Cellular Structure, Proteins, and Metabolism
87
the concentrations of glucose or pyruvate in a cell and in the
control the enzymes exert in the irreversible steps in the path-
way. This control is carried out via various hormones that alter
the concentrations and activities of these key enzymes. For
example, if blood sugar levels fall below normal, certain hor-
mones are secreted into the blood and act on the liver. There,
the hormones preferentially induce the expression of the glu-
coneogenic enzymes, thus favoring the formation of glucose.
Fat Metabolism
Fat Catabolism
Triglyceride (fat) consists of three fatty acids bound to glycerol
(Chapter 2). Fat accounts for approximately 80 percent of the
energy stored in the body (
Table 3–11
). Under resting condi-
tions, approximately half the energy used by muscle, liver, and
the kidneys is derived from the catabolism of fatty acids.
Although most cells store small amounts of fat, the pre-
ponderance of the body’s fat is stored in specialized cells known
as
adipocytes.
Almost the entire cytoplasm of each of these
cells is fi lled with a single, large fat droplet. Clusters of adipo-
cytes form
adipose tissue,
most of which is in deposits under-
lying the skin or surrounding internal organs. The function of
adipocytes is to synthesize and store triglycerides during periods
of food uptake and then, when food is not being absorbed from
the intestinal tract, to release fatty acids and glycerol into the
blood for uptake and use by other cells to provide the energy
needed for ATP formation. The factors controlling fat storage
and release from adipocytes during different physiological states
will be described in Chapter 16. Here we will emphasize the
pathway by which most cells catabolize fatty acids to provide
the energy for ATP synthesis, and the pathway by which other
fuel molecules are used to synthesize fatty acids.
Figure 3–49
shows the pathway for fatty acid catabo-
lism, which is achieved by enzymes present in the mitochon-
drial matrix. The breakdown of a fatty acid is initiated by
linking a molecule of coenzyme A to the carboxyl end of the
fatty acid. This initial step is accompanied by the breakdown
of ATP to AMP and two P
i
.
The coenzyme-A derivative of the fatty acid then pro-
ceeds through a series of reactions, collectively known as
beta
oxidation,
which split off a molecule of acetyl coenzyme A
from the end of the fatty acid and transfer two pairs of hydro-
gen atoms to coenzymes (one pair to FAD and the other to
NAD
+
). The hydrogen atoms from the coenzymes then enter
the oxidative phosphorylation pathway to form ATP.
When an acetyl coenzyme A is split from the end of a
fatty acid, another coenzyme A is added (ATP is not required
for this step), and the sequence is repeated. Each passage
through this sequence shortens the fatty acid chain by two
carbon atoms until all the carbon atoms have transferred to
coenzyme A molecules. As we saw, these molecules then lead
to production of CO
2
and ATP via the Krebs cycle and oxida-
tive phosphorylation.
How much ATP is formed as a result of the total catabo-
lism of a fatty acid? Most fatty acids in the body contain 14 to
Glycerol
Glucose
Triglyceride
metabolism
Phosphoenolpyruvate
Glucose 6-phosphate
Amino acid
intermediates
Lactate
CO
2
Citrate
Krebs
cycle
CO
2
CO
2
Oxaloacetate
Amino acid
intermediates
CO
2
CO
2
Pyruvate
Acetyl coenzyme A
Figure 3–48
Gluconeogenic pathway by which pyruvate, lactate, glycerol, and
various amino acid intermediates can be converted into glucose in
the liver. Note the points at which each of these precursors, supplied
by the blood, enters the pathway.
Table 3–11
Fuel Content of a 70-kg Person
Total-
Body
Content,
kg
Energy
Content,
kcal/g
Total-
Body
Energy
Content
kcal
%
Triglycerides
15.6
9
140,000
78
Proteins
9.5
4
38,000
21
Carbohydrates
0.5
4
2,000
1
previous page 115 Vander's Human Physiology The Mechanisms of Body Function read online next page 117 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off