36
UNIT 1
Organization of the Body
2
the bonds of glycogen molecules are broken to release simpler
molecules of glucose sugar (Figure 2.11b).
Exchange
, or
displacement
,
reactions
involve both synthe-
sis and decomposition. Bonds are both made and broken. In
an exchange reaction, parts of the reactant molecules change
partners, so to speak, producing different product molecules:
AB
1
C
S
AC
1
B
and
AB
1
CD → AD
1
CB
An exchange reaction occurs when ATP reacts with glucose
and transfers its end phosphate group (indicated by a circled P
in Figure 2.11c) to glucose, forming glucose-phosphate. At the
same time, the ATP becomes ADP. ±is important reaction oc-
curs whenever glucose enters a body cell, and it effectively traps
the glucose fuel molecule inside the cell.
Another group of important chemical reactions in living
systems is
oxidation-reduction reactions
, called
redox reac-
tions
for short. Oxidation-reduction reactions are decomposi-
tion reactions in that they are the basis of all reactions in which
food fuels are broken down for energy (that is, in which ATP is
produced). ±ey are also a special type of exchange reaction be-
cause electrons are exchanged between the reactants. ±e reac-
tant losing the electrons is referred to as the
electron donor
and
is said to be
oxidized
. ±e reactant taking up the transferred
electrons is called the
electron acceptor
and is said to become
reduced
.
Redox reactions also occur when ionic compounds are
formed. Recall that in the formation of NaCl (see Figure 2.6),
sodium loses an electron to chlorine. Consequently, sodium is
oxidized and becomes a sodium ion, and chlorine is reduced and
becomes a chloride ion. However, not all oxidation-reduction
molecule, as represented by its
molecular formula
(H
2
, CH
4
).
±e equation for the formation of methane may be read in terms
of molecules or moles—as
either
“four hydrogen atoms plus one
carbon atom yield one molecule of methane”
or
“four moles
of hydrogen atoms plus one mole of carbon yield one mole of
methane.” Using moles is more practical because it is impossible
to measure out one atom or one molecule of anything!
Patterns of Chemical Reactions
Most chemical reactions exhibit one of three recognizable pat-
terns: ±ey are either
synthesis
,
decomposition
, or
exchange
reactions
.
When atoms or molecules combine to form a larger, more
complex molecule, the process is a
synthesis
, or
combination
,
reaction
. A synthesis reaction always involves bond formation.
It can be represented (using arbitrary letters) as
A
1
B
S
AB
Synthesis reactions are the basis of constructive, or
anabolic
,
activities in body cells, such as joining small molecules called
amino acids into large protein molecules
(Figure 2.11a)
. Syn-
thesis reactions are conspicuous in rapidly growing tissues.
A
decomposition reaction
occurs when a molecule is bro-
ken down into smaller molecules or its constituent atoms:
AB
S
A
1
B
Essentially, decomposition reactions are reverse synthesis reac-
tions: Bonds are broken. Decomposition reactions underlie all
degradative, or
catabolic
, processes in body cells. For example,
Example
Amino acids are joined together to
form a protein molecule.
Example
Glycogen is broken down to release
glucose units.
Example
ATP transfers its terminal phosphate
group to glucose to form glucose-phosphate.
(c)
Exchange reactions
Bonds are both made and broken
(also called displacement reactions).
(b)
Decomposition reactions
Bonds are broken in larger
molecules, resulting in smaller,
less complex molecules.
(a)
Synthesis reactions
Smaller particles are bonded
together to form larger,
more complex molecules.
Amino acid
molecules
Protein
molecule
Glucose
molecules
Glycogen
Glucose
Adenosine triphosphate (ATP)
Adenosine diphosphate (ADP)
P
P
P
P
Glucose-
phosphate
P
P
+
+
O
O
O
O
~
~
~
Figure 2.11
Patterns of chemical reactions.
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