Prokaryotic Cell Vocabulary

Overview and Classification of Prokaryotic Cells

  • Representative Organisms: Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
  • General Characteristics:
    • Prokaryotic cells are generally smaller and multiply more rapidly than eukaryotic cells.
    • They exhibit a wide variety of shapes and functions, yet the fundamental organisation of the prokaryotic cell is remarkably similar across species.
  • Size and Dimension Metrics:
    • Typical Bacteria: 1−2 μm1-2\,\mu m
    • PPLO (Pleuro Pneumonia Like Organisms): about 0.1 μm0.1\,\mu m
    • Typical Eukaryotic Cell: 10−20 μm10-20\,\mu m
    • Viruses: 0.02−0.2 μm0.02-0.2\,\mu m
  • Four Basic Shapes of Bacteria:
    • Bacillus: Rod-like shape.
    • Coccus: Spherical shape.
    • Vibrio: Comma-shaped.
    • Spirillum: Spiral shape.

Genetic Material and Internal Cellular Structures

  • Cytoplasm and Nucleus:
    • The fluid matrix filling the cell is the cytoplasm.
    • There is no well-defined nucleus present in prokaryotic cells.
    • The genetic material is basically naked and is not enveloped by a nuclear membrane (unlike eukaryotic cells, which possess a nuclear membrane).
  • Genomic DNA vs. Plasmids:
    • Genomic DNA: Consists of a single chromosome or circular DNA.
    • Plasmid DNA: Many bacteria possess small circular DNA molecules outside the genomic DNA, known as plasmids.
    • Functions of Plasmids:
      • Confer unique phenotypic characters to bacteria, such as resistance to antibiotics.
      • Used in advanced biological monitoring to track bacterial transformation with foreign DNA.
  • Organelles and Inclusions:
    • Prokaryotes lack organelles similar to those found in eukaryotes, with the sole exception of ribosomes.
    • Prokaryotes contain unique internal structures termed inclusions.

Cell Envelope Structure and Modifications

  • Cell Envelope Organization:
    • Most prokaryotic cells, particularly bacterial cells, possess a chemically complex cell envelope.
    • The cell envelope consists of a tightly bound, three-layered structure that acts together as a single protective unit:
      1. Glycocalyx: The outermost layer.
      2. Cell Wall: The middle layer.
      3. Plasma Membrane: The innermost layer.
  • Glycocalyx Variations:
    • Differs in composition and thickness among different bacteria.
    • Slime Layer: Glycocalyx present as a loose sheath.
    • Capsule: Glycocalyx present as a thick and tough layer.
  • Cell Wall Properties:
    • Surrounds the cell membrane in all prokaryotes except mycoplasma (mycoplasma lacks a cell wall).
    • Determines the shape of the cell.
    • Provides strong structural support to prevent the bacterium from bursting or collapsing.
  • Plasma Membrane Properties:
    • Selectively permeable in nature and interacts directly with the outside world.
    • Structurally similar to the plasma membrane of eukaryotic cells.
  • Gram Staining Classification:
    • Bacteria are classified into two groups based on differences in their cell envelopes and their response to the staining procedure developed by Gram:
      • Gram Positive Bacteria: Bacteria that take up the Gram stain.
      • Gram Negative Bacteria: Bacteria that do not take up the Gram stain.

Specialized Membranous Extensions

  • Mesosomes:
    • A specialized, differentiated form of the cell membrane characteristic of prokaryotes, formed by infoldings or extensions of the plasma membrane into the cell.
    • Structural Forms: Extensions occur in the form of vesicles, tubules, and lamellae.
    • Functions of Mesosomes:
      • Cell wall formation.
      • DNA replication and distribution of genetic material to daughter cells.
      • Respiration processes.
      • Secretion processes.
      • Increasing the surface area of the plasma membrane and enhancing its enzymatic content.
  • Chromatophores:
    • Membranous extensions into the cytoplasm found in certain prokaryotes, such as cyanobacteria.
    • Contain pigments responsible for photosynthesis or coloration.

Surface Appendages and Motility

  • Flagella:
    • Bacterial cells may be motile or non-motile.
    • Motile bacteria possess thin filamentous extensions from their cell wall called flagella.
    • Bacteria show wide variations in the number and arrangement of flagella.
    • Three Parts of a Bacterial Flagellum:
      1. Filament: The longest portion, extending from the cell surface to the outside.
      2. Hook: The attachment region connecting filament to basal body.
      3. Basal Body: Anchors the flagellum to the cell envelope.
  • Non-Motile Surface Structures:
    • Pili and fimbriae are surface structures that do not play a role in bacterial motility.
    • Pili: Elongated tubular structures made of a special protein.
    • Fimbriae: Small bristle-like fibres sprouting out of the cell.
    • Function of Fimbriae: In certain bacteria, fimbriae assist in attaching the bacteria to rocks in streams as well as to host tissues.