Rock Eaters ■ 93“If you could convert electrical energy, through
microbial processes, to a liquid fuel, that could be
a really valuable way of storing energy,” says Rowe.
This electricity-to-fuel scenario could also be run
in reverse, using microbes to digest a fuel source
such as raw sewage and convert it into electricity.
There’s also the intriguing scenario that
someday our electronic devices will be powered
by microorganisms that manipulate the flow
of electrons. In this rapidly evolving field, the
possibilities are endless. And so the hunt for
rock eaters continues. “We don’t know how
many organisms do or don’t do it,” adds Spor-
mann. “It’s entirely possible that rock eating is
more widespread than previously thought. We
just don’t know because it’s so new.”Bacterial Batteries
Because cell membranes are insulated, research-
ers long believed that cells could not transport elec-
trons directly across their membranes, but now it
appears they can. In 2010, in fact, Rowe’s post-
doctoral adviser, Mohamed El-Naggar, a pioneer
in the rock breather field, found that the tiny
hairlike structures called pili (singular “pilus”) on
some bacteria are conductive (Figure 5.13)—just
as conductive, in fact, as silicon, the basis of most
electronics. One single nano-sized pilus, which
researchers refer to as a bacterial “nanowire,”
can transport about 106 electrons per second—
enough to sustain the respiration of a whole cell.
Today, Rowe, Spormann, and others are
focusing on how to use these unique bacteria in
“bacterial batteries.” Energy absorbed during the
day from a solar panel currently has to be either
instantly used or siphoned off into the electrical
grid. But what if it could be passed into a tank of
bacteria instead, where the microbes would gobble
up the electricity and store it in chemical bonds in
the form of methane or another natural gas? In
that case the gas could be stored until needed.
● (^) The sun is the source of energy fueling most
living organisms. Plants, algae, and some bacteria
gain energy from their environment through
photosynthesis. Most organisms use cellular
respiration to extract usable energy from sugar
molecules. In chemical terms, photosynthesis is the
opposite of cellular respiration.
● (^) All the many chemical reactions involved in the
capture, storage, and use of energy by living
organisms are collectively known as metabolism.
Energy-releasing breakdown reactions like cellular
respiration are catabolism; energy-requiring
synthesis reactions like photosynthesis are
anabolism.
● (^) A metabolic pathway is a multistep sequence of
chemical reactions, with each step catalyzed by a
different enzyme.
● (^) Energy carriers store energy and deliver it for cellular
activities. ATP is found in all cells and is the most
commonly used energy carrier.
● (^) Photosynthesis takes place in chloroplasts and
occurs in two stages. In the light reactions, energy
is absorbed using pigment molecules that include
REVIEWING THE SCIENCE
chlorophyll as electrons flow along the electron
transport chain. The light reactions create the
energy carriers ATP and NADPH, splitting water
molecules and releasing oxygen gas.● (^) The energy carriers are then used to convert carbon
dioxide into sugar molecules during the light-
independent reactions, or Calvin cycle. In the first
of the reactions, the enzyme rubisco catalyzes the
fixation of CO 2.
● (^) Enzymes are biological catalysts, usually small
proteins, that speed up chemical reactions.
An enzyme’s function is based on its chemical
characteristics and the three-dimensional shape of
its active site.
● (^) Cellular respiration occurs in three stages:
(1) glycolysis, which yields small amounts of ATP and
NADH; (2) the Krebs cycle, which releases carbon
dioxide and produces NADH, FADH 2 , and ATP; and
(3) oxidative phosphorylation, which generates many
molecules of ATP.
● (^) In the absence of oxygen, fermentation breaks down
the products of glycolysis into alcohol or lactic acid.
Figure 5.13
The rock eater Methanococcus
maripaludis, with electrically
conductive hairlike pili