molecules. This is of great importance for physiology, because
the shape of large molecules often determines their functions.
For example, the ability of a speciﬁ c cell membrane “receptor”
to recognize a large protein depends partly on the shape of
both the protein and the receptor.
Water is the most common molecule in the body. Out of every
100 molecules, 99 are water. The covalent bonds linking the
two hydrogen atoms to the oxygen atom in a water molecule
are polar. Therefore, the oxygen in water has a slight nega-
tive charge, and each hydrogen has a slight positive charge.
The positively polarized regions near the hydrogen atoms of
one water molecule are electrically attracted to the negatively
polarized regions of the oxygen atoms in adjacent water mol-
ecules by hydrogen bonds (see Figure 2–4).
At body temperature, water exists as a liquid because the
weak hydrogen bonds between water molecules are continu-
ously forming and breaking. If the temperature is increased,
the hydrogen bonds break more readily, and molecules of
water escape into the gaseous state. However, if the tempera-
ture is lowered, hydrogen bonds break less frequently, so larger
and larger clusters of water molecules form until at 0°C water
freezes into a continuous crystalline matrix—ice.
Water molecules take part in many chemical reactions of
the general type:
—OH + H—R
In this reaction, the covalent bond between R
one between a hydrogen atom and oxygen in water are broken,
and the hydroxyl group and hydrogen atom are transferred
, respectively. Reactions of this type are known
as hydrolytic reactions, or
Many large molecules
in the body are broken down into smaller molecular units by
hydrolysis, usually with the assistance of a class of molecules
called enzymes. These reactions are usually reversible, a pro-
cess known as
In dehydration, one net water
molecule is removed to combine two small molecules into one
larger one. Dehydration reactions are responsible for, among
other things, building proteins and other polymers required
by the body.
Other properties of water that are of importance in phys-
iology include the colligative properties—those that depend
on the number of dissolved substances, or
For example, water moves between ﬂ
uid compartments by
the process of osmosis, which you will learn about in detail in
Chapter 4. In osmosis, water moves from regions of low solute
concentrations to regions of high solute concentrations. The
characteristics of solutes and solutions are described next.
Substances dissolved in a liquid are known as solutes, and the
liquid in which they are dissolved is the
solve in a solvent to form a
Water is the most abun-
dant solvent in the body, accounting for
60 percent of total
body weight. A majority of the chemical reactions that occur
in the body involve molecules that are dissolved in water,
either in the intracellular or extracellular ﬂ
uid. However, not
all molecules dissolve in water.
In order to dissolve in water, a substance must be electrically
attracted to water molecules. For example, table salt (NaCl) is a
solid crystalline substance because of the strong electrical attrac-
tion between positive sodium ions and negative chloride ions.
This strong attraction between two oppositely charged ions is
known as an
When a crystal of sodium chloride is
placed in water, the polar water molecules are attracted to the
charged sodium and chloride ions (
). Clusters of
water molecules surround the ions, allowing the sodium and
chloride ions to separate from the salt crystal and enter the
water—that is, to dissolve.
Molecules having a number of polar bonds and/or ion-
ized groups will dissolve in water. Such molecules are said to be
or “water-loving.” Thus, the presence of ionized
groups (such as carboxyl and amino groups) or of polar groups
(such as hydroxyl groups) in a molecule promotes solubility in
water. In contrast, molecules composed predominantly of car-
bon and hydrogen are insoluble in water because their electri-
cally neutral covalent bonds are not attracted to water molecules.
These molecules are
When hydrophobic molecules are mixed with water,
two phases form, as occurs when oil is mixed with water. The
strong attraction between polar molecules “squeezes” the
nonpolar molecules out of the water phase. Such a separation
is never 100 percent complete, however, so very small amounts
of nonpolar solutes remain dissolved in the water phase.
Five water molecules. Note that polarized covalent bonds link
the hydrogen and oxygen atoms within each molecule and that
hydrogen bonds occur between adjacent molecules. Hydrogen
bonds are represented in diagrams by dashed or dotted lines, and
covalent bonds by solid lines. The
symbol means that a partial
charge exists on that atom due to the unequal sharing of electrons
between hydrogens and oxygen within a molecule.