Biology Now, 2e

Ingredients for Life ■ 49

Getting the Right Mix

Miller never stopped trying to make complex

molecules from simple ones. Though his 1953

experiment was originally met with fanfare,

scientists later began to dispute the usefulness

of his experiments. Methane and ammonia,

they argued, didn’t exist in large amounts on the

early Earth. Instead, new evidence suggested

that the atmosphere contained nitrogen gas

(N 2 ) and carbon dioxide (CO 2 ). “Most people

agree now [that Miller and Urey] didn’t have the

right composition,” says Sandford. So, in 1983,

30 years after his original experiment, Miller

repeated it using nitrogen and carbon diox-

ide. But instead of a deep red/brown broth, the

liquid produced was clear and seemingly barren.

The experiment looked like a failure.

In 2007, Bada and Cleaves decided to revisit

that experiment to see what had gone wrong.

Instead of just reanalyzing samples, they redid

the experiment and discovered that the reac-

tions between the new gases were producing

chemicals called nitrites, which destroy amino

acids. The solution also became acidic because

of the presence of nitrous acid (HNO 2 ). An acid

is a hydrophilic compound that dissolves in

water and loses one or more hydrogen ions (H+).

By donating H+ ions to water, acids increase

the concentration of free H+ ions in an aqueous

solution. H+ ions are extremely reactive and can

disrupt or alter other chemical reactions. The

acidic solution, Bada and Cleaves realized, was

preventing amino acids from forming.

To counteract that acidity, Bada added a

base. Acids and bases are chemical opposites.

Unlike acids, bases accept hydrogen ions from

aqueous surroundings. Because a base removes

H+ ions from solution, it has the overall effect of

reducing the concentration of free H+ ions in an

aqueous solution (strong bases, like strong acids,

can be dangerous because they disrupt chemi-

cal reactions important to life). Acids react with

bases to have an overall neutralizing effect,

reducing the concentration of reactive H+ ions.

Hydrogen ion concentration is commonly

expressed on a scale from 0 to 14, where 0

represents an extremely high concentration

of free H+ ions and 14 represents the lowest

concentration. This scale, called the pH scale, is

logarithmic: each pH unit represents a 10-fold

increase or decrease in the concentration of

hydrogen ions (Figure 3.9). Pure water is said to

be neutral at pH 7, in the middle of the pH scale.

The addition of acids to pure water raises the

concentration of free hydrogen ions, making the

solution more acidic and pushing the pH below

Figure 3.9

The pH scale indicates hydrogen ion concentration

Q1: Which has a higher concentration of free hydrogen ions: vinegar,

pH 2.8; or milk, pH 6.5?

Q2: What happens to the concentration of free hydrogen ions in your

stomach when you drink a glass of milk?

Q3: Black coffee has a pH of 5. Does adding coffee to water (pH 7)

increase or decrease the concentration of free hydrogen ions in the

liquid?

14

13

Oven cleaner (13.5)

12

Household

ammonia (11.7)

Basic

Acidic

11

Antacids (10.5)

10

9

Borax (9.5)

Baking soda (8.3)

8

Human blood (7.4)

7 NeutralNeutral

Seawater (7.5–8.3)

6

Milk (6.5)

Pure water

5

Natural rainwater (5.6)

4

Tomatoes (4.5)

3

Oranges (3.5)

2

Lemons (2.3)

1

0

Stomach acid (1.5–2.0)

Values above 7

indicate basic

solutions; the

higher the value,

the more basic

the solution.

Values below 7

indicate acidic

solutions; the

lower the value,

the more acidic

the solution.

A pH of 7 means

that the solution

is neutral.

Lowest free

H+ ion

concentration

Highest free

H+ ion

concentration

A solution with a

pH of 10 is 100

times more basic

than a solution

with a pH of 8.

A solution with a

pH of 3 is 10

times more acidic

than a solution

with a pH of 4.