Life on Earth is solar-powered. The chloroplasts of plants capture light energy that has traveled 160 million kilometers from the sun and convert it to chemical energy stored in sugar and other organic molecules. The process is called photosynthesis. In this chapter, you will learn how photosynthesis works. We begin by placing photosynthesis in its ecological context.

Photoautotrophs: Producers for most ecosystems.

These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (usually) water. they feed not only themselves, but the entire living world.

(A)

(B)

(A) On land, plants are the predominant producers of food. Three major groups of land plants - mosses, ferns, and flowering plants - are represented in theis scene. In oceans, ponds, lakes, and other aquatic enviroments, photosynthetic organisms include (B) multicellular algae, such as kelp; (C) some unicellular protists, such as Euglena; (D) and the prokaryotes called cyanobacteria; and (E) other photosynthetic prokaryotes, such as these purple sulfur bacteria (C, D, E; LMs).

(C)

(D)

(E)

Plants and other autotrophs are the producers of the biosphere

Photosynthesis nourishes almost all of the living world directly or indirectly. An organism acquires the organi, compounds it uses for energy and carbon skeletons one of two major modes: autotrophic or heterotrophi nutrition. At first, the term autotrophic (Gr. autos, "self," and trophos, "feed") may seem to contradict the principle that organisms are open systems, taking in resource from their environment. Autotrophs are not totally self sufficient, however; they are self-feeders only in the sense that they sustain themselves without eating or decomposing other organisms. They make their organic molecules from inorganic raw materials obtained from the environment. It is for this reason that biologists refer to autotrophs as the producers of the biosphere. Plants are autotrophs; the only nutrients they require are carbon dioxide from the air, and water and minerals from the soil. Specifically, plants are photoautotrophs, organisms that use light as a source of energy to synthesize lipids, proteins, and other organic substances. Photosynthesis also occurs in algae, including certain protists, and in some prokaryotes (FIGURE io.1). In this chapter, our emphasis will be on plants. Variations in photosynthesis that occur in algae and bacteria will be discussed in Unit Five. A much rarer form of self-feeding is unique to those bacteria that are chemoautotrophs. These organisms produce their organic compounds without the help of light, obtaining their energy by wigdizing inorganic substances, such as sulfur or ammonia. (We will postpone further discussion of this type of autotrophic nutrition until Chapter 25.)

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; heterotrophs are the biosphere's consumers. The most obvious form of this "other-feeding" (hetero means "other, different") is when an animal eats plants or other animals. But heterotrophic nutrition may be more subtle. Some heterotrophs do not kill prey, but instead decompose and feed on organic litter-such as carcassesces, and fallen leaves-and thus are known as decomposers. Most fungi and many types of bacteria get their nourishment this way. Almost all heterotrophs, including humans, are completely dependent on photoautotrophs for food, and also for oxygen, a by-product of photosynthesis. Thus, we can trace the food we eat and the oxygen we breathe to the chloroplast.