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Chapter 2 Chemistry, Matter, and Life 29
each type of atom must be the same in both the reactants and
the products; that is, the equations must be balanced. In the
above example, there is 1 carbon atom in the reactants and
1 carbon atom in the products. There are 4 hydrogen atoms
and 4 oxygen atoms in the reactants and the same number
in the products. Because changing a subscript in a formula
indicates a different type of molecule, equations can be balanced
only by adjusting the numerals before the molecular
formulas.
The reaction between glucose and oxygen is an important
one in the body. It provides much of the energy needed for
body processes and daily activities:
C 6
H 12
O 6
+ 6O 2
→ 6CO 2
+ 6H 2
O
Glucose Oxygen Carbon dioxide Water
This balanced chemical equation indicates that 1 molecule of glucose
reacts with 6 molecules of oxygen to yield 6 molecules of carbon
dioxide and 6 molecules of water. It is balanced because there
are 6 carbon atoms, 12 hydrogen atoms, and 18 oxygen atoms on
both the reactant and the product side of the equation.
Types of Chemical Reactions
Synthesis Reactions
When two or more simple reactants combine to form a new, more
complex product, the reaction is called synthesis, combination,
or composition. These are the anabolic reactions in the body.
This is represented symbolically by the following equation:
A + B → AB
For example, oxygen and hydrogen combine to form water
in a synthesis reaction. Two simple molecules combine to form
a more complex molecule as indicated in this equation:
2H 2
+ O 2
→ 2H 2
O
When two simple molecules combine to form a more complex
molecule by the removal of water, the reaction is called dehydration
synthesis. Many anabolic reactions in the body, for example,
the conversion of glucose to glycogen for storage, are of this
type.
Decomposition Reactions
When the bonds in a complex reactant break to form new,
simpler products, the reaction is decomposition. In the body,
these are the catabolic reactions of metabolism. When water is
used to break the bonds, the reaction is called hydrolysis. The
digestion of food involves hydrolysis reactions. Decomposition
reactions, represented symbolically by the following equation,
are the reverse of synthesis reactions:
AB → A + B
Single Replacement Reactions
Single replacement reactions, also called single displacement
reactions, occur when one element in a compound is replaced
by another. The general pattern for this type of reaction is represented
symbolically as follows:
A + BC → AC + B
Double Replacement Reactions
Double replacement reactions (also called double displacement
or exchange reactions) occur when substances in two
different compounds replace each other. These reactions are
partially decomposition and partially synthesis. The bonds in
the original reactants must break (decomposition) before the
new products can be formed (synthesis). The general equation
pattern for a double replacement reaction is the following:
AB + CD → AD + CB
where A has replaced C and C has replaced A from the original
reactants. A and C have exchanged places.
Exergonic and Endergonic Reactions
Chemical reactions are important in the body because this is the
way in which molecules are produced when they are needed. The
reactions are also important for the energy changes that occur
when bonds break and new bonds form. Energy is stored in the
chemical bonds of molecules. In exergonic (eks-er-GAHN-ik)
reactions, there is more energy stored in the reactants than in the
products. The extra energy is released. In other words, Reactants
→ Products + Released Energy. Some of the energy is released in
the form of heat, which helps maintain body temperature. A common
exergonic reaction that occurs in the body involves adenosine
triphosphate (ATP), which breaks down to adenosine diphosphate
(ADP) and a phosphate group, with the release of energy:
ATP → ADP + phosphate + energy
Endergonic (en-der-GAHN-ik) reactions have more energy
stored in the products than in the reactants. An input of energy
from exergonic reactions is needed to drive these reactions. The
products of endergonic reactions store energy in their chemical
bonds. For these reactions, Reactants + Energy → Products.
In the human body, the large carbohydrate, lipid, and protein
molecules are synthesized by endergonic reactions.
Reaction Rates
Chemical reactions occur at different rates. Some are very slow, like
the rusting of iron or the tarnishing of silver. Other reactions occur
much faster, such as the setting of epoxy cement or the burning
of paper. Some reactions occur so fast that they become explosive,
like dynamite or the gasoline in a car. The rate at which chemical
reactions occur is influenced by the nature of the reacting substances,
temperature, concentration, catalysts, and surface area.
Certain substances are more reactive than others, depending
on how readily bonds are broken and formed. Reactions involving
ions are extremely fast because there are no bonds to break.
Reactions in which covalently bonded molecules are involved
require that bonds be broken and that new ones be formed.
These occur more slowly. When hydrogen gas is mixed with
oxygen gas, the reaction to produce water proceeds very slowly
because the covalent bonds between the hydrogen atoms in the
molecules of hydrogen gas and the covalent bonds between the
oxygen atoms in the oxygen gas must first be broken. If a spark
is introduced into this mixture of hydrogen and oxygen gas,
the reaction occurs very rapidly because the spark supplies sufficient
energy to break the covalent bonds.