4 Prokaryotic cell

Key Characteristics of Prokaryotes

  • Domain: Prokaryotes encompass two main groups: Bacteria and Archaea

  • Biomass: Dominates total planet biomass with over 6,000 known species

  • Genomic Resources: Over 9,400,000 16S rRNA gene entries published

  • Cell Structure:

    • Lack membrane-bound organelles

    • Have "naked DNA" without true chromosomes

    • Show little to no internal compartmentalization

    • Size range: 0.5 to 10 µm diameter

    • Possess peptidoglycan in their cell walls

    • Ribosomes are smaller (70S)

Classification of Prokaryotes

  • Bacteria:

    • Classified based on phylogenetic characteristics derived from nucleic acids and physiological or morphological aspects

    • Reference: "Bergey’s Manual of Determinative Bacteriology" available in Glucksman library

  • Evolutionary Model:

    • Model shows the origin of Eukaryotes from ancestral forms of Bacteria and Archaea

Morphology of Bacteria

  • Shapes:

    • Cocci: Spherical or ovoid

    • Bacilli: Rod-shaped

    • Spiral: Various forms including Spirillum and Spirochaete

    • Other forms: Filaments and Hyphae

    • Budding bacteria found as well

Comparison of the Three Domains

  • Sizes:

    • Bacteria and Archaea: < 2 µm

    • Eukarya: > 10 µm

  • Cell Membrane Lipids:

    • Bacteria & Eukarya: Fatty acid ester

    • Archaea: Isoprene ether

  • Cell Wall:

    • Bacteria: Peptidoglycan

    • Archaea: Pseudopeptidoglycan or other polysaccharides

    • Eukarya: Cellulose (plants) and Chitin (fungi)

  • DNA Structure:

    • Circular, single molecule in Bacteria and Archaea; linear in Eukarya

  • Organelles:

    • Absent in Bacteria and Archaea, present in Eukarya

  • Ribosomes:

    • 70S in Bacteria and Archaea, 80S in Eukarya

Cytoplasmic Membrane Function

  • Roles:

    • Boundary of the cell

    • Selective permeability

    • Protein synthesis

    • Water and nutrient transport

    • Participation in respiration/photosynthesis

    • Secretion of proteins

  • Structure:

    • Composed of a phospholipid bilayer

Membrane Proteins and Their Functions

  • Transport Systems:

    • Group translocation, ABC transporter (ATP-dependent)

  • Functions of Membrane Proteins:

    • Lipid synthesis

    • Cell wall synthesis

    • Nutrient transport

    • Protein secretion

    • Chemotaxis

    • Electron transport

Cytoplasm of Prokaryotes

  • Composition:

    • Circular genome (chromosome analog)

    • Extrachromosomal DNA (plasmids)

    • mRNA, rRNA, tRNA present

    • Jelly-like consistency containing enzymes, nutrients, salts, proteins, and waste products

Transcription and Translation in Prokaryotes

  • Process:

    • Coupled transcription and translation

    • As the mRNA is synthesized, it is immediately available for ribosomal binding and subsequent protein synthesis

    • Polysomes formed when multiple ribosomes simultaneously translate mRNA

  • Importance:

    • This coupling is key for regulating gene expression

Storage Compounds in Prokaryotes

  • Types:

    • Glycogen

    • Polyhydroxyalkanoates

  • Additional Components:

    • Vacuoles: Designed for storage and buoyancy in cyanobacteria

    • Storage granules: Energy sources such as starch and PHB, and elemental sulfur globules

Cell Wall Structure

  • General Function:

    • Provides structure, withstands turgor pressure, and allows permeability to solutes

  • Types of Cell Walls:

    • Gram Positive: Thick peptidoglycan layer

    • Gram Negative: Thin peptidoglycan with an outer membrane

Gram Staining Technique

  • Technique Overview:

    • Developed by Hans Christian Gram (1884) for classifying bacteria into two groups based on cell wall characteristics

  • Process:

    • Sequentially washed with crystal violet, iodine, alcohol, and a red counterstain

    • Gram Positives retain the initial stain (dark blue/violet)

    • Gram Negatives lose the stain and take on a red appearance following counterstaining

Components of Gram-Positive Cell Envelope

  • Teichoic Acid:

    • Ribitol-phosphate or glycerol-phosphate polymers

  • Teichuronic Acid:

    • Important for cell surface charge, support, and protection from autolysins

Components of the Gram-Negative Cell Envelope

  • Lipopolysaccharide (LPS):

    • Composed of Lipid A, core sugars, and O-antigen

  • Functions of LPS:

    • Stabilizes outer membrane

    • Assists in attachment and biofilm formation

    • Creates a permeability barrier to certain substances

    • O-antigen triggers immune response; Lipid A acts as an endotoxin