THE LIVING WORLD
Unit two. The Living Cell
4.3. Prokaryotic Cells
There are two major kinds of cells: prokaryotes and eukaryotes. Prokaryotes have a relatively uniform cytoplasm that is not subdivided by interior membranes into separate compartments. They do not, for example, have special membrane-bounded compartments, called organelles, or a nucleus (a membrane-bounded compartment that holds the hereditary information). As discussed in chapter 1, figure 1.1, the two main groups of prokaryotes are bacteria and archaea; all other organisms are eukaryotes.
Prokaryotes are the simplest cellular organisms. Over 5,000 species are recognized, but doubtless many times that number actually exist and have not yet been described. Although these species are diverse in form, their organization is fundamentally similar: They are single-celled organisms; the cells are small (typically about 1 to 10 micrometers thick); the cells are enclosed by a plasma membrane; and there are no distinct interior compartments (figure 4.4). Outside of almost all bacteria and archaea is a cell wall, composed of different molecules in different groups (see table 15.1 and section 16.3). In some bacteria another layer called the capsule encloses the cell wall. Archaea are an extremely diverse group that inhabit diverse environments (figure 4.5a). Bacteria are abundant and play critical roles in many biological processes. Bacteria assume many shapes, like the sausage or spiral shapes shown in figure 4.5b, c. They can also adhere in chains and masses like the spherical cells in figure 4.5d, but in these cases the individual cells remain functionally separate from one another.
Figure 4.4. Organization of a prokaryotic cell.
Prokaryotic cells lack internal compartments. Not all prokaryotic cells have a flagellum or a capsule like the one illustrated here, but all have a nucleoid region, ribosomes, a plasma membrane, cytoplasm, and a cell wall.
Figure 4.5. Diversity of prokaryotes.
(a) Methanococcus is only able to survive in environments with no oxygen. (b) Bacillus is a rodshaped bacterium. (c) Treponema is a coil-shaped bacterium; rotation of internal filaments produces a corkscrew movement. (d) Streptomyces is a more or less spherical bacterium in which the individuals adhere in chains.
The interior of a prokaryotic cell has little or no structural support (the cell wall supports the cell’s shape), but scattered throughout the cytoplasm are small structures called ribosomes. Ribosomes are the sites where proteins are made, but they are not considered organelles because they lack a membrane boundary. Prokaryotic DNA is found in a region of the cytoplasm called the nucleoid region, which is not enclosed within an internal membrane. Some prokaryotes use a flagellum (plural, flagella) to move. Flagella are long, threadlike structures, made of protein fibers that project from the surface of a cell. They are used in locomotion and feeding. There may be none, one, or more per cell depending on the species. Bacteria can swim at speeds of up to 20 cell diameters per second, rotating their flagella like screws. Pili (singular, pilus) are short flagella (only several micrometers long, and about 7.5 to 10 nanometers thick) that occur on the cells of some prokaryotes. Pili help the prokaryotic cells attach to appropriate substrates and aid in the exchange of genetic information between cells.
Key Learning Outcome 4.3. Prokaryotic cells lack a nucleus and do not have an extensive system of interior membranes.