Structure and Function

How do prokaryotes vary in their structure and function?

Because prokaryotes are so small, it may seem hard to tell them apart.

Prokaryotes vary in their size and shape, in the way they move, and in the way they obtain and release energy.

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FIGURE 20–9 Prokaryotic Shapes Prokaryotes usually have one of three basic shapes: bacilli (left), cocci (middle), or spirilla (right).

Size, Shape, and Movement Prokaryotes range in size from 1 to 5 micrometers, making them much smaller than most eukaryotic cells. Prokaryotes come in a variety of shapes, as shown in Figure 20–9. Rod-shaped prokaryotes are called bacilli (buh SIL eye; singular: bacillus). Spherical prokaryotes are called cocci (KAHK sy; singular: coccus). Spiral and corkscrew-shaped prokaryotes are called spirilla (spy RIL uh; singular: spirillum). You can also distinguish prokaryotes by whether they move and how they move. Some prokaryotes do not move at all. Others are propelled by flagella. Some glide slowly along a layer of slimelike material they secrete.

Nutrition and Metabolism Like all organisms, prokaryotes need a supply of chemical energy, which they store in the form of fuel molecules such as sugars. Energy is released from these fuel molecules during cellular respiration, fermentation, or both. The diverse ways prokaryotes obtain and release energy are summarized in Figure 20–10. Notice that some species are able to change their method of energy capture or release depending on the conditions of their environment.

FIGURE 20–10 Energy Capture and Release by Prokaryotes Prokaryotes vary in the ways they obtain energy and the ways they release it. Interpret Tables What is the term for a prokaryote that uses only light as its energy source?

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