122
UNIT ONE
This book presents the theoretical yield of the aerobic reactions—up to
36 ATP per glucose molecule. In fact, more energy may be required to
complete these reactions than once thought. Estimates taking this into
account indicate a yield of ATP less than the theoretical maximum.
The aerobic reactions begin with pyruvic acid produced
in glycolysis moving from the cytosol into the mitochondria
(f
g. 4.11)
. From each pyruvic acid molecule, enzymes inside
the mitochondria remove two hydrogen atoms, a carbon
atom, and two oxygen atoms, generating NADH and a CO
2
and leaving a 2-carbon acetic acid. The acetic acid then com-
bines with a molecule of coenzyme A to form acetyl CoA.
CoA “carries” the acetic acid into the citric acid cycle.
Citric Acid Cycle
The citric acid cycle begins when a 2-carbon acetyl CoA mol-
ecule combines with a 4-carbon oxaloacetic acid molecule to
form the 6-carbon citric acid and CoA (± g. 4.11). The citric
acid is changed through a series of reactions back into oxalo-
acetic acid. The CoA can be used again to combine with ace-
tic acid to form acetyl CoA. The cycle repeats as long as the
mitochondrion receives oxygen and pyruvic acid.
The citric acid cycle has three important consequences:
1. One ATP is produced directly for each citric acid
molecule that goes through the cycle.
2. For each citric acid molecule, eight hydrogen atoms with
high-energy electrons are transferred to the hydrogen
carriers NAD
+
and the related FAD (fl
avine adenine
dinucleotide):
NAD
+
+ 2H
NADH + H
+
FAD + 2H
FADH
2
3. As the 6-carbon citric acid reacts to form the 4-carbon
oxaloacetic acid, two carbon dioxide molecules are
produced.
The carbon dioxide produced by the formation of acetyl
CoA and in the citric acid cycle dissolves in the cytoplasm,
diffuses from the cell, and enters the bloodstream. Eventually,
the respiratory system excretes the carbon dioxide.
Electron Transport Chain
The hydrogen and high-energy electron carriers (NADH and
FADH
2
) generated by glycolysis and the citric acid cycle now
hold most of the energy contained in the original glucose
molecule. To couple this energy to ATP synthesis, the high-
energy electrons are handed off to the electron transport
chain, a series of enzyme complexes that carry and pass elec-
trons along from one to another. These complexes dot the
folds of the inner mitochondrial membranes (see chapter 3,
p. 84), which, if stretched out, may be forty-five times
as long as the cell membrane in some cells. The electron
transport chain passes each electron along, gradually
Human muscle cells working so strenuously that their production of
pyruvic acid exceeds the oxygen supply produce lactic acid. In this
“oxygen debt,” the muscle cells use solely the anaerobic pathway,
which provides fewer ATPs per glucose molecule than do the aero-
bic reactions. The accumulation of lactic acid contributes to muscle
fatigue and cramps. Walking after cramping can increase bloodF
ow
that hastens depletion of lactic acid, easing the pain.
Aerobic Reactions
If enough oxygen is available, the pyruvic acid generated by
glycolysis can continue through the aerobic pathways (see
fig. 4.9). These reactions include the synthesis of
acetyl
coenzyme A
(as
e
˘-til ko-en
zı¯m A) or acetyl CoA, the citric
acid cycle, and the electron transport chain. In addition to
carbon dioxide and water, the aerobic reactions yield up to
thirty-six ATP molecules per glucose.
Phase 1
priming
Phase 2
cleavage
Phase 3
oxidation and
formation of
ATP and release
of high energy
electrons
2 ADP
2
NADH + H
+
2 NAD
+
2
NADH + H
+
2 NAD
+
P
ATP
P
P
P
Glyceraldehyde
phosphate
Glucose
Dihydroxyacetone
phosphate
2
4 ADP
ATP
4
Fructose-1,6-diphosphate
O
2
O
2
2 Pyruvic acid
2 Lactic acid
To citric acid cycle
and electron transport
chain (aerobic pathway)
Carbon atom
Phosphate
P
P
FIGURE 4.10
Glycolysis breaks down glucose in three stages: (1)
phosphorylation, (2) splitting, and (3) production of NADH and ATP.
Each glucose molecule broken down by glycolysis yields a net gain of
2 ATP.
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