Life on Earth is solar powered. The

Life on Earth is solar powered. The chloroplasts of plants capture light energy that has traveled 150 million kilometers from the sun and convert it to chemical energy that is stored in sugar and other organic molecules. This conversion process is called
photosynthesis. Let’s begin by placing photosynthesis in its ecological context. Photosynthesis nourishes almost the entire living world directly or indirectly. An organism acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs are “self-feeders” (automeans “self,” and trophos means “feeder”); they sustain themselves without eating anything derived from other living beings. Autotrophs produce their organic molecules from CO2 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 require are water and minerals from the soil and carbon dioxide from the air. Specifically, plants are photoautotrophs, organisms 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 second major mode of nutrition. Unable to make their own food, they live on compounds produced by other organisms
(heteromeans “other”). Heterotrophs are the biosphere’s consumers
. The most obvious form of this “other-feeding” occurs when an animal eats plants or other animals. But heterotrophic 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 photoautotrophs for food—and also for oxygen, a by-product of photosynthesis.