Ingredients for Life ■ 55the information stored in our genes would be
stuck there, like a blueprint in a foreign language
that no one can decipher. RNA comes in many
forms and plays many roles, but its most import-
ant job is providing a readable genetic language
that enables genes to be expressed as proteins.
A final valuable type of biomolecule is the
lipids, which are better known as fats, oils, and
steroids. Lipids are not polymers, because their
structure is not composed of a chain of monomers
(Figure 3.11, bottom right). Nevertheless, the
lipids form a diverse group of biomolecules made
up of combinations of hydrocarbons (carbon-
and hydrogen-only molecules), fatty acids, or
glycerol molecules. For example, triglycerides—
composed of three fatty acid molecules linked to
a glycerol molecule—provide long-term energy
storage in both plants (as oils) and animals (as
fats), and serve as insulation against the cold in
animals. And phospholipids, which have a hydro-
philic half and a hydrophobic half, are the main
component of all cellular membranes (see Chap-
ter 4 for more on phospholipids).
Miller, Bada, and other scientists have not yet
been able to identify any biomolecules in their
prebiotic soups. “It’s still very much of an unknown
how we go from simple to complex molecules,” says
Bada. Lipids and carbohydrate-like compounds,
however, have been found in meteorites, and
Sandford’s team at NASA is now attempting to
create carbohydrates using its space ice simula-
tions. Other scientists are trying to re-create lipids
and nucleic acids in the lab (see Chapter 4).Fifty More Years
Sadly, Miller died in 2007, never having seen the
results of Bada’s reanalysis. Today, researchers
continue to perform spark discharge experiments
with equipment that is essentially the same as in
Miller’s original design. “We’re not done with it,”
says Jim Cleaves. “There are just so many varia-
tions to look at—different combinations of gases,
changing the pH, adding metals, et cetera.”
“This experiment lives on,” agrees Bada.
“Not many people can say that an experiment
done over 50 years ago is still being investigated
today, and finding a wealth of new informa-
tion.” And with better and better instruments
to detect the results, who knows what scientists
might discover over the next 50 years?● (^) The physical world is composed of matter, which
is anything that has mass and occupies space.
There are 98 distinct elements found naturally on
Earth. An atom is the smallest unit of an element
that maintains its unique properties; it contains
positively charged protons, uncharged neutrons, and
negatively charged electrons.
● (^) The chemical interactions that cause atoms to
associate with each other are known as chemical
bonds. When an atom loses or gains electrons, it
becomes, respectively, a positively or negatively
charged ion. Ions of opposite charge are held
together by ionic bonds.
● (^) Covalent bonds are formed by the sharing of
electrons between atoms. A molecule contains at
least two atoms that are held together by covalent
bonds.
● (^) Hydrogen bonds are weak associations between two
molecules such that a partially positive hydrogen
atom within one molecule is attracted to a partially
negative region of the other molecule.
● (^) Partial electrical charges result from the unequal
sharing of electrons between atoms, giving rise to
polar molecules. The polarity of individual water
REVIEWING THE SCIENCE
molecules and the hydrogen bonding across water
molecules explain nearly all of the special properties
of water.● (^) A solution is any combination of a dissolved
substance, known as the solute, and a fluid into
which the solute has dissolved, known as the
solvent.
● (^) Ions and polar molecules are hydrophilic; they
readily dissolve in water. Nonpolar molecules cannot
associate with water and are therefore hydrophobic.
● (^) In chemical reactions, bonds between atoms are
formed or broken. The participants in a chemical
reaction (reactants) are modified to give rise to new
ions or molecules (products).
● (^) The concentration of free hydrogen ions in water is
expressed by the pH scale and reflects whether a
solution is acidic, basic, or neutral.
● (^) Chemical compounds are molecules that contain
atoms from at least two different elements. Carbon
atoms can link with each other and with other atoms
to generate a great diversity of chemical compounds
called organic molecules. The four main types
of large organic molecules, or biomolecules, are
proteins, carbohydrates, nucleic acids, and lipids.