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Microorganisms typically face the world as single cells rather than the multi-cellular assemblies of higher organisms. Therefore, each cell must contain all the structures necessary for managing its internal state and dealing with the outside environment. Not surprisingly, this evolutionary process results in similar structures and processes to solve similar needs in different microorganisms. However, prokaryotes have been on this earth for a long time, and this has allowed them to differentiate into a dizzying number of different species. Eukaryotic microbes are not quite so diverse, but they still display a remarkable range of properties. No one cell contains all the structures that we describe here, but we will explore the more common structures (Figure 3.1) observed by scientists in the past 150 years. A distinction in this discussion must be made between the two types of prokaryotes: the Archaea and their cousins, the Bacteria. We will initially focus on Bacteria since we know the most about them. Many of the structures we will examine are present in Bacteria and Archaea, but there are significant differences. We will cover these at the end of the chapter. Finally, we will talk about the features that are distinctive among the microbial eukaryotes.

So how did scientists find out so much about such tiny organisms? As you might guess, many techniques come into play when tackling a subject as complex as bacterial structure. Microscopes help scientists visualize the location of structures and how they are arranged spatially in the microbe. Bacterial genetics and molecular biology identify and analyze the genes necessary for the synthesis and regulation of these structures. Biochemistry permits the detailed examination of each part separately, with implications for its role in the living bacterium. The powerful combination of these disciplines provides a deep understanding of how a bacterium is put together, but there is still much to learn.

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