121
CHAPTER FOUR
Cellular Metabolism
Under anaerobic conditions, however, the electron trans-
port chain has nowhere to unload its electrons, and it can no
longer accept new electrons from NADH. As an alternative,
NADH + H
+
can give its electrons and hydrogens back to
pyruvic acid in a reaction that forms
lactic acid.
Although
this regenerates NAD
+
, the buildup of lactic acid eventually
inhibits glycolysis, and ATP production declines. The lactic
acid diffuses into the blood, and when oxygen levels return
to normal the liver converts the lactic acid back into pyruvic
acid, which can F
nally enter the aerobic pathway.
PRACTICE
15
What is the role of oxygen in cellular respiration?
16
Under what conditions does a cell produce lactic acid?
PRACTICE
12
What are the F
nal products of cellular respiration?
13
What are aerobic and anaerobic reactions?
14
What is the result of glycolysis?
Anaerobic Reactions
±or glycolysis to continue, NADH + H
+
must be able to
deliver electrons to the electron transport chain, replenishing
the cellular supply of NAD
+
. In the presence of oxygen, this is
exactly what happens. Oxygen acts as the F nal electron accep-
tor at the end of the electron transport chain, enabling the
chain to continue processing electrons and recycling NAD
+
.
1
3
4
2
Glycolysis
Cytosol
Mitochondrion
ATP
2
Glucose
High-energy electrons (e
)
High-energy electrons (e
)
High-energy electrons (e
)
2e
and 2H
+
ATP
2
H
2
O
O
2
Electron
transport
chain
ATP
32–34
CO
2
Pyruvic acid
Pyruvic acid
2 CO
2
Acetyl CoA
Citric acid
Citric acid
cycle
Oxaloacetic acid
The 6-carbon sugar glucose is broken down in the cytosol
into two 3-carbon pyruvic acid molecules with a net gain
of 2 ATP and the release of high-energy electrons.
Glycolysis
The 3-carbon pyruvic acids generated by glycolysis enter
the mitochondria. Each loses a carbon (generating CO
2
)
and is combined with a coenzyme to form a 2-carbon
acetyl coenzyme A (acetyl CoA). More high-energy
electrons are released.
Each acetyl CoA combines with a 4-carbon oxaloacetic
acid to form the 6-carbon citric acid, for which the cycle
is named. For each citric acid, a series of reactions
removes 2 carbons (generating 2 CO
2
’s), synthesizes
1 ATP, and releases more high-energy electrons.
The figure shows 2 ATP, resulting directly from 2
turns of the cycle per glucose molecule that enters
glycolysis.
The high-energy electrons still contain most of the
chemical energy of the original glucose molecule.
Special carrier molecules bring the high-energy electrons
to a series of enzymes that convert much of the remaining
energy to more ATP molecules. The other products are
heat and water. The function of oxygen as the final electron
acceptor in this last step is why the overall process is called
aerobic respiration.
Citric Acid Cycle
Electron Transport Chain
1
/
2
FIGURE 4.9
Glycolysis occurs in the cytosol and does not require oxygen. Aerobic respiration occurs in the mitochondria and only in the presence
of oxygen. The products include ATP, heat, carbon dioxide, and water. Two ATP are generated by glycolysis, 2 result directly from the citric acid cycle,
and 32–34 are generated by the electron transport chain. Thus, the total yield of ATP molecules per glucose molecule is 36–38, depending on the
type of cell.
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