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Mastering Concepts 16.1 1. What are two domains that contain prokaryotes? Bacteria and Archaea are the two domains that include prokaryotes. 2. List several ways that prokaryotes have influenced evolution. Prokaryotes have influenced evolution by contributing O2 to the atmosphere, helping to create the ozone layer, and becoming incorporated into eukaryotic cells as mitochondria and chloroplasts. 3. In what habitats do bacteria and archaea live? Prokaryotes live in every conceivable habitat. 4. Why is the term prokaryote controversial? The term prokaryote is controversial because it encompasses both bacteria and archaea and implies that they share close evolutionary relationships, when evidence shows a closer relationship between the archaea and the eukaryotes. 16.2 1. What are the three most common cell shapes among microbes? The three most common shapes are spherical (coccus), rod (bacillus), and spiral (spirillum). 2. What are plasmids, and how are they important? Plasmids are small loops of DNA that are not part of the prokaryotic chromosome. They are important because they are easily transmitted from one cell to another, so that prokaryotes can rapidly acquire new traits from their neighbors. They are also important because they help biologists ferry genes from one prokaryotic cell to another. 3. What does the Gram stain reveal about a cell? The basis of Gram staining is differences in the structure of the cell wall. In particular, Gram positive bacteria have a thicker cell wall that retains more crystal violet stain than the cell wall of Gram negative bacteria. 4. What are the functions of a glycocalyx, pili, flagella, and endospores? The glycocalyx helps in attachment to a surface, keeps the cell from drying out, and protects it from immune system cells. Attachment pili allow bacteria to attach to surfaces, while sex pili allow transfer of DNA from cell to cell. Flagella help the cell to move toward or away from