Biology Now, 2e

Ingredients for Life ■ 45

table salt, sodium chloride (NaCl), is composed

of sodium and chlorine held together by ionic

bonds. Unlike a covalent bond, no electrons are

shared in an ionic bond.

The third means of attaching atoms to one

another is called a hydrogen bond. Hydrogen

bonds are weak electrical attractions between

a hydrogen atom with a partial positive charge

and a neighboring atom with a partial negative

charge. Molecules of water bind to each other

through hydrogen bonds because the nega-

tively charged oxygen end of one water molecule

weakly attracts one of the positively charged

hydrogen ends of another water molecule. A

single hydrogen bond is about 20 times weaker

than a covalent bond, but water makes up for

that lack of strength with sheer quantity. The

collective cross-linking of many, many water

molecules through hydrogen bonds amounts to

a potent force.

The polarity of water molecules and hydrogen

bonding explain nearly all of the special prop-

erties of water, which were critical in Miller’s

experiments. The foremost of these was that

water was able to break apart the compounds in

the flask. As you may have noticed the last time

you soaked a dirty dish, water has an incredi-

ble ability to dissolve other materials. This is

because water molecules form hydrogen bonds

with other polar molecules, like sugars or, in

Miller’s experiment, ammonia. The formation

of hydrogen bonds with polar molecules causes

those compounds to dissolve in water. Such

compounds are said to be soluble; that is, they

mix completely with the water.

A solution is any combination of a solute (a

dissolved substance, such as sugar) and a solvent

(the fluid, such as water, into which the solute

has dissolved). Water is called the “universal sol-

vent” because it successfully dissolves so many

substances. The polar nature of water molecules,

however, means that they will not interact with

uncharged or nonpolar substances, such as fat

or oil. Molecules that are soluble in water (such

as salt) are called hydrophilic (“water-loving”);

molecules that don’t dissolve well in water (such

as oil) are called hydrophobic (“water-fearing”).

Figure 3.5 shows these processes in action.

When Bada and his colleagues published

their results from reanalyzing Miller’s vials,

some scientists proposed that Miller’s second

experiment, in which he shot a jet of steam

Figure 3.5

Hydrophilic substances dissolve in water, but hydrophobic

substances do not

Q1: Describe what will happen to the molecules of olive oil if you

shake the bottle and then leave it alone for an hour. What about the

molecules of vinegar?

Q2: What would happen if you added another fat to the bottle, such

as bacon grease, and shook it?

Q3: Given how sugar behaves when it is mixed into coffee or tea,

would you predict that it is hydrophobic or hydrophilic?

into the spark, had resulted in more amino

acids because the hot water enabled a wider

variety of chemical reactions. Whether or not

that was the case, water was central to Miller’s

success.

Oil molecules are hydrophobic.

They are excluded from water

and tend to clump together.

Vinegar molecules

are hydrophilic. They

are held in solution

by water molecules.

Olive

oil

VinegarVinegar

JEFFREY BADA

Jeffrey Bada is a chemist at the Scripps Institution

of Oceanography at UC San Diego. With Jim

Cleaves, he closely analyzed and then duplicated

the Miller-Urey experiments. He is also a leading

scientist studying organic compounds beyond

Earth, including in meteorites and on Mars.