44 ■ CHAPTER 03 Chemistry of Life
CELLS
The World of Water
The major difference between the two exper-
iments was steam—hot water vapor shooting
through the gas chamber. Water is essential for
life because of its unique chemical properties,
which enable molecules to dissolve and interact
in special ways through chemical reactions.
A water molecule is made up of two hydro-
gen atoms and one oxygen atom held together
by shared electrons—that is, by covalent bonds.
Electrons moving around an atom’s nucleus
have different energy levels, so we can think of
the electrons as segregated into rings or shells,
each of which can contain up to a fixed number
of electrons. The innermost shell, closest to the
nucleus, can hold up to two electrons. Moving
outward, the next two shells can hold up to eight
electrons each.
A shell needs to be full to be stable, so in an
effort to fill their shells, atoms may form cova-
lent bonds. In a covalent bond, atoms share elec-
trons in their outermost shell of electrons, also
called the valence shell. In a water molecule, the
oxygen atom uses an electron from each of two
hydrogen atoms to fill its outer shell, increas-
ing its count of electrons from six to eight. The
hydrogen atoms also benefit from the bond: they
fill their outer (and only) shell with the two elec-
trons needed to be stable.
However, the electrons are not shared
equally; they spend more time near the oxygen
atom than near the hydrogen atoms. Because
electrons are negatively charged particles, the
oxygen end of a water molecule therefore takes
on a slightly negative charge, and the hydrogen
ends become slightly positively charged. This
lopsided electron sharing means that water is a
polar molecule (Figure 3.4).
In addition to covalent bonds, two other
types of chemical bonds attach atoms to one
another: ionic bonds and hydrogen bonds.
Atoms that have lost or gained electrons are
called ions. Since electrons are negatively
charged, an atom that has gained an electron
is a negative ion, and an atom that has lost an
electron is a positive ion. When a negatively
charged ion and a positively charged ion are in
the same vicinity, they will chemically attract
each other and form an ionic bond. CommonBada decided to analyze the contents of the
vials. He sent the samples to Jason Dworkin,
also one of Miller’s former students, at NASA’s
Goddard Space Flight Center in Maryland. The
Goddard Center is home to an advanced mass
spectrometer, an instrument that measures the
weights of tiny amounts of matter as a way to
identify them.
The results were stunning. In 1953, Miller
had identified just five amino acids, three of
which were common constituents of proteins.
In 2008, analyzing the same samples with more
sophisticated techniques, Bada’s team identified
14 amino acids in the original experiment and a
whopping 22 in the unpublished steam experi-
ment. Bada was intrigued. Why did the second
experimental apparatus result in more kinds of
amino acids?Figure 3.4
Water molecules are polar
The polarity of water molecules is the cause of water’s unique properties.
Q1: Where are the covalent bonds in this figure?Q2: This figure shows a water molecule (H 2 O). A hydrogen molecule
(H 2 ) consists of two hydrogen nuclei that share two electrons. Draw a
simple diagram of a hydrogen molecule indicating the positions of the
two electrons.Q3: When table salt (sodium chloride, NaCl) dissolves in water, it
separates into a sodium ion (Na+) and a chloride ion (Cl–). Which portion
of a water molecule would attract the sodium ion, and which portion
would attract the chloride ion?Partial
negative chargePartial
positive chargePartial
positive charge