116
UNIT ONE
bohydrates, lipids, and proteins. A water molecule is used
for each bond that is broken. Hydrolysis of a disaccharide,
for instance, yields two monosaccharide molecules (see F
g.
4.1; read from right to left). The bond between the simple
sugars breaks, and the water molecule supplies a hydrogen
atom to one sugar molecule and a hydroxyl group to the
other. Hydrolysis is the reverse of dehydration synthesis.
Hydrolysis breaks down carbohydrates into monosac-
charides; fats into glycerol and fatty acids (see F
g. 4.2; read
from right to left); proteins into amino acids (see F g. 4.3;
read from right to left); and nucleic acids into nucleotides. It
does not occur automatically, even though in the body water
molecules are readily available to provide the necessary —H
and —OH. ±or example, water-soluble substances such as
the disaccharide sucrose (table sugar)
dissolve
in a glass of
water but do not undergo hydrolysis. Like dehydration syn-
thesis, hydrolysis requires speciF
c enzymes, discussed in the
next section, Control of Metabolic Reactions.
The reactions of metabolism are often reversible.
However, the enzyme that speeds, or catalyzes, an anabolic
reaction is often different from that which catalyzes the cor-
responding catabolic reaction.
Both catabolism and anabolism must be carefully con-
trolled so that the breakdown or energy-releasing reactions
occur at rates adjusted to the requirements of the building
up or energy-utilizing reactions. Any disturbance in this bal-
ance is likely to damage or kill cells.
this case, three hydrogen atoms are removed from a glycerol
molecule, and an —OH group is removed from each of three
fatty acid molecules, as
f
gure 4.2
shows (read from left to
right). The result is three water molecules and a single fat
molecule whose glycerol and fatty acid portions are bound
by shared oxygen atoms.
In cells, dehydration synthesis also builds protein mol-
ecules by joining amino acid molecules. When two amino
acid molecules are united, an —OH from the —COOH group
of one and an —H from the —NH
2
group of another are
removed. A water molecule forms, and the amino acid mol-
ecules join by a bond between a carbon atom and a nitrogen
atom (
f g. 4.3
; read from left to right). This type of bond,
called a
peptide bond,
holds the amino acids together. Two
such bound amino acids form a
dipeptide,
and many joined
in a chain form a
polypeptide.
Generally, a polypeptide
consisting of 100 or more amino acid molecules is called a
protein,
although the boundary between polypeptides and
proteins is not precisely deF ned. Some proteins consist of
more than one polypeptide chain.
Nucleic acids are also formed by dehydration synthesis.
This process is described later in the chapter.
Catabolism
Metabolic processes that break down larger molecules into
smaller ones constitute catabolism. An example of catabo-
lism is
hydrolysis
(hi-drol
ı˘-sis), which can decompose car-
CH
2
OH
H
H
OH
O
HO
H
Monosaccharide
+
H
HO
H
OH
CH
2
OH
H
H
OH
O
HO
H
Monosaccharide
H
HO
H
OH
CH
2
OH
H
H
OH
O
HO
H
Disaccharide
H
2
O
Water
+
H
HO
H
CH
2
OH
H
H
OH
O
HO
H
H
O
H
OH
FIGURE 4.1
Building up and breaking down molecules. A disaccharide is formed from two monosaccharides in a dehydration synthesis reaction
(arrows to the right). In the reverse reaction, hydrolysis, a disaccharide is broken down into two monosaccharides (arrows to the left).
H
C
H
Glycerol
3 fatty acid molecules
+
OH
HO
H
C
OH
HO
H
C
C
C
C
(CH
2
)
14
CH
3
(CH
2
)
14
CH
3
(CH
2
)
14
CH
3
OH
HO
O
H
O
O
C
C
C
(CH
2
)
14
CH
3
(CH
2
)
14
CH
3
(CH
2
)
14
CH
3
O
O
O
H
C
H
Fat molecule (triglyceride)
3 water
molecules
+
H
C
H
C
O
O
O
H
H
2
O
H
2
O
H
2
O
FIGURE 4.2
Forming a fat. A
glycerol molecule and three fatty acid
molecules react, yielding a fat molecule
(triglyceride) in a dehydration synthesis
reaction (arrows to the right). In the
reverse reaction, hydrolysis, a triglyceride
is broken down into three fatty acids and
a glycerol (arrows to the left).
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