Essential Cell Biology 5th edition

48 CHAPTER 2 Chemical Components of Cells
noncovalent bonds, such as the hydrogen bonds just discussed. Although
these noncovalent bonds are individually quite weak, their energies can
sum to create an effective force between two molecules.
The ionic bonds that hold together the Na + and Cl – ions in a salt crystal
(see Figure 2–12) represent a second form of noncovalent bond called an
electrostatic attraction. Electrostatic attractions are strongest when the
atoms involved are fully charged, as are Na + and Cl – ions. But a weaker
electrostatic attraction can occur between molecules that contain polar
covalent bonds (see Figure 2–11). Like hydrogen bonds, electrostatic
attractions are extremely important in biology. For example, any large
molecule with many polar groups will have a pattern of partial positive
and negative charges on its surface. When such a molecule encounters
a second molecule with a complementary set of charges, the two will
be drawn to each other by electrostatic attraction. Even though water
greatly reduces the strength of these attractions in most biological settings,
the large number of weak noncovalent bonds that form on the
surfaces of large molecules can nevertheless promote strong and specific
binding (Figure 2–14).
Figure 2–14 A large molecule, such as
a protein, can bind to another protein
through noncovalent interactions on the
surface of each molecule. In the aqueous
environment of a cell, many individual weak
interactions could cause the two proteins
to recognize each other specifically and
form a tight complex. Shown here is a
set of electrostatic attractions between
complementary positive and negative
charges.
ECB5 e2.13/2.14
A third type of noncovalent bond, called a van der Waals attraction,
comes into play when any two atoms approach each other closely. These
nonspecific interactions spring from fluctuations in the distribution of
electrons in every atom, which can generate a transient attraction when
the atoms are in very close proximity. These weak attractions occur in all
types of molecules, even those that are nonpolar and cannot form ionic
or hydrogen bonds. The relative lengths and strengths of these three
types of noncovalent bonds are compared to the length and strength of
covalent bonds in Table 2–1.
The fourth effect that often brings molecules together is not, strictly speaking,
a bond at all. In an aqueous environment, a hydrophobic force is
generated by a pushing of nonpolar surfaces out of the hydrogen-bonded
water network, where they would otherwise physically interfere with the
highly favorable interactions between water molecules. Hydrophobic
forces play an important part in promoting molecular interactions—in
particular, in building cell membranes, which are constructed largely
from lipid molecules with long hydrocarbon tails. In these molecules, the
H atoms are covalently linked to C atoms by nonpolar bonds (see Panel
2–1, pp. 66–67). Because the H atoms have almost no net positive charge,
they cannot form effective hydrogen bonds to other molecules, including
water. As a result, lipids can form the thin membrane barriers that keep
the aqueous interior of the cell separate from the surrounding aqueous
environment.
All four types of weak chemical interactions important in biology are
reviewed in Panel 2−3 (pp. 70–71).
TABLE 2–1 LENGTH AND STRENGTH OF SOME CHEMICAL BONDS
Bond Type Length* (nm) Strength (kJ/mole)
In Vacuum
In Water
Covalent 0.10 377 [90]** 377 [90]
Noncovalent: ionic bond 0.25 335 [80] 12.6 [3]
Noncovalent: hydrogen bond 0.17 16.7 [4] 4.2 [1]
Noncovalent: van der Waals
attraction (per atom)
0.35 0.4 [0.1] 0.4 [0.1]
*The bond lengths and strengths listed are approximate, because the exact
values will depend on the atoms involved.
**Values in brackets are kcal/mole. 1 kJ = 0.239 kcal and 1 kcal = 4.184 kJ.