Cellular Structure, Proteins, and Metabolism
91
Fuel Metabolism Summary
Having discussed the metabolism of the three major classes of
organic molecules, we can now briefl y review how each class is
related to the others and to the process of synthesizing ATP.
Figure 3–53
illustrates the major pathways we have discussed
and the relationships between the common intermediates. All
three classes of molecules can enter the Krebs cycle through
some intermediate, and thus all three can be used as a source of
energy for the synthesis of ATP. Glucose can be converted into
fat or into some amino acids by way of common intermediates
such as pyruvate, oxaloacetate, and acetyl coenzyme A. Similarly,
some amino acids can be converted into glucose and fat. Fatty
acids cannot be converted into glucose because of the irrevers-
ibility of the reaction converting pyruvate to acetyl coenzyme
A, but the glycerol portion of triglycerides can be converted
into glucose. Fatty acids can be used to synthesize portions of
the keto acids used to form some amino acids. Metabolism is
thus a highly integrated process in which all classes of molecules
can be used, if necessary, to provide energy, and in which each
class of molecule can provide the raw materials required to syn-
thesize most but not all members of other classes.
Essential Nutrients
About 50 substances required for normal or optimal body
function cannot be synthesized by the body or are synthe-
sized in amounts inadequate to keep pace with the rates at
which they are broken down or excreted. Such substances are
known as
essential nutrients
(
Table 3–12
). Because they are
all removed from the body at some fi nite rate, they must be
continually supplied in the foods we eat.
The term
essential nutrient
is reserved for substances that
fulfi
ll
two
criteria: (1) they must be essential for health, and
(2) they must not be synthesized by the body in adequate
amounts. Thus, glucose, although “essential” for normal
metabolism, is not classifi ed as an essential nutrient because
the body normally can synthesize all it needs, from amino
acids, for example. Furthermore, the quantity of an essential
nutrient that must be present in the diet to maintain health is
not a criterion for determining whether the substance is essen-
tial. Approximately 1500 g of water, 2 g of the amino acid
methionine, but only about 1 mg of the vitamin thiamine are
required per day.
Water is an essential nutrient because the body loses far
more water in the urine and from the skin and respiratory
tract than it can synthesize. (Recall that water forms as an end
product of oxidative phosphorylation as well as from several
other metabolic reactions.) Therefore, to maintain water bal-
ance, water intake is essential.
The mineral elements are examples of substances the
body cannot synthesize or break down but that the body
continually loses in the urine, feces, and various secretions.
The major minerals must be supplied in fairly large amounts,
whereas only small quantities of the trace elements are
required.
Amino acids
Glucose
Glycerol
Fatty acids
Protein
Glycogen
Fat
R
NH
2
Glycolysis
Pyruvate
Acetyl coenzyme A
Krebs cycle
Coenzyme
2H
Oxidative
phosphorylation
ATP
O
2
H
2
O
CO
2
CO
2
ATP
ATP
NH
3
Urea
Figure 3–53
The relationships between the pathways for the metabolism of carbohydrate, fat, and protein.
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