Life on Earth is solar powered. The

L
ife on Earth is solar powered. The chloroplasts of plants
capture light energy that has traveled 150 million kilo-
meters from the sun and convert it to chemical energy that
is stored in sugar and other organic molecules. This conver-
sion process is called
photosynthesis
. Let’s begin by plac-
ing photosynthesis in its ecological context.
Photosynthesis nourishes almost the entire living world
directly or indirectly. An organism acquires the organic com-
pounds it uses for energy and carbon skeletons by one of
two major modes: autotrophic nutrition or heterotrophic
nutrition.
Autotrophs
are “self-feeders” (
auto-
means “self,”
and
trophos
means “feeder”); they sustain themselves with-
out eating anything derived from other living beings. Auto-
trophs produce their organic molecules from CO
2
and other
inorganic raw materials obtained from the environment.
They are the ultimate sources of organic compounds for all
nonautotrophic organisms, and for this reason, biologists
refer to autotrophs as the
producers
of the biosphere.
Almost all plants are autotrophs; the only nutrients they re-
quire are water and minerals from the soil and carbon dioxide
from the air. Specifically, plants are
photo
autotrophs, organ-
isms that use light as a source of energy to synthesize organic
substances
(Figure 10.1)
. Photosynthesis also occurs in algae,
certain other protists, and some prokaryotes
(Figure 10.2)
. In
this chapter, we will touch on these other groups in passing,
but our emphasis will be on plants. Variations in autotrophic
nutrition that occur in prokaryotes and algae will be described
in Chapters 27 and 28.
Heterotrophs
obtain their organic material by the sec-
ond major mode of nutrition. Unable to make their own
food, they live on compounds produced by other organisms
(
hetero-
means “other”). Heterotrophs are the biosphere’s
consumers
. The most obvious form of this “other-feeding” oc-
curs when an animal eats plants or other animals. But hetero-
trophic nutrition may be more subtle. Some heterotrophs
consume the remains of dead organisms by decomposing
and feeding on organic litter such as carcasses, feces, and
fallen leaves; they are known as decomposers. Most fungi
and many types of prokaryotes get their nourishment this
way. Almost all heterotrophs, including humans, are com-
pletely dependent, either directly or indirectly, on photoau-
totrophs for food—and also for oxygen, a by-product of
photosynthesis.