Prokaryotic Cytology: Structure and Function of Prokaryotic Cells

Chapter 3: Prokaryotic Cytology

Cell Theory and General Features

Cell theory is foundational to microbiology, describing the basic unit of life and the properties shared by all living organisms. Prokaryotic cells, which include bacteria and archaea, exhibit unique structural and functional characteristics.

• All living organisms consist of one or more cells.

• Cells are the basic unit of life.

• All cells have a plasma membrane that separates the cell from its environment.

• Cytoplasm: Water-filled area containing dissolved sugars, proteins, salts, and other molecules.

Major Cell Types

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Eukaryotic cells:

  • Generally larger

  • Contain DNA in the form of chromosomes within a nucleus

  • Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum)

  • Complex internal structure

• Prokaryotic cells:

  • Smaller, simpler structure

  • DNA is typically a single circular chromosome located in the nucleoid

  • No membrane-bound organelles

  • Cell wall provides structural support and shape

Prokaryotic Cell Structure

Prokaryotic cells have specialized structures that enable survival in diverse environments.

• Cytoplasm: Contains dissolved salts, sugars, gases, and nonmembrane-bound organelles.

• Nucleoid: Region containing the circular DNA chromosome.

• Plasma membrane: Phospholipid bilayer with embedded proteins; described by the Fluid Mosaic Model.

• Cell wall: Maintains cellular integrity, especially under osmotic stress.

Osmosis and Cell Wall Function

Osmosis is the movement of water across a semipermeable membrane from low to high solute concentration.

• Isotonic solution: No net movement of water; cell remains stable.

• Hypertonic solution: Water leaves the cell; cell shrinks (plasmolysis).

• Hypotonic solution: Water enters the cell; cell may burst (lysis) if the wall is compromised.

Cell Wall Composition

The cell wall provides shape and protection. Its composition varies between bacteria and archaea.

• Bacterial cell wall:

  • Composed of peptidoglycan: Interlinked polysaccharide chains.

  • Peptidoglycan contains N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Archaeal cell wall:

  • Structure is totally different from peptidoglycan.

Types of Bacterial Cell Walls

Bacteria are classified based on their cell wall structure, which affects staining and antibiotic susceptibility.

• Gram-negative cell wall:

  • Thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharide (LPS).

  • LPS acts as a virulence factor and can trigger immune responses.

• Gram-positive cell wall:

  • Thick peptidoglycan layer (15-80 nm) without an outer membrane.

  • May contain teichoic acids.

• Acid-fast cell wall:

  • Contains mycolic acids; found in Mycobacterium species.

Capsules and Biofilms

Some bacteria produce capsules and biofilms for protection and enhanced survival.

• Capsules:

  • Prevent phagocytosis; act as virulence factors.

• Biofilms:

  • Sticky matrix containing various microbes living together.

  • Responsible for persistent contamination of medical devices.

Surface Appendages

Prokaryotic cells possess specialized appendages for motility and attachment.

• Flagella:

  • Long, whip-like structures for motility.

  • Composed of flagellin; anchored in the cell wall and membrane.

  • Movement is powered by a rotary motor mechanism.

• Pili and fimbriae:

  • Short, hair-like structures for attachment.

  • Conjugation pili (sex pili) facilitate DNA transfer between cells.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria for survival under adverse conditions.

• Formed by: Bacillus and Clostridium genera.

• Resistance: Heat, drying, radiation, chemicals.

• Structure: Core surrounded by protective layers.

• Importance: Food microbiology and sterilization processes depend on endospore resistance.

Summary Table: Comparison of Gram-Positive and Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick (15-80 nm)	Thin (2-7 nm)
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Teichoic Acids	Present	Absent
Antibiotic Susceptibility	More susceptible to penicillin	Less susceptible to penicillin

Key Equations

• Osmosis: Water movement across a semipermeable membrane: $\text{Osmosis Rate} = k \times (C_{\text{outside}} - C_{\text{inside}})$

Examples and Applications

• Dental plaque: Biofilm formed by Streptococcus mutans on teeth.

• Medical device contamination: Biofilms on catheters and implants.

• Food sterilization: Endospore resistance impacts canning and preservation methods.

Additional info: Academic context and expanded explanations have been added to clarify and supplement the original notes for exam preparation.