Essential Cell Biology 5th edition

68
PANEL 2–2 THE CHEMICAL PROPERTIES OF WATER
HYDROGEN BONDS
Because they are polarized, two
adjacent H 2 O molecules can form
a noncovalent linkage known as a
hydrogen bond. Hydrogen bonds
have only about 1/20 the strength
of a covalent bond.
Hydrogen bonds are strongest when
the three atoms lie in a straight line.
δ +
δ +
H
H
δ _ δ +
O
H O δ _
H hydrogen
δ + bond
H
H
bond lengths
O
hydrogen
bond
0.17 nm
H
O
0.10 nm
covalent bond
WATER
Two atoms connected by a covalent bond may exert different attractions for
the electrons of the bond. In such cases, the bond is polar, with one end
slightly negatively charged (δ _ ) and the other slightly positively charged (δ + ).
WATER STRUCTURE
Molecules of water join together transiently
in a hydrogen-bonded lattice.
H δ
+
electropositive
region
O
δ +
H
δ _ δ _
electronegative
region
Although a water molecule has an overall neutral charge (having the same
number of electrons and protons), the electrons are asymmetrically distributed,
making the molecule polar. The oxygen nucleus draws electrons away from
the hydrogen nuclei, leaving the hydrogen nuclei with a small net positive charge.
The excess of electron density on the oxygen atom creates weakly negative
regions at the other two corners of an imaginary tetrahedron. On these pages,
we review the chemical properties of water and see how water influences the
behavior of biological molecules.
The cohesive nature of water is
responsible for many of its unusual
properties, such as high surface tension,
high specific heat capacity, and high heat
of vaporization.
HYDROPHILIC MOLECULES
Substances that dissolve readily in water are termed hydrophilic. They include
ions and polar molecules that attract water molecules through electrical charge
effects. Water molecules surround each ion or polar molecule and carry it
into solution.
H
H H
O
H O
O
δ _ O
δ _ H
Hδ + H
H
δ +
δ _ Na + H H
δ + Cl _
O
δ _ δ _ O O H H
δ + δ +
H
H O
H O H
H
O H
H
Ionic substances such as sodium chloride
dissolve because water molecules are
attracted to the positive (Na + ) or negative
(Cl _ ) charge of each ion.
H
O
O
H
H
H
H
H
O
H
O
C
O
H
H
H
N
N
H
H
H
O
H
O
H
H
H
Polar substances such as urea
dissolve because their molecules
form hydrogen bonds with the
surrounding water molecules.
HYDROPHOBIC MOLECULES
Substances that contain a preponderance
of nonpolar bonds are usually insoluble
in water and are termed hydrophobic.
Water molecules are not attracted to such
hydrophobic molecules and so have little
tendency to surround them and bring them
into solution.
H
H O
H H
H H
H
O
H
C
H
C
C
H
H
H
Hydrocarbons, which contain many
C–H bonds, are especially hydrophobic.
O
H
O
O
H
H
H
H
O
O
H
H
H