K E Y C O N C E P T S

4.1 Organic chemistry is the study of carboncompounds

4.2 Carbon atoms can form diverse molecules bybonding to four other atoms

4.3 A few chemical groups are key to the functioningof biological molecules

O V E R V I E W

Carbon: The Backbone of LifeWater is the universal medium for life on Earth, but living or-ganisms, such as the plants and Roosevelt elk in Figure 4.1, aremade up of chemicals based mostly on the element carbon. Car-bon enters the biosphere through the action of plants. Plantsuse solar energy to transform atmospheric CO2 into the mol-ecules of life, which are then taken in by plant-eating animals.

Of all chemical elements, carbon is unparalleled in its abil-ity to form molecules that are large, complex, and varied,

making possible the diversity of organisms that have evolvedon Earth. Proteins, DNA, carbohydrates, and other moleculesthat distinguish living matter from inanimate material are allcomposed of carbon atoms bonded to one another and toatoms of other elements. Hydrogen (H), oxygen (O), nitrogen(N), sulfur (S), and phosphorus (P) are other common ingredi-ents of these compounds, but it is the element carbon (C) thataccounts for the enormous variety of biological molecules.

Large biological molecules, such as proteins, are the mainfocus of Chapter 5. In this chapter, we investigate the prop-erties of smaller molecules. We will use these small mol-ecules to illustrate concepts of molecular architecture thatwill help explain why carbon is so important to life, at thesame time highlighting the theme that emergent propertiesarise from the organization of matter in living organisms.

C O N C E P T 4.1Organic chemistry is the studyof carbon compoundsFor historical reasons, compounds containing carbon are saidto be organic, and the branch of chemistry that specializes inthe study of carbon compounds is called organic chemistry.Organic compounds range from simple molecules, such asmethane (CH4), to colossal ones, such as proteins, with thou-sands of atoms. Most organic compounds contain hydrogenatoms in addition to carbon atoms.

The overall percentages of the major elements of life—C, H,O, N, S, and P—are quite uniform from one organism to an-other. Because of carbon’s versatility, however, this limited as-sortment of atomic building blocks can be used to build aninexhaustible variety of organic molecules. Different speciesof organisms, and different individuals within a species, aredistinguished by variations in their organic molecules.

Since the dawn of human history, people have used otherorganisms as sources of valued substances—from foods andmedicines to fabrics. The science of organic chemistry origi-nated in attempts to purify and improve the yield of suchproducts. By the early 1800s, chemists had learned to makemany simple compounds in the laboratory by combining el-ements under the right conditions. Artificial synthesis of thecomplex molecules extracted from living matter seemed im-possible, however. At that time, the Swedish chemist JönsJakob Berzelius made the distinction between organic com-pounds, those thought to arise only in living organisms, andinorganic compounds, those found only in the nonlivingworld. Vitalism, the belief in a life force outside the jurisdic-tion of physical and chemical laws, provided the foundationfor the new discipline of organic chemistry.

Chemists began to chip away at the support for vitalismwhen they finally learned to synthesize organic compounds inthe laboratory. In 1828, Friedrich Wöhler, a German chemistwho had studied with Berzelius, tried to make an “inorganic”

! Figure 4.1 What properties make carbonthe basis of all life?

4Carbon andthe MolecularDiversity of Life

58 U N I T O N E The Chemistry of Life

C O N C E P T C H E C K 4.11. Why was Wöhler astonished to find he had made urea?2. When Miller tried his experiment with-

out the electrical discharge, no organic compoundswere found. What might explain this result?

For suggested answers, see Appendix A.

WHAT IF?

C H A P T E R 4 Carbon and the Molecular Diversity of Life 59

salt, ammonium cyanate, by mixing solutions of ammoniumions (NH4

!)and cyanate ions (CNO"). Wöhler was astonishedto find that instead he had made urea, an organic compoundpresent in the urine of animals. Wöhler challenged the vital-ists when he wrote, “I must tell you that I can prepare ureawithout requiring a kidney or an animal, either man or dog.”However, one of the ingredients used in the synthesis, thecyanate, had been extracted from animal blood, and the vital-ists were not swayed by Wöhler’s discovery. A few years later,however, Hermann Kolbe, a student of Wöhler’s, made the or-ganic compound acetic acid from inorganic substances thatcould be prepared directly from pure elements. Vitalism crum-bled completely after several decades of laboratory synthesis ofincreasingly complex organic compounds.

Organic Molecules and the Originof Life on Earth

In 1953, Stanley Miller, a graduate student ofHarold Urey’s at the University of Chicago, helped bring theabiotic (nonliving) synthesis of organic compounds into thecontext of evolution. Study Figure 4.2 to learn about hisclassic experiment. From his results, Miller concluded thatcomplex organic molecules could arise spontaneously underconditions thought to have existed on the early Earth. Milleralso performed experiments designed to mimic volcanicconditions, with roughly similar results. In 2008, a formergraduate student of Miller’s discovered some samples fromthese experiments. Reanalyzing them using modern equip-ment, he identified additional organic compounds that hadnot been found by Miller. Although the jury is still out, theseexperiments support the idea that abiotic synthesis of or-ganic compounds, perhaps near volcanoes, could have beenan early stage in the origin of life (see Chapter 25).

The pioneers of organic chemistry helped shift the main-stream of biological thought from vitalism to mechanism, theview that physical and chemical laws govern all natural phe-nomena, including the processes of life. Organic chemistrywas redefined as the study of carbon compounds, regardlessof origin. Organisms produce most of the naturally occurringorganic compounds, and these molecules represent a diversityand range of complexity unrivaled by inorganic compounds.However, the rules of chemistry apply to all molecules. Thefoundation of organic chemistry is not some intangible lifeforce, but the unique chemical versatility of the element carbon.

EVOLUTION

! Figure 4.2 INQUIRYCan organic molecules form under conditionsestimated to simulate those on the early Earth?

EXPERIMENT In 1953, Stanley Miller set up a closed system to mimicconditions thought to have existed on the early Earth. A flask of watersimulated the primeval sea. The water was heated so that some vapor-ized and moved into a second, higher flask containing the “atmo-sphere”—a mixture of gases. Sparks were discharged in the syntheticatmosphere to mimic lightning.

H2O“sea”

Sample forchemical analysis

Cooled “rain”containingorganicmolecules

Condenser

Coldwater

Water vaporElectrode

CH4

NH3 H2

“Atmosphere”

2 The “atmosphere” contained a mixture of hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor.

The water mixture in the ”sea” flask was heated; vapor entered the ”atmosphere”flask.

Sparks were discharged to mimic lightning.

3

As material cycled through the apparatus, Miller periodically collected samples for analysis.

A condenser cooled the atmosphere, raining water and any dissolved molecules down into the sea flask.

1

5 4

RESULTS Miller identified a variety of organic molecules that are com-mon in organisms. These included simple compounds, such as formalde-hyde (CH2O) and hydrogen cyanide (HCN), and more complex molecules,such as amino acids and long chains of carbon and hydrogen known ashydrocarbons.

CONCLUSION Organic molecules, a first step in the origin of life, mayhave been synthesized abiotically on the early Earth. (We will explorethis hypothesis in more detail in Chapter 25.)

SOURCE S. L. Miller, A production of amino acids under possible prim-itive Earth conditions, Science 117:528–529 (1953).

If Miller had increased the concentration of NH3 in his ex-periment, how might the relative amounts of the products HCN andCH2O have differed?

WHAT IF?