Cell, prokaryotic

All cells contain DNA and are separated from their environment by a cell membrane. Prokaryotic cells do not have significant internal membranes, but the larger cells of eukaryotes have an extensive membrane system. The internal membranes mark off the organelles, portions of the cell with a specific function. 

Although no well-defined nucleus is present in prokaryotes, the DNA of the cell is concentrated in one region called the nuclear region. This part of the cell directs the workings of the cell, much as the eukaryotic nucleus does. 

Nuclear region (lighter area toward center of cell) 

In a prokaryotic cell, the cytosol (the fluid portion of the cell outside the nuclear region) frequently has a slightly granular appearance because of the presence of ribosomes. Because these consist of RNA and protein, they are also called ribonucleoprotein particles they are the sites of protein synthesis in aU organisms. The presence of ribosomes is the main visible feature of prokaryotic cytosol. (Membrane-bound organelles, characteristic of eukaryotes, are not found in prokaryotes.) 

the flocculating Z. with the change of morphology not only faciUtated 

The flocculation mechanism ofZ. mobilis ZM401 has not been understood so far. However, research has shown that the enzyme cellulase could effectively defloc-culate the cell floes of ZM401, and enhanced cellulose production facilitated more stable flocculent behavior in ZM401 [33]. This indicates that the self-flocculation of ZM401 cells might be attributed to the enhanced cellulose biosynthesis in this Z. mobilis mutant. 

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Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

In comparison to prokaryotic cells, eukaryotic cells are much greater in size, typically having cell volumes 10 to 10 times larger. Also, they are much more complex. These two features require that eukaryotic cells partition their diverse... 

Even the plasma membranes of prokaryotic cells (bacteria) are complex (Figure 9.1). With no intracellular organelles to divide and organize the work, bacteria carry out processes either at the plasma membrane or in the cyto-... 

Although the interior of a prokaryotic cell is not subdivided into compartments by internal membranes, the cell still shows some segregation of metabolism. For example, certain metabolic pathways, such as phospholipid synthesis and oxidative phosphorylation, are localized in the plasma membrane. Also, protein biosynthesis is carried out on ribosomes. 

The situation in bacteria is somewhat different. Prokaryotic cells need not carry out ATP/ADP exchange. Thus, bacteria have the potential to produce approximately 38 ATP per glucose. 

In terms of evolutionary biology, the complex mitotic process of higher animals and plants has evolved through a progression of steps from simple prokaryotic fission sequences. In prokaryotic cells, the two copies of replicated chromosomes become attached to specialized regions of the cell membrane and are separated by the slow intrusion of the membrane between them. In many primitive eukaryotes, the nuclear membrane participates in a similar process and remains intact the spindle microtubules are extranuclear but may indent the nuclear membrane to form parallel channels. In yeasts and diatoms, the nuclear membrane also remains intact, an intranuclear polar spindle forms and attaches at each pole to the nuclear envelope, and a single kinetochore microtubule moves each chromosome to a pole. In the cells of higher animals and plants, the mitotic spindle starts to form outside of the nucleus, the nuclear envelope breaks down, and the spindle microtubules are captured by chromosomes (Kubai, 1975 Heath, 1980 Alberts et al., 1989). 

Fig. 3-7 A thin section through a prokaryotic cell. Note that the nuclear material (N) is not bound by a membrane, but is free in the cytoplasm. Mitochondria and other intracytoplasmic structures are absent. (Reprinted with permission from J. J. Cardamone, Jr., Univ. of Pittsburgh/Biological Photo Service.)...

M. Duckworth, in I. Sutherland (Ed.), Surface Carbohydrates of the Prokaryotic Cell, Academic Press, London, 1977, pp. 177-208. 

Busby S, Ebright RH Promoter structure, promoter recognition, and transcription activation in prokaryotes. Cell 1994 79 ... 

Eukaryotic DNA that is in an active region of chromatin can be transcribed. As in prokaryotic cells, a... 

Prokaryotic cells express hundreds to thousands of proteins while higher eukaryotes express thousands to tens of thousands of proteins at any given time. If these proteins are to be individually identified and characterized, they must be efficiently fractionated. One-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) has typically been use to study protein mixtures of <100 proteins. Onedimensional electrophoresis is useful because nearly all proteins are soluble in SDS, molecules ranging from approximately 10,000 to 300,000 molecular weight can be resolved, and extremely basic or acidic proteins can be visualized. The major disadvantage to one-dimensional gels is that they are not suitable for complex mixtures such as proteins from whole cell lysates. 

Physically, all these prokaryotes are small, diameter about 1.0 pm and are of rigid, simple shape. They usually have little or no internal structure so that chemical diffusion is relatively rapid. Secondary compartments are rare but vesicles and vacuoles (even nuclei) are found in a very few large bacteria. We shall see that all the prokaryote cells have controlled, autocatalytic, internal metabolism, but are relatively little affected by external circumstances, except by shortage of nutrients. 

From the very earliest times these anaerobic prokaryote cells had assembled fundamental organic chemicals as outlined in Table 4.4, but as we have stressed in Chapters 3 and 4, to be viable, each cell type had to have also certain inorganic systems in place and had developed features such as... 

Thus prokaryote cells have a very limited sense of the environment and a limited search apparatus (see Harold in Further Reading). 

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