Class 11 Biology Cell : The Unit of Life Prokaryotic cells

Prokaryotic cells

  • The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
  • The four basic shapes of bacteria are bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).


Fig. Various forms of cells 

  • The fluid matrix filling the cell is the cytoplasm.
  • The genetic material is basically naked, not enveloped by a nuclear membrane.
  • In addition to the genomic DNA, many bacteria have small circular DNA outside the genomic DNA called
  • Prokaryotes have something unique in the form of inclusions.
  • A special membranous structure is the mesosome which is formed by the extensions of plasma membrane into the cell form of vesicles, tubules and lamellae.
  • Bacteria have a chemically complex cell envelope consists of a tightly bound three layered structure
  • the outermost glycocalyx
  • cell wall
  • plasma membrane
  • Bacteria can be classified into two groups on the basis of the differences in the cell envelopes and the manner in which they respond to the staining procedure developed by Gram
  • The bacteria that take up the gram stain are Gram positive.
  • The bacteria that do not take up the Gram stain are Gram negative.
  • Glycocalyx differs in composition and thickness among different bacteria, which could be a loose sheath called the slime layer in some, while in others it may be thick and tough, called the
  • In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.
  • Bacterial cells may be motile or non-motile, if motile they have flagella.
  • Bacterial flagellum is composed of three parts – filament, hook and basal body.
  • Besides flagella, pili and fimbriae are also surface structures of the bacteria.
  • The pili are elongated tubular structures made of a special protein.
  • The fimbriae are small bristle like fibres sprouting out of the cell.
  • In prokaryotes ribosomes are about 15 nm by 20 nm in size and are made of two subunits - 50S and 30S units which when present together form 70S prokaryotic ribosomes.
  • Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome.
  • Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies, which lie free in the cytoplasm. Example- phosphate granules, cyanophycean granules.
  • Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.


Fig. Prokaryotic cell

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