Prokaryotic Cells

Prokaryotes can live off more different and diverse energy sources than any other living creatures, and they inhabit greater environmental extremes, such as very hot springs and very salty water. The vast diversity within the prokaryotic domains is the subject of Chapter 27.

Prokaryotic cells are generally smaller than eukaryotic cells, ranging from 0.25 x 1.2 |im to 1.5 x 4 |im. Each prokaryote is a single cell, but many types of prokaryotes are usually seen in chains, small clusters, or even clusters containing hundreds of individuals. In this section, we will first consider the features that cells in the domains Bacteria and Archaea have in common. Then we will examine structural features that are found in some, but not all, prokaryotes.

Prokaryotic cells share certain features

All prokaryotic cells have the same basic structure:

► The plasma membrane encloses the cell, regulating the traffic of materials into and out of the cell and separating it from its environment.

► A region called the nucleoid contains the hereditary material (DNA) of the cell.

The rest of the material enclosed in the plasma membrane is called the cytoplasm. The cytoplasm is composed of two parts: the liquid cytosol, and insoluble suspended particles, including ribosomes.

► The cytosol consists mostly of water that contains dissolved ions, small molecules, and soluble macromole-cules such as proteins.

► Ribosomes are granules about 25 nm in diameter that are sites of protein synthesis.

The cytoplasm is not a static region. Rather, the substances in this aqueous environment are in constant motion. For example, a typical protein moves around the entire cell within a minute, and encounters many molecules along the way.

Although structurally less complicated than eukaryotic cells, prokaryotic cells are functionally complex, carrying out thousands of biochemical transformations.

Some prokaryotic cells have specialized features

As they evolved, some prokaryotes developed specialized structures that gave a selective advantage to those cells that had them. These structures include a protective cell wall, an internal membrane for compartmentalization of chemical reactions, and flagella for cell movement through the watery environment. These features are shown in Figures 4.5 and 4.6.

cell walls. Most prokaryotes have a cell wall located outside the plasma membrane. The rigidity of the cell wall

Pseudomonas Aeruginosa Capsule

Capsule

200 nm

4.5 A Prokaryotic Cell The bacterium Pseudomonas aeruginosa illustrates typical prokaryotic cell structures. Note the existence of several protective structures external to the plasma membrane.

Cytoplasm Ribosomes Nucleoid Plasma membrane

Cell Peptidoglycan wall Outer membrane

Capsule

200 nm

4.5 A Prokaryotic Cell The bacterium Pseudomonas aeruginosa illustrates typical prokaryotic cell structures. Note the existence of several protective structures external to the plasma membrane.

supports the cell and determines its shape. The cell walls of most bacteria, but not archaea, contain peptidoglycan, a polymer of amino sugars, cross-linked by covalent bonds to form a single giant molecule around the entire cell. In some bacteria, another layer—the outer membrane (a polysac-charide-rich phospholipid membrane)—encloses the pep-tidoglycan layer. Unlike the plasma membrane, this outer membrane is not a major permeability barrier, and some of its polysaccharides are disease-causing toxins.

Enclosing the cell wall in some bacteria is a layer of slime, composed mostly of polysaccharides and referred to as a capsule. The capsules of some bacteria may protect them from attack by white blood cells in the animals they infect. The capsule helps keep the cell from drying out, and sometimes it helps the bacterium attach to other cells. Many pro-karyotes produce no capsule, and those that do have capsules can survive even if they lose them, so the capsule is not essential to cell life.

As you will see later in this chapter, eukaryotic plant cells also have a cell wall, but it differs in composition and structure from the cell walls of prokaryotes.

internal membranes. Some groups of bacteria—the cyanobacteria and some others—carry on photosynthesis. In these photosynthetic bacteria, the plasma membrane folds into the cytoplasm to form an internal membrane system that contains bacterial chlorophyll and other compounds needed for photosynthesis. The development of photosynthesis, probably by such internal membranes, was

Cell Peptidoglycan wall Outer membrane

The Outer Capsule Prokaryotic Cell

an important event in the early evolution of life on Earth. Other prokaryotes have internal membrane folds that remain attached to the plasma membrane. These mesosomes may function in cell division or in various energy-releasing reactions.

flagella and pili. Some prokaryotes swim by using appendages called flagella (Figure 4.6a,c). A single flagel-lum, made of a protein called flagellin, looks at times like a tiny corkscrew. It spins on its axis like a propeller, driving the cell along. Ring structures anchor the flagellum to the plasma membrane and, in some bacteria, to the outer membrane of the cell wall (Figure 4.6c). We know that the flagella cause the motion of the cell because if they are removed, the cell cannot move.

Pili project from the surfaces of some groups of bacteria (Figure 4.6fr). Shorter than flagella, these threadlike structures help bacteria adhere to one another during mating, as well as to animal cells for protection and food.

cytoskeleton. Recent evidence suggests that some prokaryotes, especially rod-shaped bacteria, have an internal filamentous helical structure just below the plasma membrane. The proteins that make up this structure are similar in amino acid sequence to actin in eukaryotic cells, and since actin is part of the cytoskeleton in those cells (see below), it has been suggested that the helical filaments in prokaryotes play a role in cell shape.

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Responses

  • Awate Fesahaye
    Does the plasma membrane keep a prokaryotic cell wall from drying out?
    8 years ago
  • andi
    Is pseudomonas aeruginosa eukaryotic or prokaryotic?
    7 years ago
  • Donato
    What certain cell has a plasma membrane, ribosomes, and a nucleoid region?
    7 years ago
  • Brigitte Waechter
    What cell has a plasma membrane, ribosomes, and nuclecid region?
    7 years ago
  • rudi
    Is psuedomonas prokaryote?
    7 years ago

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