944
Because obtaining energy for cellular metabolism is vital,
disruptions in glycolysis or the reactions that follow it can devas-
tate health. The opening vignette to chapter 4 (p. 115) describes
how arsenic poisoning kills by blocking the products of glycoly-
sis from entering the mitochondria.
CITRIC ACID CYCLE
An acetyl coenzyme A molecule enters the citric acid cycle by
combining with a molecule of oxaloacetic acid to form citric
acid. As citric acid is produced, coenzyme A is released and thus
can be used again to form acetyl coenzyme A from pyruvic acid.
The citric acid is then changed by a series of reactions back into
oxaloacetic acid, and the cycle may repeat.
Steps in the citric acid cycle release carbon dioxide and
hydrogen atoms. More speciF cally, for each glucose molecule
metabolized in the presence of oxygen, two molecules of acetyl
coenzyme A enter the citric acid cycle. The cycle releases four
carbon dioxide molecules and sixteen hydrogen atoms. At the
same time, two more molecules of ATP form.
The released carbon dioxide dissolves in the cytoplasm and
leaves the cell, eventually entering the bloodstream. Most of the
hydrogen atoms released from the citric acid cycle, and those
released during glycolysis and during the formation of acetyl
coenzyme A, supply electrons used to produce ATP.
ATP SYNTHESIS
Note that in F gures C.1 and C.2 various metabolic reactions release
hydrogen atoms. The electrons of these hydrogen atoms contain
much of the energy associated with the chemical bonds of the orig-
inal glucose molecule. To keep this energy in a usable form, these
hydrogen atoms, with their high energy electrons, are passed in
pairs to
hydrogen carriers.
One of these carriers is NAD
+
(nico-
tinamide adenine dinucleotide). When NAD
+
accepts a pair of
hydrogen atoms, the two electrons and one hydrogen nucleus
bind to NAD
+
to form NADH, and the remaining hydrogen
nucleus (a hydrogen ion) is released as follows:
NAD
+
+ 2H
NADH + H
+
NAD
+
is a coenzyme obtained from a vitamin (niacin), and
when it combines with the energized electrons it is said to be
reduced
. Reduction results from the addition of electrons, often
as part of hydrogen atoms. Another electron acceptor, ±AD (fl
a-
vine adenine dinucleotide), acts in a similar manner, combin-
ing with two electrons and two hydrogen nuclei to form ±ADH
2
GLYCOLYSIS
Figure C.1
illustrates the chemical reactions of glycolysis. In the
early steps of this metabolic pathway, the original glucose mol-
ecule is altered by the addition of phosphate groups (
phosphory-
lation
) and by the rearrangement of its atoms. ATP supplies the
phosphate groups and the energy to drive these reactions. The
result is a molecule of fructose bound to two phosphate groups
(fructose-1,6-bisphosphate). This molecule is split through two
separate reactions into two 3-carbon molecules (glyceraldehyde-
3-phosphate). Since each of these is converted to pyruvic acid,
the following reactions, 1 through 5, must be counted twice to
account for breakdown of a single glucose molecule.
1. An inorganic phosphate group is added to glyceraldehyde-
3-phosphate to form 1,3-bisphosphoglyceric acid, releasing
two hydrogen atoms, to be used in ATP synthesis, described
later.
2. 1,3-bisphosphoglyceric acid is changed to
3-phosphoglyceric acid. As this occurs, some energy in
the form of a high-energy phosphate is transferred from
the 1,3-bisphosphoglyceric acid to an ADP molecule,
phosphorylating the ADP to ATP.
3. A slight alteration of 3-phosphoglyceric acid forms
2-phosphoglyceric acid.
4. A change in 2-phosphoglyceric acid converts it into
phosphoenolpyruvic acid.
5. ±inally, a high-energy phosphate is transferred from
the phosphoenolpyruvic acid to an ADP molecule,
phosphorylating it to ATP. A molecule of pyruvic acid remains.
Overall, one molecule of glucose is ultimately broken down
to two molecules of pyruvic acid. Also, a total of four hydrogen
atoms are released (step
a
), and four ATP molecules form (two
in step
b
and two in step
e
). However, because two molecules of
ATP are used early in glycolysis, there is a net gain of only two
ATP molecules during this phase of cellular respiration.
In the presence of oxygen, each pyruvic acid molecule is
oxidized to an acetyl group, which then combines with a mol-
ecule of coenzyme A (obtained from the vitamin pantothenic
acid) to form acetyl coenzyme A. As this occurs, two more
hydrogen atoms and one carbon dioxide molecule are released
for each molecule of acetyl coenzyme A formed. The acetyl
coenzyme A is then broken down by means of the citric acid
cycle, which
f
gure C.2
illustrates.
APPENDIX C
Cellular Respiration
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