Biology Now, 2e

84 ■ CHAPTER 05 How Cells Work

CELLS

Into the Light,

Part 1

In 2009, environmental engineers at Pennsyl-

vania State University identified the first elusive

rock eaters by exposing a mix of microbes to

a negatively charged electrode (Figure 5.5).

One of the species, Methanobacterium palus-

tre, survived on the electricity, taking in elec-

trons and using them in a metabolic pathway to

convert carbon dioxide to methane. One news

article summed up the experiment with the

headline bug eats electricity, farts biogas.

The Penn State engineers proposed, but were

unable to prove, that the bacteria (“bugs”) were

ingesting naked electrons straight from the

surface of the electrode.

A few years later, Alfred Spormann launched

his own investigation into bacteria that seemed to

be “eating” electrons. Spormann, a microbiologist

at Stanford University, has long been interested

in outliers, microbes with unusual metabolisms.

Tall and thin with a throaty German accent,

Spormann has spent more than 25 years study-

ing such microbes, from those in the human gut

that contribute to irritable bowel syndrome to

bacteria that could be used to help decontam-

inate groundwater. “I’m really interested in

exceptions to the rules, and in understanding

the plasticity of microbial metabolism,” says

Spormann. Typically, the microbes he studies

are anaerobic, growing in places with little

or no oxygen. These cells don’t need oxygen

Much of the usable energy in ATP is held in

its energy-rich phosphate bonds (Figure 5.4).

Energy is released when a molecule of ATP loses

its terminal phosphate group, breaking into a

molecule of ADP (adenosine diphosphate) and a

free phosphate. Converting ADP and phosphate

back into ATP takes metabolic energy.

ATP is not the only energy carrier that cells

rely on; NADPH, NADH, and FADH 2 are also

energy carriers. Each one is a specialist in

terms of the amount of energy it carries and the

types of chemical reactions to which it supplies

energy and from which it receives energy. ATP,

because of its phosphate bonds, carries the

most energy.

Energy

for cells

Energy

from sunlight

or food

P P

ATP

A P P P

ADP

A P P

P

A Adenosine

Phosphate

Figure 5.4

Cells store and deliver energy using ATP

Loading a high-energy phosphate (P) on ADP

transforms the otherwise sedate molecule into

the live wire that is ATP. But turning ADP and

phosphate groups into ATP—the universal energy

currency of cells—takes metabolic energy.

Q1: Define ATP in your own words.

Q2: How is ATP involved in anabolism and

catabolism? (Hint: Review Figure 5.3.)

Q3: Arsenic disrupts ATP production. Why

would this characteristic cause it to be a

potent poison?

Figure 5.5

A negatively charged electrode used to

grow “rock eaters”

Alfred Spormann is a professor of civil and

environmental engineering, and of chemical

engineering, at Stanford University. He investigates

novel microbial metabolism, including how microbes

transport electrons between themselves and surfaces.

ALFRED SPORMANN