42
UNIT 1
Organization of the Body
2
elements, as well as with other carbon atoms. As a result, car-
bon can help form long, chainlike molecules (common in
fats), ring structures (typical of carbohydrates and steroids),
and many other structures that are uniquely suited for spe-
cific roles in the body.
As you will see shortly, many biological molecules (carbo-
hydrates and proteins for example) are polymers.
Polymers
are chainlike molecules made of many similar or repeating
units (
monomers
), which are joined together by dehydra-
tion synthesis
(Figure 2.14)
. During dehydration synthe-
sis, a hydrogen atom is removed from one monomer and
a hydroxyl group is removed from the monomer it is to be
joined with. As a covalent bond unites the monomers, a wa-
ter molecule is released. Tis removal of a water molecule at
the bond site occurs each time a monomer is added to the
growing polymer chain.
compounds are defined as compounds that lack carbon. You
should be aware of a few irrational exceptions to this generaliza-
tion: Carbon dioxide and carbon monoxide, for example, con-
tain carbon but are considered inorganic compounds.
For the most part, organic molecules are very large mol-
ecules, but their interactions with other molecules typically
involve only small, reactive parts of their structure called
func-
tional groups
(acid groups, amines, and others). Te most im-
portant functional groups involved in biochemical reactions are
illustrated in Appendix B.
What makes carbon so special that “living” chemistry de-
pends on its presence? ±o begin with, no other
small
atom is
so precisely
electroneutral
. Te consequence of its electro-
neutrality is that carbon never loses or gains electrons. In-
stead, it always shares them. Furthermore, with four valence
shell electrons, carbon forms four covalent bonds with other
+
+
Glucose
O
OH
OH
HO
O
HO
H
H
CH
2
OH
H
HO
Fructose
H
2
O
Water is
released
Monomers linked by covalent bond
Monomers linked by covalent bond
Water is
consumed
H
2
O
HOCH
2
HOCH
2
HOCH
2
Sucrose
O
H
OH
H
OH H
H
HO
H
OH
O
O
OH
HO
H
H
CH
2
OH
H
HOCH
2
H
H
OH H
H
HO
H
OH
OH
O
H
2
O
(a)
Dehydration synthesis
Monomers are joined by removal of OH from one monomer
and removal of H from the other at the site of bond formation.
+
O
H
2
O
(b)
Hydrolysis
Monomers are released by the addition of a water molecule, adding OH to one monomer and H to the other.
(c)
Example reactions
Dehydration synthesis of sucrose and its breakdown by hydrolysis
Monomer 1
Monomer 2
Monomer 1
Monomer 2
HO
Figure 2.14
Dehydration synthesis and hydrolysis.
Biological molecules are formed
from their monomers, or units, by dehydration synthesis and broken down to the monomers
by hydrolysis reactions.
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