Ingredients for Life ■ 51the neutral value of 7. Adding a base lowers the
concentration of free hydrogen ions in the solu-
tion, making the resulting solution more basic
and raising the pH above 7.
Bada added a simple base, calcium carbonate,
to the experiment in order to raise the pH of his
solution. This time, the resulting brew was burst-
ing with amino acids, suggesting that the build-
ing blocks of life could indeed have originated
on Earth and not been solely delivered to Earth
by meteorites or comets. Today, many scientists
agree that it is likely that both processes—amino
acids arriving from space and originating on
Earth—contributed to life as we know it. But no
matter where amino acids came from, the logical
follow-up question is one that continues to stump
scientists to this day: What happened next?
Life’s First Steps
“No one really knows how life got started,” says
Sandford. But researchers do agree on what life
requires. If all the water in any living organ-
ism were removed, four major classes of large
organic molecules, or biomolecules (sometimes
called “macromolecules”), would remain, all of
them critical for living cells: proteins, carbohy-
drates, nucleic acids, and lipids.
Each of these biologically important mole-
cules is built on a framework of covalently
bonded carbon atoms. Carbon is the predom-
inant element in living systems, partly because
it can form large molecules that contain thou-
sands of atoms. A single carbon atom can form
strong covalent bonds with up to four other
atoms (Figure 3.10). Carbon atoms can also
bond to other carbon atoms, forming long
chains, branched molecules, and even rings. No
other element is as versatile as carbon in the
sheer diversity of complex molecules that can be
assembled from it. In fact, while there are only
about 4,500 known naturally occurring inor-
ganic molecules (molecules that do not contain
a carbon atom) on Earth, the number of known
organic molecules is in the range of millions, and
the number of those that are not yet uncharacter-
ized is likely several orders of magnitude larger.
Proteins, carbohydrates, and nucleic acids
are polymers, long strands of repeating units of
small molecules called monomers. Amino acids
are the monomers making up proteins, simple
sugars are the monomers in carbohydrates, and
nucleotides are the basis of nucleic acid polymers.
All three of these polymers have essential
functions for every life-form. Proteins are
known to be the most numerous and versatile of
the biomolecules. Different combinations of the
twenty amino acid monomers allow for countless
proteins that vary in size and shape, and there-
fore function (Figure 3.11, top left). For exam-
ple, enzymatic proteins, like polymerases, enable
us to copy our DNA (see Chapter 9 for more on
DNA replication). Structural proteins give our
cells shape. Hormone and receptor proteins, like
insulin and its receptor, allow our cells to take up
sugars for use as energy. Other equally import-
ant protein categories include membrane trans-
port proteins that help move substances into
and out of cells, antibody molecules that protect
us from disease, storage proteins like LDL and
HDL (low-density and high-density lipoproteins,
respectively) that carry cholesterol, and venoms
and toxins such as the tetanus toxoid.
Carbohydrates are the next-most-versatile
biomolecules. They range in size from simple sugar
monomers (monosaccharides) and two-mono-
mer sugars (disaccharides) to complex carbohy-
drates that may contain thousands of monomers
(Figure 3.11, top right). Simple sugars are the
cell’s direct fuel to make ATP (adenosine triphos-
phate), the molecular energy source essential
for all cellular work (see Chapter 4 for more on
ATP). Other carbohydrates, such as glycogen in
animals and starch in plants, are used for energy
storage. Three additional complex carbohydrates
provide structural support to cells: cellulose, also
known as fiber, helps plants to grow tall; chitin
forms a hard outer covering to protect organisms
without an internal skeleton, such as insects,
spiders, and crustaceans; and peptidoglycan is a
major component of bacterial cell walls.
The third and most crucial category of poly-
mer, the nucleic acids—DNA (deoxyribonucleic
acid) and RNA (ribonucleic acid)—form the
basis of life itself. Nucleic acids are polymers
of nucleotide monomers: DNA is composed of
deoxyribonucleotides, and RNA is composed of
ribonucleotides (Figure 3.11, bottom left). DNA
provides living organisms with long-term, stable
genetic information storage in a form that is
easily copied and passed on to future generations
(see Chapter 9). Our genes are DNA. That sounds
important, but what about RNA? Without RNA,