5. Cell Structure And Organization - part 02 - Prokaryotic cells

 


5. Cell Structure And Organization - part 02 - Prokaryotic cells


Kinds of cells :

  • Living organisms are grouped into two main categories -
  1. Prokaryotes and 
  2. Eukaryotes. 
  • The prokaryotes have simple cellular organization while eukaryotes exhibit high degree of organization.

A. Prokaryotic cells :

  • It has chemically complex protective cell envelop. 
  • It does not have well-defined nucleus and other membrane bound cell organelles.
  • Cell envelop is a three-layered structure with-
  1. outer glycocalyx
  2. middle cell wall and 
  3. inner plasma membrane. 
  • Glycocalyx is present as either slime layer (loose sheath) or capsule (tough). 
  • Bacteria are better observed when stained.
  • The most followed staining method is ‘Gram staining’ developed by Danish bacteriologist Hans Christian Gram. 
  • The cell wall is made up of peptidoglycan (in Gram positive bacteria) and murein (in Gram negative bacteria). It gives mechanical strength to the cell. 
  • Cell membrane is a phospholipid bilayer. All these structures give protection to the cell and also help in inter-cellular transport.
  • In motile bacteria either cilia or flagella are found. Both are driven by rotatory movement produced by basal body (which works as motor). 
  • Other parts are filament and hook.
  • Some other surface projections are the tubular pili (which help in inter-cellular communication) and fimbriae (for clinging to support).
  • The cell membrane shows infoldings called mesosomes, which help in cell wall formation and DNA replication. 
  • Some bacteria especially photosynthetic cyanobacteria show more longer extensions called chromatophores.They carry photosynthetic pigments. 
  • The cytoplasm contains dense particles called ribosomes helping in protein synthesis.
  • Ribosomes are described by their sedimentation rate in Svedberg units. 
  • Bacterial ribosome are 70S (composed of a larger subunit 50S + smaller subunit 30S).


  1. The term cell was first used by Robert Hooke (1665) in his book ‘‘Micrographic’’.
  2. Purkinje and Mohl (1835-37) discovered protoplasm.
  3. Camillo Golgi (1838) discovered the Golgi apparatus.
  4. Robert Brown (1881) discovered the Nucleus.
  5. Balbiani (1881) discovered chromosomes in salivary glands of Chironomus larva.
  6. Flemming (1882) studied cell division in detail and coined the term Mitosis.
  7. Porter (1945) discovered Endoplasmic Reticulum.
  8. C.Benda gave the name Mitochondria.
  9. C. de Duve (1955) discovered Lysosomes

Always remember :

  • Genetic material in bacterium is a single chromosome made up of circular and coiled DNA. It remains attached to mesosome.
  • This DNA undergoes a very typical replication pattern called as theta model of replication.
  • The DNA is not associated with histone proteins (as in eukaryotes) hence not referred to as chromatin. 
  • Besides chromosomal DNA many bacteria show plasmids which are small circular DNA molecules carrying few genes.
  • They are termed as extrachromosomal selfreplicating DNA molecules. 
  • They are of two basic types 
  1. F – plasmid for reproduction and
  2. R – plasmid for resistance against antibiotics.


  • Cytoplasm of prokaryotes is a pool of all necessary materials like water, enzymes, elements, amino acids, etc. 
  • Some inclusion bodies in form of organic (cyanophycean starch and glycogen) and inorganic granules (phosphate and sulphur) are also found.

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