Prokaryotic Cells – Definition, Structure, Characteristics
The prokaryotic cells are represented by bacteria, blue-green algas, mycoplasma and PPLO (pleuro pneumonia-like organisms).They are generally smaller in size and multiply much faster than the eukaryotic cells. They may vary greatly in shape and size but exhibit a similar basic cellular organisation.
The organisation of the prokaryotic cell is fundamentally similar even though prokaryotes exhibit a wide variety of shapes and function. Most prokaryotes have a cell wall surrounding the cell membrane. The fluid matrix filling the cell is the cytoplasm. There is no well- defined nucleus. The genetic material is basically naked, not enveloped by a nuclear membrane. In addition to the genomic DNA (the single chromosome /circular DNA), many bacteria have small circular DNA outside the genomic DNA. These smaller DNA are called plasmids. The plasmid DNA confers certain unique phenotypic characters to such bacteria. One such character is resistance to antibodies. The plasmid DNA is used to monitor bacterial transformation with foreign DNA. No organelles, like the ones in eukaryotes, are found in prokaryotic cells except for ribosomes.
Cell Envelope and its Modifications
Most prokaryotic cells, particularly the bacterial cells, have a chemically complex cell envelope. The cell envelope consists of a tightly bound three-layered structure i.e. the outermost glycocalyx followed by the cell wall and the plasma membrane.
Glycocalyx is the outermost layer comprising a coating of mucous or polysaccharides macromolecules, which protects the cells and also helps in adhesion. This layer differs in thickness and chemical composition in different bacteria. Some have a loose sheath called slime layer, which protects the cell from loss of water and nutrients.
Others may have a thick and tough covering known as capsule. The capsule and slime layer are made up of polysaccharides, but may sometimes contain proteins also. The capsule is responsible for giving gummy and sticky character to the cell. It allows bacterium to hide from hosts immune system. The cell wall determines the shape of cell and provides a strong structural support to prevent the bacterium from bursting and collapsing. This layer is rigid due to a special macromolecules called peptidoglycan (murien or mucopeptide). A number of antibiotics (e.g., penicillin) inhibits cross linking of peptidoglycan strands. Therefore, cells undergo lysis in the presence of these antibiotics.
Gram staining (developed by Christian Gram) is a special technique, which is used to classify bacteria into two groups, viz. Gram-positive and Gram-negative bacteria. Those bacteria that taka up the Gram stain are Gram positive and the others that do not are called Gram negative bacteria.
The plasma membrane is selectively permeable in nature and interacts with the outside world. The membrane is similar structural it that of the eukaryotes. A special membranous structure is the mesosome which is formed by the invagination of plasma membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in the cell wall formation, DNA replication and distribution to daughter cells. They also help in help in respiration, secretion process, to increase the surface area of the plasma membrane and enzymatic content. Mesosome is found in gram positive bacteria. In some photosynthesis prokaryotes like cyanobacteria, and purple bacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagella. Bacteria show a rang in the number and arrangement of flagella. The flagellum is composed of three parts — filaments, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. It is a hollow rigid cylindrical structure made up of the protein called flagellin. Basal body is rod – like structure which consists of rings.
Besides flagella, pili and fimbriae are also surface structure of the bacteria but do not play a role in motility. The pili are elongated tubular structures made up of a special protein I.e., pilin. True pili have been reported only in Gram-negative bacteria so far and in these from they are involved in mating process. During this process, usually partial transfer of DNA from one cell (donor cell) to anther cell (recipient cell ) takes place. Formation of pili is generally considered and is specific for a cell type as conjunction takes place between compatible bacterial cells. The fimbriae are small bristle-like fibres sprouting out of the cell . These seem to be slender tubes composed of helically arranged protein subunits, which are 3-10 nm in diameter. In some bacteria, they are known to help in attaching the bacteria to rocks in streams and also to the host Tissues.
Ribosomes and Inclusions Bodies
In prokaryotes, ribosomes are associated with the plasma membrane of the cell. They are about 15nm by 20nm in size and made up of two subunits _ 50 S and 30S units which when present together form 70 S prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Cytoplasmic ribosomes synthesize proteins, which remain within cells but the ribosomes on the plasma membrane make proteins that are transported out. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA in to proteins.
Inclusion bodies : Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bounded any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanphycean granules and glycogen granules. Some other inclusion bodies may be surrounded by a single layer non unit membrane which is 2-4 nm thick ,e.g., poly B-hydroxybutyrate granules, sulphur granules and gas vacuole. Gas vacuoles are found in blue-green algae, purple and green photosynthetic bacteria.