924
UNIT 4
Maintenance of the Body
24
During cellular respiration, most energy flows in this sequence:
Glucose
S
NADH
1
H
1
S
electron transport chain
S
proton gradient energy
S
ATP
Let’s do a little bookkeeping to summarize the net energy
gain from one glucose molecule. First, we tally the results of
substrate-level phosphorylation, giving us a net gain of 4 ATP
produced directly by substrate-level phosphorylations (2 during
glycolysis and 2 during the Krebs cycle). Next we must calculate
the much greater number of ATP molecules produced by oxida-
tive phosphorylation
(Figure 24.12)
.
For each NADH
1
H
1
that transfers a pair of high-energy
electrons to the electron transport chain, the proton gradient
generates about 2½ ATP molecules. ±e oxidation of FADH
2
is less efficient because it doesn’t donate electrons to the “top”
of the electron transport chain as does NADH
1
H
1
, but to a
lower energy level (at complex II). So, for each 2 H delivered by
FADH
2
, just about 1½ ATP molecules are produced.
Now let’s tally the results of oxidative phosphorylation. ±e
2 NADH
1
H
1
generated during glycolysis yield 5 ATP mol-
ecules. ±e 8 NADH
1
H
1
and the 2 FADH
2
produced during
the Krebs cycle are “worth” 20 and 3 ATP respectively.
Overall, complete oxidation of 1 glucose molecule to CO
2
and H
2
O by both substrate-level phosphorylation and oxida-
tive phosphorylation yields a maximum of 32 molecules of ATP
(Figure 24.12). However, there is uncertainty about the energy
yield of reduced NAD
1
that glycolysis generates
outside
the mi-
tochondria. ±e crista membrane is not permeable to reduced
NAD
1
generated in the cytosol, so NADH
1
H
1
formed dur-
ing glycolysis uses a
shuttle molecule
to deliver its extra electron
pair to the electron transport chain. It appears that cells using
the malate/aspartate shuttle harvest the whole 2½ ATP from
oxidation of reduced NAD
1
, but in cells using a different shut-
tle (the glycerol phosphate shuttle for example) the shuttle has
an energy cost. At present, the consensus is that the net energy
yield for reoxidation of reduced NAD
1
in this case is probably
the same as for FADH
2
, that is, 1½ ATP per electron pair.
So, if we deduct 2 ATP to cover the “fare” for the shuttle, our
bookkeeping comes up with a grand total of 30 ATP per glucose
as the typical energy yield. (Actually our figures are probably
still too high because, as mentioned earlier, the proton gradient
energy is also used to do other work and the electron transport
chain may not work at maximum capacity all of the time.)
Check Your Understanding
11.
Briefly, how do substrate-level and oxidative phosphorylation
differ?
12.
What happens in the glycolytic pathway if oxygen and
pyruvic acid are absent and NADH
1
H
1
cannot transfer its
“picked-up” hydrogen to pyruvic acid?
13.
What two major kinds of chemical reactions occur in
the Krebs cycle, and how are these reactions indicated
symbolically?
For answers, see Appendix H.
Glycogenesis, Glycogenolysis, and Gluconeogenesis
Although most glucose is used to generate ATP molecules, un-
limited amounts of glucose do
not
result in unlimited ATP syn-
thesis, because cells cannot store large amounts of ATP.
Mitochondrion
Cytosol
(4 ATP – 2 ATP
used for
activation
energy)
2
Acetyl
CoA
Electron transport
chain and oxidative
phosphorylation
2 NADH
+ H
+
2 NADH
+ H
+
6 NADH
+ H
+
2 FADH
2
Glucose
Glycolysis
Pyruvic
acid
by substrate-level
phosphorylation
by substrate-level
phosphorylation
by oxidative
phosphorylation
Net +2 ATP
+ about 28 ATP
+2 ATP
About
30 ATP
Typical
ATP yield
per glucose
10 NADH + H
+
× 2.5 ATP
2 FADH
2
× 1.5 ATP
Electron
shuttle across
mitochondrial
membrane
Krebs
cycle
Figure 24.12
Energy yield during cellular respiration.
(The typical energy yield of
30 ATP per glucose takes into account the 2 ATP used for shuttling NADH
1
H
1
to the electron
transport chain.)
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