82 ■ CHAPTER 05 How Cells Work
CELLS
flowing through the electrodes in the sand began
to steadily increase. Something was stealing the
electrons, consuming more and more electricity.An Unusual Pathway
Scientists in the 1980s first discovered two
types of bacteria that expel—breathe, actually—
electrons onto metal. Because it was assumed(Figure 5.2). Plants break down the glucose
made during photosynthesis, whereas nonpho-
tosynthetic organisms, including animals, eat
plants or other animals and break down sugars
in that food to release energy. Electrons are
transferred back and forth during these two
metabolic processes. Metabolism, at its core, is
a feat of juggling electrons.
Rowe spent a week on Catalina setting up
her tanks and then let them sit for 3 months,
untouched. During that time, her instruments
began to detect evidence of life: the negative charge(^1) Sunlight is the
ultimate
source of
energy in most
ecosystems.
(^2) Photosynthetic organisms capture energy
from sunlight and use it to manufacture
sugars from CO 2 and H 2 O, releasing
oxygen into the environment.
(^3) Producers and
consumers use
cellular respiration to
break down sugars,
generating useful
energy for cells and
releasing carbon
dioxide and water
into the environment.
Sunlight
Sugars and O 2 out
CO 2 and H 2 O out
Photosynthesis
Cellular respiration
Figure 5.2
From sunlight to usable energy
Photosynthesis transforms sunlight into sugar molecules within the cell, releasing oxygen as a by-
product. Without photosynthesis, we would not have access to the sun’s energy, and we wouldn’t have
oxygen in our atmosphere to support life. In a complementary process, cellular respiration breaks down
these sugar molecules, allowing organisms to access the energy stored in them.
Q1: Why is photosynthesis called “primary production”?
Q2: How does animal life depend on photosynthesis?
Q3: Explain how photosynthesis and cellular respiration are “complementary” processes.