BIO Exam 1: Ch 3

Prokaryotic Cells

Overview of Prokaryotes

Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and organelles. The two main domains of prokaryotes are Bacteria and Archaea. They are distinguished from eukaryotes (plants, animals, fungi, and protists) by their simpler cellular organization.

• Bacteria: Ubiquitous microorganisms with diverse metabolic capabilities.

• Archaea: Share some features with eukaryotes and bacteria; often inhabit extreme environments.

• Endosymbiotic Theory: Proposes that some eukaryotic organelles originated from prokaryotic cells through symbiosis.

Cell Morphology

Prokaryotic cells exhibit a variety of shapes and arrangements, which are important for classification and identification.

• Monomorphic: Cells maintain a single shape.

• Pleomorphic: Cells can vary in shape and size.

• Common Shapes:

  • Bacilli: Rod-shaped

  • Cocci: Spherical

  • Vibrio: Comma-shaped

  • Stella: Star-shaped

  • Coccobacilli: Ovoid

  • Spirochetes: Spiral-shaped, corkscrew motion

Cellular Organization

• Prokaryotes are unicellular.

• Lack a membrane-bound nucleus and organelles.

• Cell size ranges from 0.2 to 750 micrometers in diameter.

Prokaryotic Cell Structures

Cell Envelope

The cell envelope consists of several layers that protect the cell and mediate interactions with the environment.

• Plasma Membrane: Phospholipid bilayer with embedded proteins; acts as a selective barrier and site for metabolic reactions.

• Cell Wall: Provides structural support and shape; composition varies between bacteria and archaea.

• Outer Membrane: Present in some bacteria (notably Gram-negative); contains lipopolysaccharides and porins.

Plasma Membrane

• Composed of proteins (about half the mass) and phospholipids.

• Exhibits selective permeability—only certain substances can cross.

• Functions of membrane proteins:

  • Transporters

  • Anchors

  • Receptors

  • Enzymes

• Site for ATP production in prokaryotes.

• Described as a fluid-mosaic model—lipids and proteins move laterally within the layer.

Cell Wall

• Bacteria: Contain peptidoglycan (a polymer of sugars and amino acids).

• Archaea: Lack peptidoglycan; may have cell walls made of polysaccharides, proteins, or may lack a cell wall entirely.

Gram Stain and Cell Wall Types

The Gram stain differentiates bacteria based on cell wall structure:

Type	Peptidoglycan Layer	Outer Membrane	Stain Color	Other Features
Gram-Positive	Thick	Absent	Blue/Purple	More susceptible to antibiotics
Gram-Negative	Thin or none	Present	Red/Pink	Contains lipopolysaccharide; more resistant to chemicals

• Acid-Fast Stain: Used to identify bacteria with waxy cell walls (e.g., Mycobacterium species).

Outer Membrane (Gram-Negative Bacteria)

• Contains lipopolysaccharides (LPS) and porin proteins.

• Acts as an additional barrier to certain antibiotics and chemicals.

Internal Structures

• Cytoplasm: Gel-like matrix; 70-80% water; contains enzymes, nutrients, and genetic material.

• Ribosomes: Sites of protein synthesis; consist of small and large subunits.

• Cytoskeleton: Network of proteins that maintain cell shape and aid in division.

Transport Mechanisms

• Passive Transport: Does not require energy.

  • Simple Diffusion: Movement from high to low concentration.

  • Facilitated Diffusion: Movement via membrane proteins.

  • Osmosis: Diffusion of water across a membrane.

• Active Transport: Requires energy (ATP or proton motive force) to move substances against their concentration gradient.

External Structures

• Flagella: Long, whip-like structures for motility.

  • Chemotaxis: Movement in response to chemicals.

  • Phototaxis: Movement in response to light.

  • Aerotaxis: Movement in response to oxygen.

• Fimbriae: Short, bristle-like proteins for adhesion; important in biofilm formation.

• Pili: Longer, less numerous than fimbriae; involved in adhesion, motility, and gene transfer (conjugation).

Flagella Arrangements

Arrangement	Description
Monotrichous	Single flagellum at one pole
Lophotrichous	Cluster of flagella at one pole
Amphitrichous	Flagella at both poles
Peritrichous	Flagella distributed over the entire cell surface

• Gram-Positive Bacteria: Two rings anchor the flagellum.

• Gram-Negative Bacteria: Four rings anchor the flagellum.

Glycocalyx

• Sticky, carbohydrate-rich layer outside the cell wall.

• Slime Layer: Loosely organized and easily removed.

• Capsule: Well-organized, tightly attached; enhances pathogenicity by protecting against phagocytosis.

Endospores

• Dormant, tough, non-reproductive structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Protect genetic material during harsh conditions (e.g., heat, desiccation, chemicals).

• Sporulation: The process of endospore formation.

Archaea: The Other Prokaryotes

Characteristics of Archaea

• Single-celled organisms; considered a separate superkingdom (domain).

• More closely related to eukaryotes than to bacteria based on genetic and biochemical evidence.

• Cell wall composition varies:

  • Some have cell walls made entirely of polysaccharides or proteins.

  • Some lack a cell wall completely.

  • All lack true peptidoglycan.

• Many archaea can survive in extreme environments (e.g., high temperature, salinity, acidity).

Summary Table: Bacteria vs. Archaea

Feature	Bacteria	Archaea
Cell Wall	Contains peptidoglycan	No peptidoglycan; may have polysaccharides, proteins, or none
Membrane Lipids	Ester-linked	Ether-linked
Habitat	Wide range, including moderate environments	Often extreme environments
Relation to Eukaryotes	Less closely related	More closely related

Key Processes

Binary Fission

Prokaryotes reproduce asexually by binary fission, resulting in genetically identical daughter cells.

1. DNA is replicated.

2. Cell grows and elongates.

3. Chromosomes are separated to opposite ends.

4. Septum forms at the midpoint.

5. Cell divides into two daughter cells.

Diffusion and Transport

• Simple Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement via specific membrane proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement against a concentration gradient using energy (ATP or proton motive force).

Additional info:

• Prokaryotic cells are fundamental to understanding microbial life and are the focus of many microbiology courses.

• Knowledge of cell structure and function is essential for studying microbial physiology, genetics, and pathogenesis.