
1
2
+
+
+
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Phase 1
:
NADH + H
FADH
H
+
H
+
H
+
e
-
1
2
4
Mitochondrial
matrix
2
+
e
-
e
-
e
-
3
At respiratory enzyme
complex IV, electron pairs
combine with two protons
(H
+
) and a half molecule
of O
2
, forming water.
The electrons are transferred from
one complex to another in the
membrane. Each complex is reduced
and then oxidized, releasing energy
that is used to pump H
+
into the
intermembrane space. This creates an
electrochemical gradient between the
matrix and the intermembrane space.
Coenzyme Q (ubiquinone) and
cytochrome c are mobile carriers that
shuttle between the larger complexes.
Reduced coenzymes
(NADH + H
+
and FADH
2
)
deliver electrons picked up
during the oxidation of
food fuels to respiratory
enzyme complexes I and II.
Complex V, called ATP
synthase, harnesses energy of
the proton gradient to
synthesize ATP. As H
+
flows
back across the membrane
through ATP synthase, the
synthase rotor spins, causing P
i
to attach to ADP, forming ATP.
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FOCUS
Oxidative Phosphorylation
 
921
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