Prokaryotic cell

PROKARYOTIC CELL DEFINITION

All living things found on the planet earth are divided into two major groups namely, Prokaryotic and Eukaryotic based on the types of cells these organisms  possess. Bacteria and Cyanobacteria are prokaryotes Fungi, plants and animals are eukaryotes.

PROKARYOTIC CELL STRUCTURE

In general, Prokaryotes contains one closed compartment containing the cytosol and bounded by the cell wall .Although bacterial cells don have a well defined nucleus, the genetic material, DNA, is condensed into the central region of the cell. In all prokaryotic cells, most of or all the genetic information resides in a single circular DNA molecule, in the central region of the cell. This region is often referred to as incipient nucleus or nucleoid. In addition, most ribosomes, the cell’s protein synthesizing centres are found within the DNA-free region of the. cell. Some bacteria even have an invagination of the cell wall called a mesosome, which is related to synthesis of DNA and secretion of proteins. Thus we can not say that bacterial cells are completely devoid of internal organization.


Bacterial cells possess a cell wall which lies adjacent to the external side of  the plasmamembrane. The cell membrane consists of layers of peptidoglycan, a posh of proteins and oligosaccharides. It protects the cell and maintain its shape.

Some bacteria (eg E.coli) have a thin cell wall and an unusual outermembranes Separated from the cell wall by the periplasmic space. Such bacteria aren't stained by Gram staining technique and thus are classified as Gram negative bacteria.

Other bacteria (eg. Bacillus polymyxa) that have a thicker cell wall without ano outermembrane take the Gram stain and thus are classified as  Gram positive bacteria.

Ultra Structure Of A Prokaryotic Cell

The bacterium is surrounded by two definite membranes separated by the periplasmic space. The outer layer is rigid, serves for mechanical protection and is designated because the cell membrane . The chemical composition of the cell membrane is quite complex; it contains peptidoglycan, polysaccharides, lipid and protein molecules.

Source Wikipedia

One of the most abundant polypeptides, porin, forms channels that allow for thed diffusion of solutes. The plasma membrane is a lipoprotein structure serving as an molecular barrier with the surrounding medium. The plasma membrane controls the entry and exit of small molecules and ions. The enzymes involved in the oxidation of metabolites (i.e the respiratory chain) as well as the photosystems used in photosynthesis are present in the plasma membrane of prokaryotes.


The bacterial chromosome may be a single circular molecule of naked DNA tightly coiled within the nucleoid which appears within the microscope as a lighterr region of the protoplasm. it's amazing to notice that the DNA of E.coli which measures about 1mm long when uncoiled, contains all the genetic information of the organism. during this case, there's sufficient information to code for 2000 to 3000 different proteins.

The single chromosome or the DNA molecule is circular and at one point it's attached to the cell wall and it's believed that this attachment may help within the separation of two chromosomes after DNA replication.


In addition to a chromosome, certain bacteria contain a small, extra-chromosomal circular DNA called plasmid. The plasmid is responsible for the antibiotic resistance in some bacteria.

These plasmids are considerably utilized in gene-splicing ,where the plasmids are separated and reincorporated, genes (specific pieces of DNA) are often inserted into plasmids, which are then transplanted into bacteria using the techniques of gene-splicing .

Surrounding the DNA within the darker region of the protoplasm are 20,000 to 30,000 particles called ribosomes. These are composed of RNA and proteins and are the sites of protein synthesis. Ribosomes exist in groups called polyribosomes or polysomes. Each ribosome consists of a large and a small sub unit. Ther remainderof the cell is filled with H2O , various RNAs, protein molecules(including enzymes) and various  smaller molecules.


Certain motile bacteria have numerous, thin hair like processes of variable lenghth called flagella. Flagella are used for locomotion. In contrast with the flagella of eukaryotic cells which contain 9+2 micortubles each flagellum in bacteria is formed of one fibril.

Prokaryotes are successively classified into two major sub-groups.

1) the Archae bacteria and
2) Eubacteria.

Cyanobacteria are included in the group Eubacteria.

The Cyanobacterial prokaryotes, commonly called blue-green algae, are photosynthetic. In cyanobacterial cells, the photosynthetic,
respiratory and genetic apparatuses are present but not delimited from each other by any bounding membrane of their own. No sharp boundaries divide the cell into special regions. But, there are several cell components with characteristic spectrum line . These are distributed throughout the cell in patterns varying from species to and also in several developmental stages within the same species.

These cyanobacterial cells have an elaborate photosynthetic membranes systems composed of straightforward thylakoids and a central nucleoplasmic area which is typically fibrillar or granular or both. The cell also includes various kinds of granular inclusions, a rigid, several layered cell wall and a fibrous sheath over the cell wall. The characteristic collective properties of Cyanobacteria include oxygenicp  chromatic adaptation, nitrogen fixation and a capacity for cellular differentiation by the formation of heterocysts , akinetes and hormogonia.