87

As they evolved, some prokaryotes developed specialized structures that gave them a selective advantage: cells with these structures were better able to survive and reproduce in particular environments than cells lacking them.

CELL WALLS Most prokaryotes have a cell wall located outside the cell membrane. The rigid wall supports the cell and determines its shape. The cell walls of most bacteria, but not archaea, contain peptidoglycan, a polymer of amino sugars that is linked at regular intervals to short peptides. Cross-linking among these peptides results in a single giant molecule around the entire cell. In some bacteria, another layer, the outer membrane (a polysaccharide-rich phospholipid membrane), encloses the peptidoglycan layer (see Figure 5.4). Unlike the cell membrane, this outer membrane is not a major barrier to the movement of molecules across it.

Enclosing the cell wall in some bacteria is a slimy layer composed mostly of polysaccharides and referred to as a capsule. In some cases these capsules protect the bacteria from attack by white blood cells in the animals they infect. Capsules also help keep the cells from drying out, and sometimes they help bacteria attach to other cells.

INTERNAL MEMBRANES Some groups of bacteria—including the cyanobacteria—carry out photosynthesis: they use energy from the sun to convert carbon dioxide and water into carbohydrates. These bacteria have an internal membrane system that contains molecules needed for photosynthesis. The development of photosynthesis, which requires membranes, was an important event in the early evolution of life on Earth. Other prokaryotes have internal membrane folds that are attached to the cell membrane. These folds may function in cell division or in various energy-releasing reactions.

FLAGELLA AND PILI Some prokaryotes swim by using appendages called flagella, which sometimes look like tiny corkscrews (Figure 5.5A). In bacteria, the filament of the flagellum is made of a protein called flagellin. (As you will see in Key Concept 5.3, the flagella of eukaryotes are quite different in structure, but similar in function.) A complex motor protein spins the bacterial flagellum on its axis like a propeller, driving the cell along. The motor protein is anchored to the cell membrane and, in some bacteria, to the outer membrane of the cell wall (Figure 5.5B). We know that the flagella cause the motion of cells because if they are removed, the cells do not move.

Pili are structures made of protein that project from the surfaces of some types of bacterial cells. These hairlike structures are shorter than flagella and are used for adherence. Conjugative pili (sex pili) help bacteria join to one another to exchange genetic material. Fimbriae are composed of the same proteins as pili but are shorter, and help cells adhere to surfaces such as animal cells, for food and protection.

CYTOSKELETON The cytoskeleton is the collective name for protein filaments that play roles in cell division, cell movement, and in maintaining the shapes of cells. One such protein forms a ring structure that constricts during cell division, whereas another forms helical structures that extend down the lengths of rod-shaped cells, helping maintain their shapes. In the past it was thought that only eukaryotic cells had cytoskeletons (see Key Concept 5.3), but more recently, biologists have recognized that cytoskeletal components are also widely distributed among prokaryotes.