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Prokaryotic Cells

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Prokaryotic Cells - Lesson Summary

Cells that possess a nucleiod but no membrane-bound organelles are called prokaryotic cells. Such organisms are called prokaryotes. All the members of Kingdom Monera are prokaryotes. Most prokaryotes possess a cell envelope ― consisting of the outermost glycocalyx, followed by the cell wall and then the plasma membrane. In some prokaryotes, glyocalyx occurs as a thick and tough capsule, while in others it is a loose slimy layer. The glycocalyx and the cell wall give shape and structural support to the cell.

The cell wall structure differs in different prokaryotes. In certain prokaryotes such as pseudomonas, the cell wall is composed of inner peptidoglycan and an outer lipopolysaccharide and lipoprotein. When such prokaryotes are subjected to the gram-staining procedure, they do not take up the gram stain but appear red or pink when counter-stains such as safrannin are added. Hence, they are called gram-negative bacteria. Whereas in some prokaryotes such as Bacillus anthracis, the cell wall is composed of peptidoglycan and teichoic acids, which turns violet when subjected to the gram-staining procedure. Hence, they are called gram-positive bacteria.
 
The cell wall is followed by the semi-permeable plasma membrane which controls the entry and exit of small molecules. Often, the plasma membrane extends into the cell, forming the mesosome.
 
Mesosomes help in cell wall formation, respiration, secretion processes, expansion of the surface area of the plasma membrane and enzymatic content, DNA replication and distribution to daughter cells. Some prokaryotes also have photosynthetic lamellae that contain photosynthetic pigments. Motile bacterial cells have a thin filamentous extension from their cell wall called a flagellum. Motile as well as non-motile prokaryotes possess pili and fimbriae, which attach themselves to rocks in streams and also to host tissues. The cytoplasm hosts a single chromosome made up of circular DNA. Many prokaryotes also have plasmids outside their genomic DNA. The cytoplasm also contains 70S ribosomes.   
 
These ribosomes attach to mRNA, and translate mRNA into proteins. Cytoplasm also contains inclusion bodies such as phosphate granules, cyanophycean granules and glycogen granules, which are used for storing reserve materials. Certain prokaryotes such as blue-green, purple and green photosynthetic bacteria also have other inclusion bodies such as gas vacuoles, which help them float in water. 

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