Cellular Structure, Proteins, and Metabolism
93
transferred from a phosphorylated metabolic intermediate
directly to ADP.
d. During aerobic glycolysis, NADH + H
+
transfers hydrogen
atoms to the oxidative phosphorylation pathway.
II. The Krebs cycle catabolizes molecular fragments derived from
fuel molecules and produces carbon dioxide, hydrogen atoms,
and ATP. The enzymes that mediate the cycle are located in
the mitochondrial matrix.
a. Acetyl coenzyme A, the acetyl portion of which is derived
from all three types of fuel molecules, is the major substrate
entering the Krebs cycle. Amino acids can also enter at several
places in the cycle by being converted to cycle intermediates.
b. During one rotation of the Krebs cycle, two molecules of
carbon dioxide are produced, and four pairs of hydrogen
atoms are transferred to coenzymes. Substrate-level
phosphorylation produces one molecule of GTP, which can
be converted to ATP.
III. Oxidative phosphorylation forms ATP from ADP and P
i
,
using the energy released when molecular oxygen ultimately
combines with hydrogen atoms to form water.
a. The enzymes for oxidative phosphorylation are located on
the inner membranes of mitochondria.
b. Hydrogen atoms derived from glycolysis, the Krebs cycle,
and the breakdown of fatty acids are delivered, most bound
to coenzymes, to the electron transport chain. The electron
transport chain then regenerates the hydrogen-free forms
of the coenzymes NAD
+
and FAD by transferring the
hydrogens to molecular oxygen to form water.
c. The reactions of the electron transport chain produce
a hydrogen-ion gradient across the inner mitochondrial
membrane. The fl ow of hydrogen ions back across the
membrane provides the energy for ATP synthesis.
d. Small amounts of reactive oxygen species, which can
damage proteins, lipids, and nucleic acids, are formed
during electron transport.
Carbohydrate, Fat, and Protein Metabolism
I. The aerobic catabolism of carbohydrates proceeds through the
glycolytic pathway to pyruvate. Pyruvate enters the Krebs cycle
and is broken down to carbon dioxide and to hydrogens, which
are then transferred to coenzymes.
a. About 40 percent of the chemical energy in glucose can
be transferred to ATP under aerobic conditions; the rest is
released as heat.
b. Under aerobic conditions, 38 molecules of ATP can
form from 1 molecule of glucose: 34 from oxidative
phosphorylation, 2 from glycolysis, and 2 from the Krebs
cycle.
c. Under anaerobic conditions, 2 molecules of ATP can form
from 1 molecule of glucose during glycolysis.
II. Carbohydrates are stored as glycogen, primarily in the liver and
skeletal muscles.
a. Different enzymes synthesize and break down glycogen.
The control of these enzymes regulates the fl ow of glucose
to and from glycogen.
b. In most cells, glucose 6-phosphate is formed by glycogen
breakdown and is catabolized to produce ATP. In liver and
kidney cells, glucose can be derived from glycogen and
released from the cells into the blood.
III. New glucose can be synthesized (gluconeogenesis) from some
amino acids, lactate, and glycerol via the enzymes that catalyze
reversible reactions in the glycolytic pathway. Fatty acids
cannot be used to synthesize new glucose.
IV. Fat, stored primarily in adipose tissue, provides about 80
percent of the stored energy in the body.
a. Fatty acids are broken down, two carbon atoms at a time, in
the mitochondrial matrix by beta oxidation, to form acetyl
coenzyme A and hydrogen atoms, which combine with
coenzymes.
b. The acetyl portion of acetyl coenzyme A is catabolized to
carbon dioxide in the Krebs cycle, and the hydrogen atoms
generated there, plus those generated during beta oxidation,
enter the oxidative phosphorylation pathway to form ATP.
c. The amount of ATP formed by the catabolism of 1 g of fat
is about 2
1
2
times greater than the amount formed from 1 g
of carbohydrate.
d. Fatty acids are synthesized from acetyl coenzyme A
by enzymes in the cytosol and are linked to
α
-glycerol
phosphate, produced from carbohydrates, to form triglycerides
by enzymes in the smooth endoplasmic reticulum.
V. Proteins are broken down to free amino acids by proteases.
a. The removal of amino groups from amino acids leaves keto
acids, which can either be catabolized via the Krebs cycle to
provide energy for the synthesis of ATP or converted into
glucose and fatty acids.
b. Amino groups are removed by (1) oxidative deamination,
which gives rise to ammonia, or by (2) transamination, in
which the amino group is transferred to a keto acid to form
a new amino acid.
c. The ammonia formed from the oxidative deamination of
amino acids is converted to urea by enzymes in the liver and
then excreted in the urine by the kidneys.
VI. Some amino acids can be synthesized from keto acids derived
from glucose, whereas others cannot be synthesized by the
body and must be provided in the diet.
Essential Nutrients
I. Approximately 50 essential nutrients are necessary for health
but cannot be synthesized in adequate amounts by the body
and must therefore be provided in the diet.
II. A large intake of water-soluble vitamins leads to their rapid
excretion in the urine, whereas large intakes of fat-soluble
vitamins lead to their accumulation in adipose tissue and may
produce toxic effects.
SECTION E KEY TERMS
α
-glycerol phosphate
88
acetyl coenzyme A (acetyl
CoA)
80
adipocyte
87
adipose tissue
87
aerobic
79
anaerobic
79
beta oxidation
87
chemiosmotic hypothesis
83
citric acid cycle
79
cytochrome
83
electron transport chain
83
essential amino acid
90
essential nutrient
91
fat-soluble vitamin
92
gluconeogenesis
86
glycogen
86
glycogenolysis
86
glycolysis
78
hydrogen peroxide
84
hydroxyl radical
84
keto acid
89
Krebs cycle
79
lactate
79
negative nitrogen balance
90
oxidative deamination
89
oxidative phosphorylation
82
positive nitrogen balance
90
protease
89
proteolysis
89
pyruvate
78
substrate-level
phosphorylation
79
superoxide anion
84
transamination
89
tricarboxylic acid cycle
79
urea
90
water-soluble vitamin
92
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