Detailed structure of prokaryotic cell

What is a prokaryotic cell?

Prokaryotic cells are unicellular organism that lacks a defined nucleus surrounded by a complex nuclear membrane and membrane bound organelles. They generally contain a single, circular chromosome located in an area of the cell called as nucleoid. Unicellular prokaryotic organisms are classified under the Domain Archaea and Bacteria where the cell wall differs significantly.

Features of prokaryotic cell:

Prokaryote is derived from the Greek word, “before nucleus” which means they have unbound nucleus.

Prokaryotic cell size ranges from 0.2 µm to 2.0 µm in diameter and 1 µm to 6 µm in length. But there are some exceptional organisms that may be as long as 250 µm (Spirochaeta) and may be as large as 600 X 800 µm (Epulopiscium fishelsoni) and may be as small as 0.05 µm (ultramicrobacteria or nannobacteria).

The cell size of bacteria depends on the rate of diffusion within the cell.

Cell shapes also varies according to the bacteria. Some may be uniform spherical or coccoid cells (Streptococcus pyogenes), rod shaped bacilli (Bacillius subtilis, Fusobacterium nucleatum), curve shaped (Vibrio cholerae).

Some lack a cell wall (Mycoplasma), Corynebacterium diptheriae lacks a characteristic shape and Streptomyces albus contains a typical bacterial cell shape.

Prokaryote lacks membrane bound organelles therefore, invagination of the plasma membrane to form simple membranous structure may occur.

The interior of the prokaryotic cell contains a genetic material located near the nucleoid and ribosomes, inclusion bodies, plasmids, mesosomes.

The exterior of the prokaryotic cell contains cell wall, flagella, fimbriae, pili, capsule or slime layer.

Reproduction of prokaryotic cell are asexual i.e, they reproduce by binary fission or by recombination.

Diagram of Prokaryotic cell:

Prokaryotic cell

Structure and functions:

Exterior part of the prokaryotic cell:

Cell wall:

Cell wall is mostly present in bacteria and is composed of peptidoglycan layer which differs between gram negative and gram positive bacteria.

This difference in cell wall helps in the identification between these two bacteria by Gram’s staining technique.

But Archeal cell contains pseudopeptidoglycan instead of peptidoglycan.

The main function of the cell wall is that it protects the cell from external injuries and provides shape.

Bacterial Cell wall

Plasma membrane:

  • All prokaryotic cells have a plasma membrane which is made up of phospholipid bilayer.
  • In archea, phospholipid layer is monolayer instead of bilayer.
  • The main function of the plasma membrane is it allows selective permeability to molecules to allow in and out of the cell.

Photosynthetic membrane structures:

  • Some prokaryotic cells (cyanobacteria and photosynthetic bacteria) have membrane structure that performs photosynthesis.
  • Photosynthetic membrane structure contains a pigment which is known as chlorophyll in cyanobacteria and chromatophores in photosynthetic bacteria.

Capsule:

  • A capsule is an additional outer layer usually composed of polysaccharides or proteins.
  • Capsule protects the cell against foreign invaders and allows adhesion to the surface.

Flagella:

  • Flagella are long thread like structure that helps the cell to move in aqueous environment.
  • Flagella may be monotrichous, amphtrichous, lophotrichous or peritrichous depending upon the bacteria.
Flagella

Fimbriae and pili:

  • Fimbriae and pili are structurally similar that enables a cell to attach to the surface.
  • Fimbriae is short projecting from the cell whereas pili is long.

Filamentous appendages:

  • Protein appendages are present in many bacterial cells that allow interaction with the environment.
  • It helps in the attachment, transfer of DNA or helps in movement of the cell.

Interior part of the prokaryotic cell

Nucleoid:

  • The nucleus of the prokaryotic cell is circular, haploid and unbound by a complex nuclear membrane.
  • The genome of the prokaryotic cell is smaller than the eukaryotic genome.
  • Prokaryotic DNA interacts with nucleoid-associated proteins (NAPs) that packages and organizes the chromosome.

Plasmids:

  • Prokaryotic cells may contain plasmids, an extra chromosomal DNA.
  • They are commonly found in bacteria confering an advantageous trait such as antibiotic resistance.
  • Plasmids are important for the survival of the organism.

Ribosomes:

  • Prokaryotic ribosomes are present in cell cytoplasm and are responsible for protein synthesis.
  • Bacterial and archeal ribosomes are found in 70s size but have different proteins and rRNA molecules.
Ribosomes

Endospores:

  • When environmental condition seem unfavourable, bacteria forms endospore that protect them from dormant state.
  • Endospores are resistant to chemicals, extreme temperatures and even radiation.
Endospores

Cytoplasm:

  • Cytoplasm is the space inside the cell where cytosol and water based minerals and ions are present that is essential for the cell.
  • They also contain ribosomes and chromosomes.

Inclusion bodies:

  • Inclusion bodies are found in cytoplasm and remains freely without a membrane.
  • They function in storing materials and present as a gas vacuoles in cyanobacteria and photosynthetic bacteria.

Examples of prokaryotic cell:

  1. Bacteria (E. coli, Pseudomonas, Bacillus etc)
  2. Cyanobacteria (Nostoc, Oscillatoria, Gleocapsa)
  3. Archeal cell (Halobacterium, Thermoplasma, Sulfolobus etc)

References:

  1. https://courses.lumenlearning.com/microbiology/chapter/unique-characteristics-of-prokaryotic-cells/
  2. https://open.oregonstate.education/microbiology/chapter/3-3unique-characteristics-of-prokaryotic-cells/
  3. https://www.eolss.net/Sample-Chapters/C03/E6-51-04-01.pdf
  4. https://courses.lumenlearning.com/boundless-biology/chapter/structure-of-prokaryotes/

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