945
APPENDIX C
The molecules that act as electron carriers comprise an
electron transport chain
described in Chapter 4 (pp. 122–123).
As electrons are passed from one carrier to another, the carriers
are alternately reduced and oxidized as they accept or release
electrons. The transported electrons gradually lose energy as
they proceed down the chain.
Among the members of the electron transport chain are sev-
eral proteins, including a set of iron-containing molecules called
cytochromes.
The chain is located in the inner membranes of
the mitochondria (F
g. C.3; also see chapter 3, p. 84). The folds
(F g. C.2). In their reduced states, the hydrogen carriers NADH
and ±ADH
2
now hold most of the energy once held in the bonds
of the original glucose molecule.
±igure 4.12 shows that NADH can release the electrons and
hydrogen nucleus. Since this reaction removes electrons, the
resulting NAD
+
is said to be
oxidized
. Oxidation results from the
removal of electrons, often as part of hydrogen atoms; it is the
opposite of reduction. The two electrons this reaction releases
pass to a series of electron carriers. The regenerated NAD
+
can
once again accept electrons, and is recycled.
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
ATP
ADP
ATP
ADP
ADP
ADP
ADP
ADP
Fructose-1, 6-bisphosphate
Dihydroxyacetone phosphate
1, 3-Bisphosphoglyceric acid
P
P
P
P
P
Glyceraldehyde-3-phosphate
P
P
P
P
1, 3-Bisphosphoglyceric acid
ATP
ATP
ATP
ATP
P
P
3-Phosphoglyceric acid
P
3-Phosphoglyceric acid
P
P
P
2H
Glyceraldehyde-3-phosphate
Pyruvic acid
Pyruvic acid
2-Phosphoglyceric acid
P
P
2-Phosphoglyceric acid
Phosphoenolpyruvic acid
P
P
Phosphoenolpyruvic acid
2H
NADH + H
+
NAD
+
NADH + H
+
NAD
+
FIGURE C.1
Chemical reactions of glycolysis. There is a net production of 2 ATP molecules from each glucose molecule. The four
hydrogen atoms released provide high-energy electrons that may be used to generate ATP in the electron transport chain, described later.
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