Biology Now, 2e

88 ■ CHAPTER 05 How Cells Work

CELLS

and each carbon atom is released into the envi-

ronment in a molecule of CO 2 , with water (H 2 O)

as a by-product. Thus, cellular respiration is the

reciprocal process of photosynthesis, as illus-

trated earlier, in Figure 5.2. In most eukaryotes,

cellular respiration is a multistep process that

occurs in the cell’s mitochondria (Figure 5.10).

Cellular respiration is the main way that animal

cells, which do not perform photosynthesis,

obtain energy. We digest glucose from food

sources; break it down into carbon dioxide,

hydrogen, and electrons; and then process those

captured electrons for energy.

Simply put, cellular respiration consists of

three stages: glycolysis, the Krebs cycle, and

oxidative phosphorylation (Figure 5.11). The

first stage, glycolysis, takes place in the cyto-

plasm of the cell. During glycolysis, sugars

(mainly glucose) are split to make a three-carbon

compound called pyruvate. This process results

in two useful molecules of ATP and two mole-

cules of NADH for each glucose molecule that

is split (Figure 5.11, top left). In other words,

glycolysis converts some of the chemical energy

of glucose into the chemical energy of NADH

and ATP.

From an evolutionary standpoint, glycoly-

sis was probably the earliest means of produc-

ing ATP from food molecules, and it is still

the primary means of energy production in

many prokaryotes, including some of Rowe’s

microbes. However, because glucose is only

partially broken down through this process, the

Into the Light,

Part 2

As Spormann was unraveling the mechanisms

of how electron-eating life works, Rowe’s pros-

pecting trip to Catalina uncovered more species

of rock eaters (Figure 5.9). From her tanks,

Rowe identified 30 new varieties of microbes

sucking electrons from the electrodes. She was

even able to grow a few of them on plates in

the laboratory, using minerals like sulfur and

iron as electron sources for the microbes to

munch. Not all the species relied exclusively on

electrons as a food source; that is, they didn’t

just live off mineral sediments. Like eukary-

otic cells, most microbial cells use the tradi-

tional route of cellular respiration for obtaining

energy, converting sugars into energy through

catabolic reactions.

During cellular respiration, the carbon-

carbon bonds in glucose molecules are broken,

Figure 5.9

Sediment containing rock eaters,

collected off the coast of Santa

Catalina Island

Outer Matrix

membrane

Inner

membrane

Intermembrane

space

Mitochondrion

Figure 5.10

Mitochondria are the site of cellular

respiration in eukaryotes