4.2.1 Study Notes Prokaryotic Cells

• Three different types of cells exist in nature:

  • Prokaryotic cells lack a nucleus and are smaller and simpler than eukaryotic cells (which have a nucleus)

    • Prokaryotes are structurally less complicated than eukaryotes, but their metabolic capabilities exceed the eukaryotes.

    • The functions of bacteria vary

      • some cause serious diseases

      • some major component to ecosystems by decomposing dead organisms

  • Eukaryotic cells have a nucleus and organelles

  • Archaeans have qualities of both prokaryotic and eukaryotic cells.

A. The Structure of Prokaryotes

• Prokaryotes are extremely small

  • average size is 1–1.5 μm wide and 2–6 μm long .

• Prokaryotes occur in three basic shapes

  • spherical coccus

  • rod-shaped bacillus

  • spiral spirillum (if rigid) or spirochete (if flexible). 

• Cell Envelope

  • the cell envelope includes the plasma membrane, the cell wall, and the glycocalyx.

    • The plasma membrane is a lipid bilayer with imbedded and peripheral proteins

    •  it regulates the movement of substances into and out of the cell

    • The plasma membrane can form internal pouches called mesosomes

      • increase the internal surface area of the membrane for enzyme attachment

  • The cell wall maintains the shape of the cell and is strengthened by peptidoglycan.

    • The glycocalyx is a layer of polysaccharides on the outside of the cell wall

    • it is called a capsule if organized and not easily removed or a slime layer if it is not well-organized and is easily removed

• Cytoplasm

  • The cytoplasm is a semifluid solution containing water, inorganic and organic molecules, and enzymes. 

  • The nucleoid is a region that contains the single, circular DNA molecule.

  • Plasmids are small accessory (extrachromosomal) rings of DNA; they are not part of the bacterial genetic material.

  • Ribosomes are particles with two RNA- and protein-containing subunits that synthesize proteins.

  • Inclusion bodies in the cytoplasm are granules of stored substances.

    • Cyanobacteria (also called blue-green bacteria) are bacteria that photosynthesize

      • they lack chloroplasts but have thylakoids containing chlorophyll and other pigments.

• Appendages

  • Motile bacteria usually have flagella

    • the filament, hook, and basal body work to rotate the flagellum like a propeller to move through fluid medium

  • Fimbriae are small, bristle like fibers that attach to an appropriate surface.

  • Conjugation pili are tubes used by bacteria to pass DNA from cell to cell.