92
Chapter 3
We have already noted that 9 of the 20 amino acids are
essential. Two fatty acids, linoleic and linolenic acid, which
contain a number of double bonds and serve important roles
in chemical messenger systems, are also essential nutrients.
Three additional essential nutrients—inositol, choline, and
carnitine—have functions that will be described in later chap-
ters but do not fall into any common category other than
being essential nutrients. Finally, the class of essential nutri-
ents known as vitamins deserves special attention.
Vitamins
Vitamins are a group of 14 organic essential nutrients required
in very small amounts in the diet. The exact chemical struc-
tures of the fi
rst vitamins to be discovered were unknown,
and they were simply identifi ed by letters of the alphabet.
Vitamin B turned out to be composed of eight substances now
known as the vitamin B complex. Plants and bacteria have the
enzymes necessary for vitamin synthesis, and we get our vita-
mins by eating either plants or meat from animals that have
eaten plants.
The vitamins as a class have no particular chemical
structure in common, but they can be divided into the
water-
soluble vitamins
and the
fat-soluble vitamins.
The water-
soluble vitamins form portions of coenzymes such as NAD
+
,
FAD, and coenzyme A. The fat-soluble vitamins (A, D, E,
and K) in general do not function as coenzymes. For example,
vitamin A (retinol) is used to form the light-sensitive pigment
in the eye, and lack of this vitamin leads to night blindness.
The specifi c functions of each of the fat-soluble vitamins will
be described in later chapters.
The catabolism of vitamins does not provide chemical
energy, although some of them participate as coenzymes in
chemical reactions that release energy from other molecules.
Increasing the amount of a vitamin in the diet beyond a certain
minimum does not necessarily increase the activity of those
enzymes for which the vitamin functions as a coenzyme. Only
very small quantities of coenzymes participate in the chemical
reactions that require them, and increasing the concentration
above this level does not increase the reaction rate.
The fate of large quantities of ingested vitamins varies
depending upon whether the vitamin is water-soluble or fat-
soluble. As the amount of water-soluble vitamins in the diet
is increased, so is the amount excreted in the urine; thus the
accumulation of these vitamins in the body is limited. On the
other hand, fat-soluble vitamins can accumulate in the body
because they are poorly excreted by the kidneys and because
they dissolve in the fat stores in adipose tissue. The intake of
very large quantities of fat-soluble vitamins can produce toxic
effects.
SECTION E SUMMARY
Cellular Energy Transfer
I. The end products of glycolysis under aerobic conditions
are ATP and pyruvate; the end products under anaerobic
conditions are ATP and lactate.
a. Carbohydrates are the only major fuel molecules that can
enter the glycolytic pathway, and the enzymes that facilitate
this pathway are located in the cytosol.
b. During anaerobic glycolysis, hydrogen atoms are transferred
to NAD
+
, which then transfers them to pyruvate to form
lactate, thus regenerating the original coenzyme molecule.
c. The formation of ATP in glycolysis occurs by substrate-level
phosphorylation, a process in which a phosphate group is
Table 3–12
Essential Nutrients
Water
Mineral Elements
7 major mineral elements (see Table 2–1)
13 trace elements (see Table 2–1)
Essential Amino Acids
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Tyrosine
Valine
Essential Fatty Acids
Linoleic
Linolenic
Vitamins
Water-soluble vitamins
B
1
: thiamine
B
2
: ribofl avin
B
6
: pyridoxine
B
12
: cobalamine
Niacin
Pantothenic acid
Folic acid
Biotin
Lipoic acid
Vitamin C
Fat-soluble vitamins
Vitamin A
Vitamin D
Vitamin E
Vitamin K
Vitamin B complex
Other Essential Nutrients
Inositol
Choline
Carnitine
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