Biology 12

12 MHR • Unit 1 Metabolic Processes

A hydrogen bond is a force between molecules,

not a chemical bond within a molecule. Hydrogen

bonds are usually weaker than chemical bonds. For

instance, a hydrogen bond may be only five percent

the strength of a covalent bond, but it is sufficient

to hold one water molecule to another in liquid

water or ice. Under normal conditions, water

molecules are attracted to each other in such a way

that they are neither attracted too strongly (to form

a solid) nor too weakly (allowing water to become a

vapour). For this reason, under normal conditions

on Earth, water exists as a liquid.

Figure 1.7Hydrogen bonding (shown via dotted lines)

between water molecules

Solubility of Substances in Water

All cells depend on liquid water. In fact, living

organisms contain more molecules of water than

any other substance; water comprises as much as

90 percent of a typical cell. Water is a perfect fluid

environment through which other molecules can

move and interact.

Sodium chloride (table salt), and many other

ionic compounds or salts, dissolve readily in water.

This occurs because the positively charged poles of

the water molecule are attracted to the anions

(chloride ions) in the salt. The negatively charged

pole of the water molecule is similarly attracted to

the cations (sodium ions) in the salt as shown in

Figure 1.8. These two attractions pull the sodium

ions and chloride ions away from each other. The

salt is now dissociated, which means that the

sodium ions and chloride ions have separated

and have dissolved in the water.

Figure 1.8The salt NaCl dissolves in water because

chloride ions and sodium ions attract water molecules.

Compounds that interact with water — for

example, by dissolving in it — are called

hydrophilic. In contrast, compounds that do not

interact with water are called hydrophobic. Non-

polar compounds are hydrophobic. They cannot

form hydrogen bonds with water in the same way

that ionic or polar compounds can. Therefore,

hydrophobic molecules are insoluble in water. For

example, when you place a drop of oil (a non-polar

compound of carbon and hydrogen) into water,

the oil does not mix with the water — they

remain separate.

In this section, you have learned that the type of

chemical bond that joins individual atoms together

determines whether the resulting compound is

ionic or covalent. Covalent molecules may be polar

or non-polar, depending on the electronegativities

of the bonded atoms and shape of the molecule.

You have learned that hydrogen bonds form

between molecules in water, which interacts very

differently with hydrophobic and hydrophilic

compounds. Hydrophobic interactions especially

have a great effect on many biological molecules.

For instance, many protein molecules have

hydrophobic regions in portions of their structure.

Interactions of these regions with water cause

the molecules to adopt specific shapes. You will

see examples of this in the next section, which

reviews the four main kinds of molecules that

make up all cells.

What you have learned about molecules and polarity may

be useful in preparing your Biology Course Challenge. How

could you use this knowledge to help you prepare for your

science symposium?

COURSE CHALLENGE

HHO HH

O

Na+

δ+ δ+ δ+ δ+

δ−

δ−

CI−

+

+

–

hydrogen

bond

δ

δ

δ