Rock Eaters ■ 81atmosphere, and a consequence of this increase
in carbon dioxide is that the surface of our
planet has begun to warm. To prevent further
climate change and lessen our dependence on
fossil fuels—many of which we pay a premium
to import from other countries—scientists and
engineers have developed clean, renewable ways
to capture energy, such as solar, wind, and water
power.
Yet to switch from a fossil fuel–based econ-
omy to renewable energy technologies, we need
efficient and inexpensive ways to store renew-
able energy. With the right chemical stabiliz-
ers, oil can be left in a barrel for decades, but a
direct current of electricity from a wind farm or
solar panels needs to be used immediately or fed
into the electrical grid. Today’s batteries aren’t a
good storage option; they are bulky, expensive,
and not terribly efficient.
Now imagine the ability to siphon electricity
from a wind farm or solar panel and transform it
into a liquid fuel. That’s where electricity-eating
microbes come in.
Energy for Life
All living cells require energy. Organisms
use energy for growth, reproduction, and
defense, and to manufacture the many chem-
ical compounds that make up living cells. They
must obtain energy from the living or nonliv-
ing components of their environment, and at
the very core of making and storing energy
is the transfer of electrons—subatomic parti-
cles with a negative charge. Electrons play an
essential role in electricity, magnetism, and
many other physical phenomena that shape
the world we know. The success of computers,
solar cells, cell phones, and other devices is due
to our ability to shape and control the flow of
electrons.
But long before electrons flowed through
computers, they moved through cells. The first
law of thermodynamics says that energy cannot
be created or destroyed, but it can be changed
from one form to another. In other words, cells
cannot create energy from nothing, so they
must utilize one form of energy and change
it to another form. However, organisms can’t
simply perform the biological equivalent of
plugging into an electrical socket and sucking
up electricity. That’s because the cell membrane
is an electrically neutral zone that prevents
charged particles from sneaking through,
including electrons. So, to move electrons into
and out of a cell, living organisms attach elec-
trons to molecules. Plants, for example, smug-
gle electrons into the cell via water molecules.
Humans and other organisms obtain elec-
trons from food (sugars, proteins, fats, etc.). As
the Nobel Prize–winning physiologist Albert
Szent-Györgyi reportedly said, “Life is nothing
but an electron looking for a place to rest.”
Cells use and store energy by transferring
electrons among molecules via chemical reac-
tions. Thousands of different types of chemical
reactions are required to sustain life in even the
simplest cell. The term metabolism describes
all the chemical reactions that occur inside
living cells, including those that store or release
energy. Most chemical reactions in a cell occur
in chains of linked events known as metabolic
pathways. Metabolic pathways produce key
biological molecules in a cell, including impor-
tant chemical building blocks like amino acids
and nucleotides.
Two metabolic pathways drive most of the
life around us. The sun is the ultimate source
of energy for most living organisms, and in the
first process, known as photosynthesis, organ-
isms capture energy from the sun and use it to
create sugars from carbon dioxide and water
(Figure 5.2). In this way, photosynthetic organ-
isms such as plants transform light energy into
chemical energy stored in the covalent bonds
of sugar molecules. These sugar molecules, for
example, glucose, fuel the cell’s activities, and
some are converted to fatty acids to help build
cell membranes and to store energy for future
needs.
The second important process is cellular
respiration, a process reciprocal to photo-
synthesis. During cellular respiration, the
cell breaks down sugars into usable energyAnnette (“Annie”) Rowe is a postdoctoral research
associate at the University of Southern California,
where she studies microbes that take up electrons
from inorganic surfaces.ANNETTE ROWE