123
CHAPTER FOUR
Cellular Metabolism
hydrogen ions (provided by the hydrogen carriers) and an
atom of oxygen to form a water molecule:
2e
+ 2H
+
+ 1/2 O
2
H
2
O
Thus, oxygen is the F nal electron “carrier.” In the absence
of oxygen, electrons cannot continue to pass through the
electron transport chain, and the aerobic reactions of cellular
respiration grind to a halt.
lowering the electron’s energy level and transferring that
energy to ATP synthase, an enzyme complex that uses
this energy to phosphorylate ADP to form ATP
(f
g. 4.12)
.
These reactions, known as oxidation/reduction reactions,
are described further in
Appendix C, pages 944–947.
Neither glycolysis nor the citric acid cycle uses oxygen
directly, although they are part of the aerobic metabolism
of glucose. Instead, the F nal enzyme of the electron trans-
port chain gives up a pair of electrons that combine with two
Citric acid cycle
ADP +
ATP
Pyruvic acid from glycolysis
Citric acid
(start molecule)
Acetyl CoA
(replenish molecule)
Acetic acid
Oxaloacetic acid
(finish molecule)
Isocitric acid
α
-Ketoglutaric acid
CO
2
CO
2
CO
2
Succinyl-CoA
Succinic acid
FAD
FADH
2
Fumaric acid
Malic acid
Cytosol
Mitochondrion
NADH + H
+
NAD
+
NADH + H
+
NAD
+
NADH + H
+
NAD
+
CoA
CoA
CoA
CoA
P
NADH + H
+
NAD
+
P
CoA
Carbon atom
Phosphate
Coenzyme A
FIGURE 4.11
Each turn of the citric acid cycle (two “turns” or citric acids per glucose) produces one ATP directly, and two CO
2
molecules. Eight
hydrogens with high-energy electrons are released.
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