Organisms have two ways of harvesting energy from their environment: They can obtain energy either from the sun or from chemical compounds (Fig. 6.1). Organisms that capture energy from sunlight are called phototrophs. Plants are the most familiar example. Plants use the energy of sunlight to convert carbon dioxide and water into sugar and oxgen (Chapter 8). Sugars, such as glucose, contain energy in their chemical bonds that can be used to synthesize ATP, which in turn can power the work of the cell.

Other organisms derive their energy directly from chemical compounds. These organisms are called chemotrophs, and animals are familiar examples. Animals ingest other organisms, obtaining organic molecules such as glucose that they break down in the presence of oxygen to produce carbon dioxide and water. In this process, the energy in the chemical bonds of the organic molecule is converted to energy carried in the bonds of ATP (Chapter 7).

In drawing this distinction between plants and animals, we have to be careful. Although sunlight provides the energy that plants use to synthesize glucose, plants still power most cellular processes by breaking down the sugar they make, just as animals do. And although chemotrophs harness energy from organic molecules, the energy in these organic molecules is generally derived from the sun. Bearing these two points in mind, we use the terms “phototroph” and “chemotroph” because they call our attention to the flow of energy from the sun to organisms and then from one organism to the next (discussed more fully in Chapter 25).

Organisms can also be classified in terms of where they get their carbon (Fig. 6.1). Some organisms are able to convert carbon dioxide (an inorganic form of carbon) into glucose (an organic form of carbon). These organisms are autotrophs, or “self feeders,” because they make their own organic carbon using inorganic carbon as the starting material. Plants again are an example, so plants are both phototrophs and autotrophs, or photoautotrophs.

Other organisms do not have the ability to convert carbon dioxide into organic forms of carbon. Instead, they obtain their carbon from organic molecules synthesized by other organisms, called preformed organic molecules. In other words, these organisms eat other organisms or molecules derived from other organisms. Such organisms are heterotrophs, or “other feeders,” as they rely on other organisms for their organic forms of carbon. Animals obtain carbon in this way, and so animals are both chemotrophs and heterotrophs, or chemoheterotrophs. In fact, animals get their energy and carbon from the same molecule. That is, a molecule of glucose supplies both energy and carbon.

As we move away from the familiar examples of plants and animals and begin in Part 2 to explore the diversity of life, we will see that not all organisms fit into the two categories of photoautotrophs and chemoheterotrophs (Fig. 6.1). Some microorganisms gain energy from sunlight but obtain their carbon from preformed organic molecules; such organisms are called photoheterotrophs. Other microorganisms extract energy from inorganic sources but build their own organic molecules; these organisms are called chemoautotrophs. They are often found in extreme environments, such as deep-sea vents, where sunlight is absent and inorganic compounds such as hydrogen sulfide are plentiful.

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