Prokaryotic Cell

Prokaryotic Cell Definition

The type of cells which do not have a well-defined nucleus and no membrane-bound organelles are known as prokaryotic cells. Organisms with prokaryotic cells are called prokaryotes and they are generally single-celled microorganisms. These organisms can be free-living or can be found in the gut of animals. The size of a prokaryotic cell can range between 0.2 to 10 microns.

Examples of prokaryotic cells include bacterial cells, which have different shapes and structures and possess extrachromosomal DNA known as a plasmid.

Archaeal cells which are similar to bacterial cells, are found in extreme environments such as hot springs and also in soil and marsh. These single-celled organisms also possess plasmids. 

Prokaryotic Cell Structure

Prokaryotic cells are not complex structures. They do not have a true nucleus and the genetic material is not contained within a membrane but it is seen as coiled in the cytoplasm of the cell. Below is a list of structures that can be observed in a prokaryotic cell.

  • Capsule: This is an outer protective coat observed in some prokaryotic cells which assist in the retention of moisture and adherence to nutrients and surfaces.

  • Cell wall: This is the outer covering of the prokaryotic cells which gives the cell its size and shape. It also protects the cell from harm.

  • Plasma Membrane: The cell membrane surrounds the cytoplasm and plays a role in the regulation of the flow of substances going in and out of the cell.

  • Cytoplasm:  It is a gel-like substance that is mainly composed of water and contains various cellular components, enzymes, organic molecules, and salts.

  • Pilli: These are hair-like structures that are observed on the surface of the cell and help the cell in attaching itself to a surface. One single structure is called a pilus and the short form of it is known as fimbriae. These structures are also known as appendages.

  • Flagella: These are long whip-like protruding structures that help in the locomotion of the cell.

  • Plasmids: Plasmids are the genetic material of a prokaryotic cell. They are circular or double-stranded DNA structures.

  • Ribosomes: The prokaryotic ribosome molecules are associated with the plasma membrane. These are 70S ribosomes which are composed of 30S smaller and 50s larger subunits. The molecules are made from messenger RNA.

  • Nucleoid Region: It is the area of the cytoplasm that contains the prokaryotic DNA molecule/ genetic material.


 Major Components of The Prokaryotic Cell:

  • Glycocalyx: In some prokaryotic cells it is a part of the cell envelope or capsule and is composed of macromolecules. 

  • Mesosomes: These are extensions of the cell or plasma membrane in the form of lamella or tubules. Their functions are:

    • Assisting in cellular respiration and secretion.

    • Cell wall formation.

    • Increasing the surface area of enzymatic content and plasma membrane.

    • Genetic material replication and distribution in daughter cells.

  • Inclusion bodies: They are free bodies observed in the cytoplasm of a prokaryotic cell and act as storage units for reserve materials.

  • Chromatophores: These membrane extensions are specifically found in cyanobacteria and have photosynthetic pigments inside.

All bacterias are prokaryotes and the above-mentioned structures can be seen in all bacterial cells.

Prokaryotic cellular organisms or prokaryotes can be divided into two domains:

The Different Types of Prokaryotic Cells

Archaea

Bacteria

Archaea can be subdivided into


  1. Euryarchaeota: Eg: Haloarchaea

  2. Crenarchaeota: Acidilobus saccharovoran

  3. Nanoarchaeota: Nanoarchaeum equitans

  4. Korarchaeota: Ignicoccus

Bacteria can be subdivided into

Proteobacteria- eg: Rhizobium

Chlamydias- eg: Chlamydia trachomatis

Spirochetes: Treponema pallidum (Syphilis causing bacteria)

Cyanobacteria: Prochlorococcus

Gram-positive bacteria: Bacillus anthracis (Anthrax causing bacteria)

Gram-negative bacteria:  Salmonella typhi (typhoid causing bacteria)

Features of Prokaryotic Cells

  • The genetic material is localised in a region known as nucleoid and it has no surrounding membrane.

  • These cells contain large numbers of the ribosome for protein synthesis

  • In some prokaryotes, the cell membrane folds to form structures known as mesosomes which assist in cell respiration.

  • Some prokaryotes have structures such as flagella and pili. Flagella helps in locomotion and pili assists in the exchange of genetic material between two cells.

Important Characteristics of Prokaryotic Cell:

  • The absence of clearly-defined membrane-limited organelles such as Golgi complex and mitochondria.

  • Histone proteins that are essential for the formation of chromosomes in the eukaryotic cells are absent.

  • The mitotic apparatus and the nucleolus are absent.

  • The cell membrane below the cell wall is produced into the cytoplasm acts as the mitochondrial membrane to carry respiratory enzymes.

Reproduction in prokaryotes:

Reproduction in a prokaryotic cell takes place in two ways

Binary fission: The process starts with the replication of the DNA molecule and two copies of the molecule attach themselves to the cell membrane.

  • Then the cell membrane begins to grow between the two molecules and once the cell is twice its original size the cell membrane begins to pinch inwards.

  • Afterward, a cell wall forms between the two DNA molecules  which divides the original cell into two identical daughter cells


Recombination: The genes of one cell are transferred to the genome of another cell. The process takes place in three ways

  • Conjugation: The process of gene transfer takes place between two cells through a protein tube-like structure called a pilus.

  • Transformation: It is a type of sexual reproduction where the cell takes genetic material from the surrounding and incorporates it into its genetic material.

  • Transduction: In this process, the genetic material is transferred to another cell with the help of viruses.

FAQ (Frequently Asked Questions)

1. What is a prokaryotic cell?

A prokaryotic cell can be defined as a cell which does not have a well-defined nucleus or membrane-bound organelles such as mitochondria or and lysosomes. An organism with a  prokaryotic cell is called a prokaryote and they are generally bacterias. 

Although most bacteria cause diseases some are beneficial. The function of these prokaryotic cells are: 

Digestion of food: Bacteria such as lactobacillus help in the digestion of lactose in the stomach

Inhibition of pathogens:  Gut flora also plays a role in defending our body against pathogens.

Development of enteric protection: The gut flora (bacteria) is established in the first two years of birth. After it is established, the intestinal lining also establishes and in a way gut bacteria against pathogens which can enter into the stomach through food.

2. What are probiotics?

Probiotics are live bacteria that are good for the human digestive system. The function of probiotics is to replenish gut flora after an infection. Some common probiotics are Lactobacillus and Bifidobacterium. Probiotics can be used for the treatment of infectious diarrhoea, inflammatory bowel syndrome, and irritable bowel syndrome.