Читать книгу Cell Biology - Stephen R. Bolsover - Страница 14

Superficially at least, cells exhibit a staggering diversity. Some have defined, geometric shapes; others have flexible boundaries; some lead a solitary existence; others live in communities; some swim, some crawl, and some are sedentary. Given these differences, it is perhaps surprising that there are only two types of cell (Figure 1.2). Prokaryotic (Greek for “before nucleus”) cells have very little visible internal organization so that, for instance, the genetic material, stored in the molecule deoxyribonucleic acid (DNA), is free within the cell. These cells are especially small, the vast majority being 1–2 μm in length. The prokaryotes are made up of two broad groups of organisms, the bacteria and the archaea (Figure 1.3). The archaea were originally thought to be an unusual group of bacteria but we now know that they are a distinct group of prokaryotes with an independent evolutionary history. The cells of all other organisms, from yeasts to plants to worms to humans, are eukaryotic (Greek for “with a nucleus”). These are generally larger (5–100 μm, although some eukaryotic cells are large enough to be seen with the naked eye; Figure 1.1) and structurally more complex. Eukaryotic cells contain a variety of specialized structures known collectively as organelles, embedded within a viscous substance called cytosol. Their DNA is held within the largest organelle, the nucleus. The structure and function of organelles will be described in detail in subsequent chapters. Table 1.1 summarizes the differences between prokaryotic and eukaryotic cells.

Figure 1.1. Dimensions of some example cells. 1 mm = 10⁻³ m; 1 μm = 10⁻⁶ m; 1 nm = 10⁻⁹ m.