Prokaryotic cell

What is a Prokaryotic Cell?

Those organisms with primitive nucleus are called as prokaryotes (pro – primitive; karyon – nucleus). These are primitive and incomplete cells. They have less developed nucleus without nuclear membrane & nucleolus. They all are single-celled microorganisms. The cell size ranges from 0.1 to 0.5 µm in diameter. The hereditary material can either be DNA or RNA.

Example: Bacteria, blue green algae, Mycoplasma, Rickettsiae and Spirochaetae.

Characteristics of Prokaryotic Cell

The characteristics of the prokaryotic cells are:

1. DNA is naked and lying coiled in cytoplasm.
2. Nuclear envelope, nucleoplasm, nucleolus and histone covering are absent.
3. Cell wall is present in bacteria but absent in mycoplasma.
4. They lack mitochondria, endoplasmic reticulum , Golgi apparatus , lysosomes, microfilaments and centrioles.
5. Ribosomes are 70S.
6. Cytoplasm does not show streaming movement.
7. Reproduction occurs by asexual means like binary fission, sporulation.
8. DNA content is low in prokaryotic cells.
9. Respiratory enzymes are usually lie in contact with cell membrane.

Prokaryotic Cell Structure

A bacterial cell contains cell envelope, mesosomes, chromatophores, cytoplasm nucleoid plasmids, flagella, Pili and fimbriae.

Cell Envelope: It is a chemically complex protective covering. It is outer covering of bacterial cell and contains 3 components- glycocalyx cell wall and cell membrane.

Many prokaryotes, especially bacteria have a 3 layered call envelope.

Outermost glycocalyx: a carbohydrate coat to protect bacteria from host's immune system

Middle layer-cell wall: Provide mechanical support & prevent bacteria from bursting collapsing

Innermost cell membrane: due to its selectively permeable nature, it decides which molecules shall go in and out of the cell (interacts with the outside world).

Glycocalyx, Cell wall, cell membrane: All 3-layers work together as a Protective unit
Mesosomes & Chromatophores
Mesosomes : They are infoldings of the bacterial cell membrane. it consists of vesicles, tubules & lamellae.   They are of 2 types septal and lateral.  It is like mitochondria of eukaryotes.

Chromatophores : Membrane bound organelles containing pigments that helps in photosynthesis in bacteria. They are present in photosynthetic prokaryotes. It is a double membrane structure and are similar to chloroplast in plants

Nucleoid: It is the region in which the genetic material of prokaryotes r present. It consists of a single circular strand of DNA. It is formed of non-membranous circular genomic DNA and Protein.

Flagella :These are thin filamentous extensions from the cell wall of motile bacteria.  Flagella are long, slender, filamentous structure that protrude from the bacterial surface and help in motility.

 Pili and Fimbriae

Pili are longer and thicker tubular structure which developed in response to fertility in gram negative bacteria it is made up of protein called pilin.  These are surface structures that have no role in motility. Pillai enables bacterial conjugation transferring genetic material between cells.,

Fimbriae are small bristle like fibres sprouting out of the cell. They facilitate bacterial attachment to surface. They are short thin structure composed of fimbrial protein. In some bacteria, they help to attach the bacteria to rocks in streams and to the host tissues.

Ribosomes: They are small membrane less microscopic. Ribosomes are of 2 types fixed and free fixed ribosomes are attached to plasma membrane and free ribosomes occurs free in the cytoplasmic matrix. Bacterial Ribosomes are 70S nature. It consists of 2 subunits 50S and 30S. Ribosomes take part in protein synthesis. They are associated with plasma membrane of prokaryotes.

 Inclusion Bodies: They are non living structure present in the cytoplasm. These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells. They are the membrane bound or membrane less organelles that contains specific proteins lipids or other molecule separating them from rest of the cell.

Prokaryotic Cell Diagram

The diagram below shows the structure of a prokaryotic cell, which is a type of bacterial cell. 

Prokaryotic cells are simpler and smaller compared to eukaryotic cells. They lack membrane-bound organelles and a true nucleus, but still perform essential functions necessary for life.

Components of prokaryotic cell:

1. No True Nucleus: Unlike eukaryotic cells, prokaryotic cells don't have a true nucleus. Instead, their genetic material (DNA) floats freely within the cell.

2. Flagellum: Some prokaryotic cells have a flagellum, which is a long, whip-like structure that helps them move.

3. Cell Wall: Prokaryotic cells have a cell wall that provides structural support and protection.

4. Plasma Membrane: The plasma membrane is a thin layer that surrounds the cell and regulates the movement of substances in and out.

5. Cytoplasm: The cytoplasm is the jelly-like substance within the cell where many metabolic processes take place.

6. Ribosomes: Ribosomes are small organelles found throughout the cytoplasm where protein synthesis occurs.

Reproduction in Prokaryotes

It occurs by three methods:

• Asexually by binary fission, sporulation
• Sexually by conjugation

Binary Fission

• It is the common method of bacterial multiplication under favourable condition.
• Under favourable conditions the cell divides into two daughter cells. 
• The nuclear material divides first, and it is followed by the formation of a simple median constriction which finally results in the separation of two cells.

Recombination

Typical sexual reproduction involving the formation and fusion of gametes is absent in bacteria. However, gene recombination can occur in bacteria by three different methods:

1. Conjugation: Conjugation is the process by which two prokaryotic cells directly transfer DNA from one cell to another. This process involves the use of a specialized structure called a conjugation pilus, which allows the cells to come into close contact and exchange DNA.

2. Transformation: Transformation is the process by which a prokaryotic cell takes up free DNA molecules from its environment. This DNA can come from other cells that have lysed or from viral infections. Once the DNA is taken up, it can be integrated into the cell's genome through recombination.

3. Transduction: Transduction is the process by which a prokaryotic cell receives DNA from a bacteriophage (a virus that infects bacteria). When a bacteriophage infects a bacterial cell, it can accidentally package some of the host cell's DNA into its own viral particles. If these viral particles, then infect another bacterial cell, they can transfer some of the host cell's DNA to the new cell.

Examples of Prokaryotic Cells

Bacteria, blue green algae, Mycoplasma, Rickettsiae and Spirochaetae.