704
UNIT FIVE
and hair; elastin and collagen of connective tissue; plasma
proteins that regulate water balance; the muscle components
actin and myosin; certain hormones; and the antibodies that
protect against infection
(f
g. 18.6)
.
Protein molecules may also supply energy. To do this,
they must F
rst be broken down into amino acids. The amino
acids then undergo
deamination,
a process in the liver
that removes the nitrogen-containing (—NH
2
) groups from
the amino acids (see fig. 2.17). These —NH
2
groups sub-
sequently react to form a waste called
urea
(u-re
ah). The
liver therefore produces urea from amino groups released by
deamination of amino acids. The blood carries urea to the
kidneys, where it is excreted in urine.
Certain kidney disorders impair the removal of urea from the blood,
raising the blood urea concentration. A blood test called blood urea
nitrogen (BUN) determines the blood urea concentration and is often
used to evaluate kidney function.
Several pathways decompose the remaining deaminated
parts of amino acids. The speciF c pathways that are followed
depend upon the particular type of amino acid being disman-
tled. Some pathways form acetyl coenzyme A, and others
more directly lead to steps of the citric acid cycle. As energy
is released from the cycle, some of it is captured in molecules
of ATP
(f g. 18.7)
. If energy is not required immediately, the
deaminated parts of the amino acids may react to form glucose
or fat molecules in other metabolic pathways (see F g. 18.6).
A few hours after a meal, protein catabolism, through the
process of gluconeogenesis (see chapter 13, p. 509), becomes
a major source of blood glucose. However, metabolism in
most tissues soon shifts away from glucose and toward fat
catabolism as a source of ATP. Thus, energy needs are met
in a way that spares proteins for tissue building and repair,
rather than being broken down and reassembled into carbo-
hydrates to supply energy. Using structural proteins to gen-
erate energy causes the tissue-wasting of starvation.
The types and locations of the chemical bonds between carbon atoms
of fatty acid molecules aF
ect how healthful the fat is. ±or example,
monounsaturated fats (such as from avocado and olives), promote
cardiovascular health, whereas saturated fats, such as those in but-
ter or lard, contribute to heart disease. The site of the double bond
that contributes to a fat’s degree of unsaturation is also important.
Omega-3 fatty acids, which have double bonds between the third
and fourth carbons, are more healthful than omega-6 fatty acids, with
double bonds between the sixth and seventh carbons. Omega-3 fatty
acids are found in ²
sh; omega-6 fatty acids are in red meat.
The amounts and types of fats required for health vary
with individuals’ habits and goals. Linoleic acid is an essential
fatty acid, so nutritionists recommend that infants fed formula
receive 3% of the energy intake in the form of linoleic acid to
prevent deF ciency conditions. ±at intake must be sufF cient to
supply and carry fat-soluble vitamins. A typical adult diet con-
sisting of a variety of foods usually provides adequate fats.
PRACTICE
10
Which foods commonly supply lipids?
11
Which fatty acids are essential nutrients?
12
What is the role of the liver in the use of lipids?
13
What are the functions of cholesterol?
18.4
PROTEINS
Proteins
are polymers of amino acids. They have a wide
variety of functions. When dietary proteins are digested, the
resulting amino acids are absorbed and transported by the
blood to cells. Many of these amino acids are used to form
new protein molecules, as speciF
ed by DNA base sequences.
These new proteins include enzymes that control the rates
of metabolic reactions; clotting factors; the keratins of skin
Hormones
Plasma
proteins
Enzymes
Structural
proteins
Proteins
Protein
Amino acids
Digestion
Energy
+
CO
2
+
H
2
O
Fat
Glucose
FIGURE 18.6
Proteins are digested to their constituent amino acids. These amino acids are then linked, following genetic instructions, to build
new proteins. ±ree amino acids are also used to supply energy under certain conditions.
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