Prokaryotic Cell Structure and Function: Study Notes

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:

$\text{Peptidoglycan} = \text{NAG} - \text{NAM} - \text{Tetrapeptide crossbridges}$

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Stain Color (Gram Stain)	Purple	Pink
Antibiotic Susceptibility	More susceptible	More resistant

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:

$\text{Phospholipid} = \text{Glycerol} + 2\ \text{Fatty Acids} + \text{Phosphate Group}$

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

Feature	Gram-Positive Bacteria	Gram-Negative Bacteria	Archaea
Peptidoglycan	Thick	Thin	Absent
Outer Membrane	No	Yes	Variable
Lipopolysaccharide	No	Yes	No
Antibiotic Resistance	Lower	Higher	Variable

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.