Prokaryotic Cell Structure

  • Learning Objectives:

    • Structural differences among bacterial, archaeal, and eukaryotic cells.
    • Features of a typical prokaryote cell:
    • Nucleoid: Zone of circular DNA, not membrane-bound, contains about 60% DNA.
    • Plasmids: Extra-chromosomal DNA, replicate independently, carry nonessential genes.
    • Cytoplasmic Membrane: Selectively permeable barrier, site for metabolic processes.
    • Peptidoglycan: Structural component giving shape and protection from osmotic stress.
    • Outer Membrane: Contains lipopolysaccharides in Gram-negative bacteria.
    • Periplasm: Space between inner and outer membrane, contains enzymes and proteins.
    • Capsule: Polysaccharide layer providing protection.
    • Ribosomes: Site of protein synthesis, targeted by antimicrobials.
    • Cytoplasm: Intracellular fluid containing cellular components.

  • Nucleoid:

    • Organized DNA zone; circular, double-stranded.

  • Plasmids:

    • Mobile DNA elements; assist in antibiotic resistance and virulence.
    • Can transfer genes between bacteria.

  • Ribosomes:

    • Comprise of RNA and proteins, crucial for translation.
    • Structural differences make them targets for antibiotics.

  • Prokaryotic Cell Envelope:

    • Comprises cytoplasmic membrane, peptidoglycan, and outer membrane (Gram-negative).
    • Gram-positive cells have thick peptidoglycan; Gram-negative have thin peptidoglycan and outer membrane.

  • Peptidoglycan Structure:

    • Composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
    • Crosslinked by peptides, critical for cell integrity and a target for certain antibiotics.

  • Lipopolysaccharide (LPS):

    • In Gram-negative bacteria; composed of lipid A, core oligosaccharide, and O antigen.
    • Contributes to immune response.

  • Capsule and Slime Layer:

    • Protect from immune response, desiccation, and can serve in attachment.

  • Endospores:

    • Formed by some Gram-positive bacteria under stress.
    • Resistant to extreme conditions; can remain dormant for years.

  • Inclusions and Microcompartments:

    • Storage for nutrients, gases, or carbon; can aid in buoyancy.
    • Include gas vesicles and magnetosomes, aiding in environmental navigation.