The Structure Of The Prokaryotic Cell

The overall structure of the prokaryotic cell is deceptively simple (figure 3.23). The cytoplasmic membrane surrounds the cell, acting as a barrier between the external environment and the interior of the cell. This membrane permits the passage of only certain molecules into and out of the cell. Enclosing the cytoplasmic membrane is the cell wall, a rigid barrier that functions as a tight corset to keep the cell contents from bursting out. Cloaking the wall may be additional layers, some of which serve to protect the cell from predators and environmental assaults. The cell may also have appendages giving it useful traits, including motility and the ability to adhere to certain surfaces.

The capsule (if present), cell wall, and cytoplasmic membrane together make up the cell envelope. Enclosed within this envelope are the contents of the cell—the cytoplasm and nucleoid. The cytoplasm is a viscous fluid composed of a variety of substances including water, enzymes and other proteins, carbohydrates, lipids, and various inorganic molecules. Structures within the cytoplasm include ribosomes and various storage granules. The nucleoid is the gel-like region where the chromosome resides. Unlike the nucleus that characterizes eukaryotic cells, the nucleoid is not enclosed within a membrane.

The structures of a prokaryote, together, enable the cell to survive and multiply in a given environment. Some structures are essential for survival and, as such, are common among all prokaryotic cells; others might be considered optional. Without these "optional" structures, the cell might be able to exist in the protected confines of a laboratory, but it may not be suited for the competitive surroundings of the outside world. The characteristics of typical structures of prokaryotic cells are summarized in table 3.3.

Table 3.3 A Summary of Prokaryotic Cell Structures

Structure

Characteristics

Extracellular

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