Living World is classified into 3 division or domains, namely:

1) Bacteria (Eubacteria)

2) Archaea (Archae-bacteria)

3) Eukaryotes

Bacteria & Archaea are grouped separately under Prokaryotes.

Since, Viruses lack metabolic apparatus & considered non-living outside host cell, they are classified separately from the above divisions.

Evolutionary tree – Prokaryotes & Eukaryotes


Bacteria – Escherichia coli, Lactobacillus, Mycoplasma, Cyanobacteria etc.

Archaea – Halobacterium, Methanobacteria, Pyrodictium etc.

Eukaryotes – Humans, Yeast, Plants etc.


Prokaryotic Cell: (pro = before; karyon = nucleus)

  • In simple words, prokaryotic cells does not possess membrane bound nucleus & sub-cellular organelles.
  • Organisms possessing prokaryotic cell are termed as prokaryotes & are single celled.

Eukaryotic Cell: (Eu = true; karyon = nucleus)

  • In simple words, eukaryotic cells possess membrane bound nucleus & other sub-cellular organelles.
  • Organisms possessing eukaryotic cells are termed as eukaryotes & can be single or multicellular.

Prokaryotic Cell

  • Prokaryotes:
    • Single celled (Unicellular).
    • Most numerous & widespread organisms on earth.
    • They can derive energy from diverse sources.
    • They can tolerate environmental extremes, like –
      • Hot Springs (Temperature ≈100C) : Thermus aquaticus
      • Salty Water (Dissolved salts ≥ 50g/L) : Archaeal or Bacterial Halophiles
  • Prokaryotic cell size range: 0.1 to 10 μm in length or diameter.
Prokaryotic Cell

Domains Bacteria & Archaea each are further sub-divided, based on type of habitats they live in:

  1. Aerobic Habitats:
    • Have abundant oxygen supply.
    • Residents (Aerobes) derive energy by transfer of electrons from fuel molecules to oxygen.
    • Example: Mycobacterium tuberculosis (Bacteria), Halobacterium (Archaea) etc.
  2. Anaerobic Habitats:
    • Devoid of proper oxygen supply.
    • Residents (Anaerobes) obtain energy by transferring electrons to nitrate (forming N2), sulfate (forming H2S), or CO2 (forming CH4).
    • Example: Escherichia coli (Bacteria), Pyrococcus (Archaea) etc.
    • Anaerobes are further divided:
      • Obligate anaerobes: Die when exposed to oxygen. Example: Peptostreptococcus (Bacteria), Methanogens (Archaea) etc.
      • Facultative anaerobes: Able to live with or without oxygen. Example: Escherichia coli (Bacteria), Haloarchaea (Archaea).

Basic features shared by all prokaryotic cells:

  1. Plasma Membrane
  2. Nucleoid
  3. Cytoplasm

1. Plasma Membrane: Encloses cell, regulate material in & out of cell. Separates cell cytoplasm from outside environment.

Bacterial Plasma Membrane
S.No.ComponentBacterial Plasma MembraneArchaeal Plasma Membrane
1PhospholipidsLipids are attached to glycerol by ester linkages.Lipids are attached to glycerol by ether linkages. (Ether linkages are more resistant to chemical attack and heat than are ester linkages).
2Lipid layersHopanoids form lipid bilayer. Some bilayers have sterols.Glycerol diethers form lipid bilayer; Glycerol tetraethers form lipid monolayer.
Difference between Bacterial & Archaeal plasma membrane

2. Nucleoid: Irregularly shaped region in the cell where the chromosomes are located.

S.No.ComponentBacterial NucleoidArchaeal Nucleoid
1  ChromosomesMost bacteria have circular double stranded deoxyribonucleic acid (dsDNA), but some bacteria have a linear chromosome.All known archaea have circular, double stranded deoxyribonucleic acid (dsDNA).
2Chromosome numberMostly single copy present. Except few bacteria like Vibrio cholerae and Borrelia burgdorferi which have more than one chromosome.Some archaea are polyploid, i.e. have more one copy of chromosome – number varying 10 to 25 chromosome copies.
Comparative Nucleoid of Bacteria & Archaea

3. Cytoplasm:

Enclosed inside plasma membrane. It has two components:

  • Cytosol: Composed mainly of water. Contains ions, soluble micro- & macro- molecules, suspended particles/structures/molecules.
  • Ribosomes: Prokaryotes have 70S sized ribosomes (S stands for Svedberg units) – made of 50S & 30S subunits. The 50S subunit contains 23S & 5S rRNA, whereas, 30S subunit contains 16S rRNA.
70S Ribosome – Structure

To be continued…

Harjeet Kaur

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