THE LIVING WORLD

Unit two. The Living Cell

 

2. The Chemistry of Life

 

2.4. Hydrogen Bonds Give Water Unique Properties

 

Three-fourths of the earth’s surface is covered by liquid water. About two-thirds of your body is water, and you cannot exist long without it. All other organisms also require water. It is no accident that tropical rain forests are bursting with life, whereas dry deserts seem almost lifeless except after rain. The chemistry of life, then, is water chemistry.

Water has a simple atomic structure, an oxygen atom linked to two hydrogen atoms by single covalent bonds. The chemical formula for water is thus H2O. It is because the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms that water is a polar molecule and so can form hydrogen bonds. Water’s ability to form hydrogen bonds is responsible for much of the organization of living chemistry, from membrane structure to how proteins fold.

The weak hydrogen bonds that form between a hydrogen atom of one water molecule and the oxygen atom of another produce a lattice of hydrogen bonds within liquid water. Each of these bonds is individually very weak and short-lived—a single bond lasts only 1/100,000,000,000 of a second. However, like the grains of sand on a beach, the cumulative effect of large numbers of these bonds is enormous and is responsible for many of the important physical properties of water (table 2.2).

 

TABLE 2.2. THE PROPERTIES OF WATER

 

Property

Explanation

Heat storage

Hydrogen bonds require considerable heat before they break, minimizing temperature changes.

Ice formation

Water molecules in an ice crystal are spaced relatively far apart because of hydrogen bonding.

High heat of vaporization

Many hydrogen bonds must be broken for water to evaporate.

Cohesion

Hydrogen bonds hold molecules of water together.

High polarity

Water molecules are attracted to ions and polar compounds.

 

Heat Storage

The temperature of any substance is a measure of how rapidly its individual molecules are moving. Because of the many hydrogen bonds that water molecules form with one another, a large input of thermal energy is required to disrupt the organization of liquid water and raise its temperature. Because of this, water heats up more slowly than almost any other compound and holds its temperature longer. That is a major reason why your body is able to maintain a relatively constant internal temperature.

 

Ice Formation

If the temperature is low enough, very few hydrogen bonds break in water. Instead, the lattice of these bonds assumes a crystal-like structure, forming a solid we call ice. Interestingly, ice is less dense than water—that is why icebergs and ice cubes float. Why is ice less dense? This is best understood by comparing the molecular structures of water and ice that you see in figure 2.9. At temperatures above freezing (0°C or 32°F), water molecules in figure 2.9a move around each other with hydrogen bonds breaking and forming. As temperatures drop, the movement of water molecules decreases, allowing hydrogen bonds to stabilize, holding individual molecules farther apart, as in figure 2.9b, making the ice structure less dense.

 

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Figure 2.9. Ice formation.

When water (a) cools below 0°C, it forms a regular crystal structure (b) that floats. The individual water molecules are spaced apart and held in position by hydrogen bonds.

 

High Heat of Vaporization

If the temperature is high enough, many hydrogen bonds break in water, with the result that the liquid is changed into vapor. A considerable amount of heat energy is required to do this—every gram of water that evaporates from your skin removes 2,452 joules of heat from your body, which is equal to the energy released by lowering the temperature of 586 grams of water 1°C. That is why sweating cools you off; as the sweat evaporates (vaporizes) it takes energy with it, in the form of heat, cooling the body.

 

Cohesion

Because water molecules are very polar, they are attracted to other polar molecules—hydrogen bonds bind polar molecules to each other. When the other polar molecule is another water molecule, the attraction is called cohesion. The surface tension of water is created by cohesion. Surface tension is the force that causes water to bead, like on the spider web in figure 2.10, or supports the weight of the water strider. When the other polar molecule is a different substance, the attraction is called adhesion. Capillary action—such as water moving up into a paper towel—is created by adhesion. Water clings to any substance, such as paper fibers, with which it can form hydrogen bonds. Adhesion is why things get “wet” when they are dipped in water and why waxy substances do not—they are composed of nonpolar molecules that don’t form hydrogen bonds with water molecules.

 

 

Figure 2.10. Cohesion.

(a) Cohesion allows water molecules to stick together and form droplets. (b) Surface tension is a property derived from cohesion—that is, water has a "strong" surface due to the force of its hydrogen bonds. Some insects, such as this water strider, literally walk on water.

 

High Polarity

Water molecules in solution always tend to form the maximum number of hydrogen bonds possible. Polar molecules form hydrogen bonds and are attracted to water molecules. Polar molecules are called hydrophilic (from the Greek hydros, water, and philic, loving), or water-loving, molecules. Water molecules gather closely around any molecule that exhibits an electrical charge, whether a full charge (ion) or partial charge (polar molecule). When a salt crystal dissolves in water as you see happening in figure 2.11, what really happens is that individual ions break off from the crystal and become surrounded by water molecules. The blue hydrogen atoms of water are attracted to the negative charge of the chloride ions and the red oxygen atoms are attracted to the positive charge of the sodium ions. Water molecules orient around each ion like a swarm of bees attracted to honey, and this shell of water molecules, called a hydration shell, prevents the ions from reassociating with the crystal. Similar shells of water form around all polar molecules, and polar molecules that dissolve in water in this way are said to be soluble in water.

 

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Figure 2.11. How salt dissolves in water.

Salt is soluble in water because the partial charges on water molecules are attracted to the charged sodium and chloride ions.

The water molecules surround the ions, forming what are called hydration shells. When all of the ions have been separated from the crystal, the salt is said to be dissolved.

 

Nonpolar molecules like oil do not form hydrogen bonds and are not water-soluble. When nonpolar molecules are placed in water, the water molecules shy away, instead forming hydrogen bonds with other water molecules. The nonpolar molecules are forced into association with one another, crowded together to minimize their disruption of the hydrogen bonding of water. It seems almost as if the nonpolar compounds shrink from contact with water, and for this reason they are called hydrophobic (from the Greek hydros, water, and phobos, fearing). Many biological structures are shaped by such hydrophobic forces, as will be discussed in chapter 3.

 

Key Learning Outcome 2.4. Water molecules form a network of hydrogen bonds in liquid and dissolve other polar molecules. Many of the key properties of water arise because it takes considerable energy to break liquid water's many hydrogen bonds.