Essential Cell Biology 5th edition

Chemical Bonds
47
Because of the favorable interaction between ions and water molecules
(which are polar), many salts (including NaCl) are highly soluble in water.
They dissociate into individual ions (such as Na + and Cl – ), each surrounded
by a group of water molecules. Positive ions are called cations
and negative ions are called anions. Small inorganic ions such as Na + , Cl – ,
K + , and Ca 2+ play important parts in many biological processes, including
the electrical activity of nerve cells, as we discuss in Chapter 12.
In aqueous solution, ionic bonds are 10–100 times weaker than the covalent
bonds that hold atoms together in molecules. But, as we will see,
such weak interactions nevertheless play an important role in the chemistry
of living things.
Hydrogen Bonds Are Important Noncovalent Bonds
for Many Biological Molecules
Water accounts for about 70% of a cell’s weight, and most intracellular
reactions occur in an aqueous environment. Thus the properties of water
have put a permanent stamp on the chemistry of living things. In each
molecule of water (H 2 O), the two covalent H–O bonds are highly polar
because the O is strongly attractive for electrons whereas the H is only
weakly attractive. Consequently, in each water molecule, there is a preponderance
of positive charge on the two H atoms and negative charge
on the O. When a positively charged region of one water molecule (that
is, one of its H atoms) comes close to a negatively charged region (that
is, the O) of a second water molecule, the electrical attraction between
them can establish a weak bond called a hydrogen bond (Figure 2–13A).
These bonds are much weaker than covalent bonds and are easily broken
by random thermal motions. Thus each bond lasts only an exceedingly
short time. But the combined effect of many weak bonds is far from
trivial. Each water molecule can form hydrogen bonds through its two
H atoms to two other water molecules, producing a network in which
hydrogen bonds are being continually broken and formed (see Panel 2–3,
pp. 70–71). It is because of these interlocking hydrogen bonds that water
at room temperature is a liquid—with a high boiling point and high surface
tension—and not a gas. Without hydrogen bonds, life as we know it
could not exist.
Hydrogen bonds are not limited to water. In general, a hydrogen bond
can form whenever a positively charged H atom held in one molecule
by a polar covalent linkage comes close to a negatively charged atom—
typically an oxygen or a nitrogen—belonging to another molecule (Figure
2–13B). Hydrogen bonds can also occur between different parts of a
single large molecule, where they often help the molecule fold into a
particular shape.
Like molecules (or salts) that carry positive or negative charges, substances
that contain polar bonds and can form hydrogen bonds also mix
well with water. Such substances are termed hydrophilic, meaning that
they are “water-loving.” A large proportion of the molecules in the aqueous
environment of a cell fall into this category, including sugars, DNA,
RNA, and a majority of proteins. Hydrophobic (“water-fearing”) molecules,
by contrast, are uncharged and form few or no hydrogen bonds,
and they do not dissolve in water. These and other properties of water
are reviewed in Panel 2–2 (pp. 68–69).
(A)
(B)
δ _
H
δ +
O
O
O
N
N
donor
atom
polar
covalent
bond
H
H
H
H
δ +
H
hydrogen
bond
O
N
O
N
O
acceptor
atom
Figure 2–13 Noncovalent hydrogen bonds
form between water molecules and
between many other polar molecules.
(A) A hydrogen bond forms between two
water molecules. The slight positive charge
associated with the hydrogen atom is
electrically ECB5 attracted e2.14/2.13 to the slight negative
charge of the oxygen atom. (B) In cells,
hydrogen bonds commonly form between
molecules that contain an oxygen or
nitrogen. The atom bearing the hydrogen
is considered the H-bond donor and the
atom that interacts with the hydrogen is the
H-bond acceptor.
QUESTION 2–4
δ _
True or false? “When NaCl is
dissolved in water, the water
molecules closest to the ions
will tend to preferentially orient
themselves so that their oxygen
atoms face the sodium ions and
face away from the chloride ions.”
Explain your answer.
H
H
δ +
δ +
Four Types of Weak Interactions Help Bring Molecules
Together in Cells
Much of biology depends on specific but transient interactions between
one molecule and another. These associations are mediated by