CH.4: Anatomy of Prokaryotic & Eukaryotic Cells

The Cell: An Overview

This section introduces the fundamental differences and similarities between prokaryotic and eukaryotic cells, as well as the major bacterial shapes and arrangements.

• Types of Cells:

  • Prokaryotes lack a membrane-bound nucleus and organelles. Their genetic material is located in a nucleoid region.

  • Eukaryotes possess a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Similarities: Both cell types have a plasma membrane, cytoplasm, ribosomes, and genetic material.

  • Differences: Eukaryotes are generally larger, more complex, and can be multicellular, while prokaryotes are usually unicellular.

• Bacterial Shapes:

  • Coccus (spherical)

  • Bacillus (rod-shaped)

  • Spirillum (spiral-shaped)

  • Vibrio (comma-shaped)

  • Spirochete (flexible spiral)

• Bacterial Arrangements:

  • Strepto-: Chains (e.g., Streptococcus)

  • Staphylo-: Clusters (e.g., Staphylococcus)

  • Diplo-: Pairs (e.g., Diplococcus)

  • Tetrads: Groups of four

• Identification: Bacteria can be identified by their shape and arrangement using microscopy and staining techniques.

• Example: A cluster of circular-shaped cells is likely Staphylococcus; a chain of rod-shaped cells may be Streptobacillus.

Structures External to the Bacterial Cell Wall

Bacteria possess several external structures that contribute to their survival, pathogenicity, and motility.

Glycocalyx

• Definition: The glycocalyx is a gelatinous polymer surrounding the cell wall, composed of polysaccharides and/or polypeptides.

• Types: Capsule (organized, firmly attached) and slime layer (unorganized, loosely attached).

• Function: Protects against desiccation, aids in adherence to surfaces, and can inhibit phagocytosis by host cells.

• Virulence: The presence of a capsule often increases bacterial virulence, making them more resistant to host defenses.

• Example: Streptococcus pneumoniae is more virulent when encapsulated.

Flagella

• Definition: Flagella are long, whip-like appendages used for motility.

• Chemotaxis: Movement toward or away from chemical stimuli.

• Arrangements:

  • Monotrichous: Single flagellum at one end

  • Lophotrichous: Tuft of flagella at one end

  • Amphitrichous: Flagella at both ends

  • Peritrichous: Flagella distributed over the entire cell surface

Fimbriae and Pili

• Fimbriae: Short, hair-like structures used for attachment to surfaces and host tissues.

• Pili: Longer than fimbriae; involved in conjugation (transfer of genetic material between bacteria) and sometimes motility.

• Visual Difference: Pili are typically fewer and longer than fimbriae.

• Conjugation Pilus: Specialized pilus for DNA transfer during conjugation.

The Cell Wall & Plasma Membrane

The cell wall and plasma membrane are essential for maintaining cell shape, protecting against osmotic pressure, and regulating transport.

The Cell Wall

• Definition: A rigid structure outside the plasma membrane, primarily composed of peptidoglycan in bacteria.

• Function: Provides structural support, maintains shape, and prevents osmotic lysis.

• Chemical Structure: Peptidoglycan is a polymer of sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by peptides.

• Gram-Positive vs. Gram-Negative:

  • Gram-Positive: Thick peptidoglycan layer, teichoic acids present, no outer membrane.

  • Gram-Negative: Thin peptidoglycan layer, outer membrane containing lipopolysaccharides (LPS), periplasmic space, porins.

• Lipoproteins, Periplasm, Porins:

  • Lipoproteins: Anchor the outer membrane to the peptidoglycan in Gram-negative bacteria.

  • Periplasm: Gel-like space between the plasma membrane and outer membrane in Gram-negative bacteria.

  • Porins: Proteins that form channels in the outer membrane for transport of molecules.

• Teichoic Acids: Found in Gram-positive cell walls; contribute to cell wall maintenance and ion regulation.

The Plasma Membrane

• Phospholipid Bilayer: Organized with hydrophilic heads facing outward and hydrophobic tails inward.

• Composition: Phospholipids, proteins, and sometimes carbohydrates.

• Function: Selective barrier regulating entry and exit of substances; site of metabolic activities.

• Selective Permeability: Only certain molecules can pass through freely; others require transport proteins.

• Fluid Mosaic Model: Describes the dynamic nature of the membrane, with proteins floating in or on the fluid lipid bilayer.

Bacterial Cell Contents

The internal structures of bacterial cells are essential for their growth, reproduction, and survival.

• Cytoplasm: Gel-like substance inside the cell containing water, enzymes, nutrients, wastes, and gases.

• Cytoskeleton: Network of protein filaments providing structural support and shape.

• Nucleoid: Region containing the bacterial chromosome (DNA), not surrounded by a membrane.

• Plasmids: Small, circular DNA molecules that replicate independently and often carry antibiotic resistance genes.

• Ribosomes: Sites of protein synthesis; in prokaryotes, they are 70S (smaller than eukaryotic 80S ribosomes).

Endospores

• Definition: Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) under adverse conditions.

• Structure: Contains a core with DNA, ribosomes, and dipicolinic acid, surrounded by a tough spore coat.

• Function: Allows bacteria to survive extreme heat, desiccation, chemicals, and radiation.

• Formation: Triggered by nutrient depletion or environmental stress.

• Example: The anthrax outbreak on Gruinard Island was due to endospore-forming Bacillus anthracis, which persisted in the environment for decades.

Additional info: Endospores are not reproductive structures but survival mechanisms. Their resistance is due to low water content, presence of dipicolinic acid, and protective spore coat.