946
APPENDIX C
hydrogen ions (formed at the beginning of the sequence) to
combine with an atom of oxygen. This process produces a water
molecule:
2e
–
+ 2H
+
+ 1/2 O
2
→
H
2
O
Thus, oxygen is the F nal electron acceptor. In the absence of
oxygen, electrons cannot pass through the electron transport chain,
NAD+ cannot be regenerated, and aerobic respiration halts.
As electrons pass through the electron transport chain,
energy is released. Some of this energy is used by a mechanism
involving the ATP synthase to combine phosphate and ADP by a
high-energy bond (phosphorylation), forming ATP.
of the inner mitochondrial membrane provide surface area on
which the energy reactions take place. In a muscle cell, the inner
mitochondrial membrane, if stretched out, may be as much as
forty-F
ve times as long as the cell membrane!
Note in
f gure C.3
, that as electrons pass through the elec-
tron transport chain, hydrogen ions are forced into the space
between the inner and outer mitochondrial membranes. This
sets up a concentration gradient for the hydrogen ions to diffuse
back into the mitochondrial matrix via the enzyme complex
ATP
synthase
. Much of their energy is used by the synthase to synthe-
size ATP.
The F
nal cytochrome of the electron transport chain (cyto-
chrome oxidase) gives up a pair of electrons and causes two
CoA
Fumaric acid
Succinic acid
FADH
2
FAD*
Succinyl CoA
Pyruvic acid
Isocitric acid
Citric acid
Oxaloacetic
acid
Malic acid
Citric
acid
cycle
ATP
NADH
+
H
+
NAD
+
CO
2
2H
NADH
+
H
+
NAD
+
2H
NADH
+
H
+
NAD
+
2H
NADH
+
H
+
NAD
+
2H
+
CO
2
2H
CO
2
Acetyl
coenzyme A
Coenzyme A
CoA
CoA
ADP
α
-Ketoglutaric acid
FIGURE C.2
Chemical reactions of the citric acid cycle. NADH and FADH
2
molecules carrying hydrogens are highlighted.
