Prokaryotes and Eukaryotes

How are prokaryotic and eukaryotic cells different?

Cells come in an amazing variety of shapes and sizes, some of which are shown in Figure 7–4. Although typical cells range from 5 to 50 micrometers in diameter, the smallest Mycoplasma bacteria are only 0.2 micrometer across, so small that they are difficult to see under even the best light microscopes. In contrast, the giant amoeba Chaos chaos can be 1000 micrometers (1 millimeter) in diameter, large enough to be seen with the unaided eye as a tiny speck in pond water. Despite their differences, all cells, at some point in their lives, contain DNA, the molecule that carries biological information. In addition, all cells are surrounded by a thin flexible barrier called a cell membrane. (The cell membrane is sometimes called the plasma membrane because many cells in the body are in direct contact with the fluid portion of the blood—the plasma.) There are other similarities as well, as you will learn in the next lesson.

Cells fall into two broad categories, depending on whether they contain a nucleus. The nucleus (plural: nuclei) is a large membrane-enclosed structure that contains genetic material in the form of DNA and controls many of the cell's activities. Eukaryotes (yoo KAR ee ohts) are cells that enclose their DNA in nuclei. Prokaryotes (pro KAR ee ohts) are cells that do not enclose DNA in nuclei.

FIGURE 7–4 Cell Size Is Relative The human eye can see objects larger than about 0.5 mm. Most of what interests cell biologists, however, is much smaller than that. Microscopes make seeing the cellular and subcellular world possible.

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