Human Physiology, 14th edition (2016)

28 Chapter 2

when they are dissolved in water. Dissociation of NaCl, for

example, yields Na^1 and Cl^2. Each of these ions attracts polar

water molecules; the negative ends of water molecules are

attracted to the Na^1 , and the positive ends of water molecules

are attracted to the Cl^2 ( fig. 2.6 ). The water molecules that sur-

round these ions, in turn, attract other molecules of water to

form hydration spheres around each ion.

It is the formation of hydration spheres that makes an

ion or a molecule soluble in water. Glucose, amino acids,

and many other organic molecules are water soluble because

hydration spheres can form around atoms of oxygen, nitrogen,

and phosphorus, which are joined by polar covalent bonds to

other atoms in the molecule. Such molecules are said to be

hydrophilic. By contrast, molecules composed primarily of

nonpolar covalent bonds, such as the hydrocarbon chains of fat

molecules, have few charges and thus cannot form hydration

spheres. They are insoluble in water and appear repelled by

water molecules (because the water molecules preferentially

bond with each other; fig. 2.7 ). For this reason, nonpolar mol-

ecules are said to be hydrophobic (“water fearing”).

Hydrogen Bonds

When a hydrogen atom forms a polar covalent bond with an

atom of oxygen or nitrogen, the hydrogen gains a slight posi-

tive charge as its electron is pulled toward the other atom. This

other atom is thus described as being electronegative. Because

the hydrogen has a slight positive charge, it will have a weak

attraction for a second electronegative atom (oxygen or nitro-

gen) that may be located near it. This weak attraction is called

a hydrogen bond. Hydrogen bonds are usually shown with

Figure 2.5 The reaction of sodium with chlorine to
produce sodium and chloride ions. The positive sodium and
negative chloride ions attract each other, producing the ionic
compound sodium chloride (NaCl).

11 P+

12N

17P+

18N

11 P+

12N

17P+

18N

Sodium atom (Na)

Sodium ion (Na+)

Chlorine atom (Cl)

Chloride ion (Cl–)

Figure 2.6 How NaCl dissolves in water. The negatively charged oxygen-ends of water molecules are attracted to the
positively charged Na^1 , whereas the positively charged hydrogen-ends of water molecules are attracted to the negatively charged Cl^2.
Other water molecules are attracted to this first concentric layer of water, forming hydration spheres around the sodium and chloride
ions.

Water molecule

Oxygen

Hydrogen

Na+ Cl–

(+) (+)

(–)