The Prokaryotic Cell

Introduction

Prokaryotic cells, which include Bacteria and Archaea, are fundamental to the study of microbiology. They are distinguished from eukaryotic cells by their simpler structure and lack of membrane-bound organelles. This guide summarizes the key features, classification, and cellular components of prokaryotes.

Classification of Prokaryotes

Main Domains of Life

Life is classified into three domains: Bacteria, Archaea, and Eukarya. Prokaryotes comprise the first two domains and are characterized by their unique cellular structures and genetic organization.

• Bacteria and Archaea dominate total planet biomass.

• Over 6,000 different bacterial species are known.

• More than 9,400,000 16S rRNA gene entries have been published.

• Prokaryotes lack membrane-bound organelles.

• They possess "naked DNA" (no real chromosome structure).

• Little to no internal compartmentalization.

• Size range: 0.5 to 10 μm in diameter.

• Cell wall contains peptidoglycan (in bacteria).

• Ribosomes are smaller (70S) than those in eukaryotes (80S).

Classification Methods

• Phylogenetic classification is based on nucleic acid sequences (e.g., 16S rRNA).

• Physiological and morphological characteristics are also used.

• Bergey's Manual of Determinative Bacteriology is a key reference for bacterial classification.

• BacDive is a database for strain phenotypic and genetic data.

Bacterial Cell Shapes

Major Morphologies

Bacteria exist as single cells and display a variety of shapes, which are important for identification and classification.

• Cocci: Spherical or ovoid cells.

• Rods (Bacilli): Cylindrical, rod-shaped cells.

• Spiral shapes: Includes spirilla, spirochetes, and vibrios.

• Filaments: Long, thread-like cells.

Example: Staphylococcus aureus is a coccus, while Escherichia coli is a rod-shaped bacterium.

Comparison of the Three Domains

Structural and Genetic Differences

The following table summarizes the key differences between Bacteria, Archaea, and Eukarya:

The Cytoplasmic Membrane

Structure and Function

The cytoplasmic membrane is a critical boundary in prokaryotic cells, controlling the movement of substances in and out of the cell and facilitating essential cellular processes.

• Boundary: Separates the cell interior from the environment.

• Selective permeability: Regulates entry and exit of molecules.

• Protein synthesis: Membrane-associated ribosomes in prokaryotes.

• Water and nutrient transport: Via membrane proteins.

• Respiration/photosynthesis: Occurs at the membrane in prokaryotes.

• Secretion of proteins: Export of enzymes and toxins.

• Phospholipid bilayer: Fundamental structural component.

Example: The bacterial cytoplasmic membrane is composed of a phospholipid bilayer with embedded proteins.

Membrane Proteins

• Lipid synthesis

• Cell wall synthesis

• Nutrient transport

• Protein secretion

• Chemotaxis

• Electron transport

Transport systems such as uniporters, symporters, antiporters, and ABC transporters facilitate the movement of molecules across the membrane, often requiring ATP.

The Cytoplasm

Contents and Organization

The cytoplasm of prokaryotic cells is a jelly-like matrix that contains essential biomolecules and cellular machinery.

• Enzymes: Catalyze metabolic reactions.

• Solutes: Nutrients, salts, proteins, and waste products.

• DNA: Includes the main genome and plasmids.

• Ribosomes: Sites of protein synthesis.

Genetic Material

• Bacteria usually have a single, double-stranded circular genome located in the nucleoid region.

• Genome is not bound by a membrane.

• Additional genetic elements include plasmids (extrachromosomal DNA).

• Bacteria also contain mRNA, rRNA, and tRNA, which differ from those in eukaryotes.

Transcription and Translation

In prokaryotes, transcription and translation are coupled processes, allowing rapid protein synthesis.

• Transcription: Synthesis of RNA from DNA.

• Translation: Synthesis of protein from RNA.

• Occurs simultaneously in the cytoplasm.

• Multiple ribosomes can translate a single mRNA molecule, forming polysomes.

Equation:

Additional info: Coupling of transcription and translation is important for gene regulation in prokaryotes.

Other Cytoplasmic Components

• Storage compounds: Glycogen, poly-hydroxy-alkanoates.

The Cell Envelope

Cell Wall Structure

The cell wall is a vital component of most bacteria, providing structural support and protection.

• Peptidoglycan (murein) is the main structural polymer in bacterial cell walls.

• Some bacteria, such as Mycoplasma, lack a cell wall.

• Functions include withstanding osmotic pressure and maintaining cell shape.

Gram-Positive vs. Gram-Negative Bacteria

Bacterial envelopes are classified based on their response to the Gram stain:

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Lipopolysaccharide (LPS)

• Found in the outer membrane of Gram-negative bacteria.

• Acts as an endotoxin and contributes to pathogenicity.

• Plays roles in immune evasion and structural integrity.

Summary

Prokaryotic cells are structurally simple but highly diverse and ecologically significant. Understanding their classification, cellular structures, and molecular processes is essential for microbiology students.