920
UNIT 4
Maintenance of the Body
24
water, and the energy released during those reactions is har-
nessed to attach P
i
groups to ADP, forming ATP. As we noted
earlier, this type of phosphorylation process is called
oxidative
phosphorylation
. Let’s peek under the hood of a cell’s power
plant and see how this rather complicated process works.
Most components of the electron transport chain are proteins
that are bound to metal atoms (known as
cofactors
). ±ese pro-
teins vary in composition and form multiprotein complexes that
are firmly embedded in the inner mitochondrial membrane as
shown in
Focus on Oxidative Phosphorylation
(Figure 24.8)
. For
Electron Transport Chain and Oxidative Phosphorylation
Like glycolysis, none of the reactions of the Krebs cycle use
oxygen directly. ±is is the exclusive function of the
electron
transport chain
, which carries out the final catabolic reactions
that occur on the mitochondrial cristae. However, because the
reduced coenzymes produced in the Krebs cycle are the sub-
strates for the electron transport chain, these two pathways are
coupled, and both are
aerobic
, meaning they require oxygen.
In the electron transport chain, the hydrogens removed dur-
ing the oxidation of food fuels are combined with O
2
to form
Electron trans-
port chain
and oxidative
phosphorylation
Glycolysis
ATP
ATP
ATP
Krebs
cycle
ATP
P
i
Krebs cycle
NAD
+
NAD
+
GDP +
NAD
+
FAD
NAD
+
NADH+H
+
Cytosol
Mitochondrion
(matrix)
NADH+H
+
FADH
2
NADH+H
+
Citric acid
(initial reactant)
Isocitric acid
α
-Ketoglutaric acid
Oxaloacetic acid
(pickup molecule)
Malic acid
Succinic acid
Succinyl-CoA
GTP
ADP
Carbon atom
Inorganic phosphate
Coenzyme A
CoA
CoA
CoA
Acetyl CoA
Pyruvic acid from glycolysis
CoA
Transitional
phase
Fumaric acid
NADH+H
+
CoA
P
i
CO
2
CO
2
CO
2
Figure 24.7
Simplified version of the Krebs (citric acid) cycle.
During
each turn of the cycle, two carbon atoms are removed from the substrates
as CO
2
(decarboxylation reactions); four oxidations by removal of hydrogen
atoms occur, producing four molecules of reduced coenzymes (3 NADH
1
H
1
and 1 FADH
2
); and one ATP is synthesized by substrate-level phosphorylation.
An additional decarboxylation and an oxidation reaction occur in the transi-
tional phase (at top) that converts pyruvic acid, the product of glycolysis, to
acetyl CoA, the molecule that enters the Krebs cycle pathway.
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