A chemical reaction occurs when new bonds form
or old bonds break between atoms. Chemical reactions
involve transfers of energy. Through chemical reactions,
body structures are built and body functions are carried
out. Let’s take a look at four types of chemical reactions.
There Are Different Types
of Chemical Reactions
Chemical reactions can be classified into four types (
Fig-
ure 2.7
):
Synthesis
—In synthesis, one or more atoms, ions, or
molecules combine to form new and larger molecules.
Decomposition
—In decomposition, a molecule is split
apart.
Exchange
—An exchange consists of both synthesis and
decomposition reactions simultaneously.
Reversible
—Reversible
reactions
can
go
in
either
direction under different conditions. A double-ended
arrow or two half arrows pointing in opposite directions
indicates a reversible reaction.
All the synthesis reactions that occur in the human body
are collectively referred to as anabolism (a-NAB-o-lizm).
An example of anabolism is combining simple amino acids
to form the large proteins that form structures within cells,
speed up chemical reactions, transport substances, and so
on. (Glycogen synthesis is an example of anabolism that will
be discussed later in this chapter.) In contrast, all the de-
composition reactions that occur in the body are collectively
referred to as catabolism (ka-TAB-o-lizm). The breakdown
of large starch molecules into many small glucose molecules
during digestion is an example of catabolism.
CONCEPT CHECK
1.
What
is the significance of the valence shell
electrons of an atom?
2.
How
are ionic and covalent bonds different?"
3.
Why
do water molecules and methane mol-
ecules have different properties?"
4.
What
type of reaction is this: 2H2O "*• 2H2 + O2?
Life Uses Important Chemicals
rWILEY^O
D
i m
LEARNING OBJECTIVES
1.
Describe
the properties of water.
2.
Define
pH
and briefly explain how the body
regulates it.
3.
Discuss
the structure and functions of carbohy-
drates.
Video
O
rganic compounds are found in all living sys-
tems. Let’s take a look at the roles these com-
pounds play in maintaining homeostasis in
the human body.
Water, Acids, and Bases Make
Up a Major Part of Body Fluids
Water is the most important and most abundant inorganic
compound in all living systems, making up 55% to 60% of
body mass in lean adults. With few exceptions, most of the
volume of cells and body fluids is water. Several of its prop-
erties explain why water is such a vital com-
pound for life:
See this in your
Wiley PLUS
course.
4.
Explain
the structure and functions of lipids.
5.
Identify
the structure and function of proteins.
6.
Describe
how enzymes work.
7.
Distinguish
between DNA, RNA, and ATP.
and wastes throughout
the body. The versatility
of water as a solvent is due to its polar covalent
bonds (see Figure 2.6), and water is often referred
to as the
universal solvent.
Polar substances dissolve
easily in water and are called hydrophilic
(hydro-
= water;
-philic =
loving). Common examples of
hydrophilic solutes are sugar
and salt. The combination of
solvent plus solute is called
a
solution.
Salt
water
is
considered a solution. In contrast
solute
A substance or
material that has been
dissolved in a solvent.
1.
Water is an excellent solvent. Water is
the solvent that carries nutrients, oxygen,
solvent
A liquid or
gas in which some
other material, a solute,
has been dissolved.
molecules that
contain
mainly
non-polar
covalent
bonds
are not very water soluble and are called
hydrophobic
(-phobic =
fearing). Examples
of hydrophobic compounds include animal
fats and vegetable oils.
Life Uses Important Chemicals 31
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