58
UNIT ONE
can decompose to yield the products hydrogen and oxygen.
Decomposition is symbolized as
AB
A + B
Synthetic reactions, which build larger molecules from
smaller ones, are particularly important in growth of body parts
and repair of worn or damaged tissues. Decomposition reac-
tions digest nutrient molecules into molecules small enough to
be absorbed into the bloodstream in the small intestine.
A third type of chemical reaction is an
exchange reac-
tion
(replacement reaction). In this reaction, parts of two dif-
ferent types of molecules trade positions as bonds are broken
and new bonds are formed. The reaction is symbolized as
AB + CD
AD + CB
An example of an exchange reaction is an acid reacting with
a base, producing water and a salt. The following section
discusses this type of reaction.
Many chemical reactions are reversible. This means
the product or products can change back to the reactant
hydrogen bonds are important in protein and nucleic acid
structure. In these cases, hydrogen bonds form between
polar regions of a single, very large molecule.
PRACTICE
8
Distinguish between a molecule and a compound.
9
What is an ion?
10
Describe two ways that atoms may combine with other atoms.
11
What is a molecular formula? A structural formula?
12
Distinguish between an ion and a polar molecule.
Chemical Reactions
Chemical reactions form or break bonds between atoms,
ions, or molecules. The starting materials changed by the
chemical reaction are called
reactants
(re-ak
tantz). The
atoms, ions, or molecules formed at the reaction’s conclu-
sion are called
products.
When two or more atoms, ions,
or molecules bond to form a more complex structure, as
when hydrogen and oxygen atoms bond to form molecules
of water, the reaction is called
synthesis
(sin
the
˘-sis). Such a
reaction can be symbolized as
A + B
AB
If the bonds of a reactant molecule break to form simpler
molecules, atoms, or ions, the reaction is called
decomposi-
tion
(de-kom
po-zish
un). For example, molecules of water
H
2
HH
O
2
OO
H
2
O
O
CO
2
H
H
C
OO
FIGURE 2.8
Water is a polar molecule. (
a
) Water molecules have
equal numbers of electrons and protons but are polar because the
electrons are shared unequally, creating slightly negative ends and
slightly positive ends. (
b
) Water molecules form hydrogen bonds with
each other.
FIGURE 2.6
Structural and molecular formulas depict molecules
of hydrogen, oxygen, water, and carbon dioxide. Note the double
covalent bonds. (Triple covalent bonds are also possible between
some atoms.)
FIGURE 2.7
A three-dimensional model represents this water
molecule (H
2
O). The white parts represent the hydrogen atoms, and
the red part represents oxygen.
Slightly negative end
Slightly positive ends
H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
Hydrogen bonds
(b)
(a)
previous page 88 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 90 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off