Prokaryotic Cell Structure and Function

Overview of Prokaryotic Cells

Prokaryotic cells are unicellular organisms that lack a membrane-bound nucleus and organelles. They include bacteria and archaea, and are characterized by their simple structure and diverse metabolic capabilities.

• Key Components: Cell wall, cell membrane, cytoplasm, nucleoid, ribosomes, flagella, glycocalyx, and various inclusions.

• Examples: Escherichia coli, Bacillus subtilis

Glycocalyx

The glycocalyx is an external layer found outside the cell wall of many prokaryotes. It can be a capsule (firmly attached) or a slime layer (loosely attached).

• Composition: Polysaccharides, polypeptides, or both.

• Functions:

  • Protection against desiccation and phagocytosis (antiphagocytic mechanism).

  • Facilitates attachment to biological surfaces.

  • Contributes to pathogenicity in some bacteria.

• Example: Capsule in Streptococcus pneumoniae enhances virulence.

Flagella

Flagella are long, whip-like appendages that provide motility to prokaryotic cells. Their structure and arrangement vary between species.

• Structure: Composed of a filament, hook, and basal body.

• Gram-positive vs. Gram-negative:

  • Gram-positive: Basal body has two rings.

  • Gram-negative: Basal body has four rings.

• Arrangements:

  • Monotrichous: Single polar flagellum.

  • Lophotrichous: Cluster of flagella at one pole.

  • Amphitrichous: Flagella at both poles.

  • Peritrichous: Flagella distributed over the entire cell surface.

• Function: Enables movement toward or away from stimuli (chemotaxis).

Pili and Fimbriae

Pili and fimbriae are hair-like structures on the surface of prokaryotic cells, primarily involved in attachment and genetic exchange.

• Fimbriae: Short, numerous; aid in attachment to surfaces.

• Pili: Longer, fewer; involved in conjugation (transfer of genetic material).

• Example: Sex pilus forms a bridge for DNA transfer between cells.

Cell Wall

The cell wall provides structural support, maintains cell shape, and protects against osmotic pressure. It is a key feature distinguishing Gram-positive and Gram-negative bacteria.

• Composition: Mainly peptidoglycan (murein), a polymer of sugars and amino acids.

• Functions:

  • Maintains cell shape.

  • Prevents osmotic lysis.

  • Contributes to pathogenicity and antibiotic resistance.

Peptidoglycan Structure

• Repeating units: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Cross-linking: Tetrapeptide bridges connect NAM units, providing strength.

Peptidoglycan formula:

Gram-Positive vs. Gram-Negative Cell Walls

Lipopolysaccharide (LPS) in Gram-Negative Bacteria

• Components: O-antigen, core oligosaccharide, lipid A.

• Function: Structural integrity, protection, and can trigger immune responses (endotoxin).

• Clinical relevance: Lipid A can cause fever, inflammation, and septic shock.

Archaeal Cell Walls

• Composition: Lack peptidoglycan; contain specialized polysaccharides and proteins.

• Gram-positive and Gram-negative Archaea: Based on cell wall thickness and presence of outer layers.

Cell Membrane

The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It regulates transport, energy conversion, and cell signaling.

• Phospholipid Structure: Glycerol backbone, two fatty acid tails, and a phosphate group.

• Archaeal Membranes: May contain ether-linked isoprenoid chains, increasing stability in extreme environments.

• Functions:

  • Anchoring proteins.

  • Energy conversions (electron transport chain, ATP synthesis).

  • Maintaining permeability barrier.

Phospholipid formula:

Transport Mechanisms

• Passive Transport: Diffusion, facilitated diffusion.

• Active Transport: Requires energy (ATP or proton motive force).

• Group Translocation: Substance is chemically modified during transport.

• ABC Transporters: ATP-binding cassette systems move substances against concentration gradients.

Cytoplasm

The cytoplasm is the internal matrix of the cell, containing water, enzymes, nutrients, ribosomes, and genetic material.

• Nucleoid: Region containing the circular DNA molecule.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes).

• Inclusions: Storage granules for nutrients (e.g., polyhydroxybutyrate).

Endospores

Endospores are highly durable, dormant structures formed by certain Gram-positive bacteria (e.g., Bacillus, Clostridium) under adverse conditions.

• Structure: Core, cortex (peptidoglycan), coat (keratin and other proteins), dipicolinic acid, high Ca2+ content.

• Function: Resistance to heat, desiccation, chemicals, and radiation.

• Example: Bacillus anthracis endospores can survive for decades.

Reproduction and Morphology

Prokaryotes reproduce asexually, primarily by binary fission. Their morphology varies and is used for classification.

• Binary Fission Steps:

  1. DNA replication.

  2. Cell elongation and DNA segregation.

  3. Formation of cross wall.

  4. Separation into two daughter cells.

• Common Shapes:

  • Cocci (spherical)

  • Bacilli (rod-shaped)

  • Spirilla (spiral-shaped)

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-), palisades.

Clinical Relevance: Antibiotic Resistance

The structure of the cell wall and membrane influences bacterial susceptibility to antibiotics. Gram-negative bacteria are generally more resistant due to their outer membrane and efflux pumps.

• Mueller-Hinton agar: Used for antibiotic susceptibility testing.

• Efflux pumps: Actively remove antibiotics from the cell.

Summary Table: Key Differences Between Prokaryotic Cell Types

Additional info: Some details, such as the specific chemical structure of archaeal membranes and the role of efflux pumps, were inferred for completeness and academic context.