Cell Structure and Function in Prokaryotes: Study Notes for Microbiology

Cell Structure and Function in Prokaryotes

Introduction

This section covers the fundamental features of prokaryotic cells, focusing on their structure, external and internal components, and the functional significance of these features. Understanding these basics is essential for studying microbial physiology, classification, and the mechanisms of action of antibiotics.

Features of Prokaryotic Cells

Definition and General Characteristics

• Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and other internal membrane-bound organelles.

• Genetic material is located in a region called the nucleoid.

• Examples include Bacteria and Archaea.

• Prokaryotes have simpler structures compared to eukaryotes.

• They reproduce primarily by binary fission.

Example: Escherichia coli is a well-studied prokaryotic bacterium.

External Structures of Prokaryotes

Overview

• Glycocalyces (capsules and slime layers)

• Flagella

• Fimbriae and Pili

Glycocalyces

• Gelatinous, sticky layer surrounding the outside of the cell.

• Composed of polysaccharides, polypeptides, or both.

• Types:

  • Capsule: Organized, firmly attached to cell surface; protects bacteria from desiccation and phagocytosis.

  • Slime layer: Loosely attached, water-soluble; aids in adherence to surfaces.

• Functions:

  • Protection from desiccation and immune system attack.

  • Facilitates attachment to surfaces (biofilm formation).

Example: The capsule of Streptococcus pneumoniae is a major virulence factor.

Flagella

• Long, whip-like structures responsible for motility.

• Composed of three parts: filament, hook, and basal body.

• Flagella rotate to propel the cell through its environment.

• 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.

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

Fimbriae and Pili (Nonmotile Extensions)

• Fimbriae: Short, bristle-like projections; aid in attachment to surfaces and other cells.

• Pili: Longer than fimbriae, usually only one or a few per cell; involved in conjugation (transfer of DNA between cells).

• Both structures are important for colonization and biofilm formation.

Example: Neisseria gonorrhoeae uses fimbriae to attach to host tissues.

Comparison: Fimbriae vs. Flagella

Feature	Fimbriae	Flagella
Length	Short	Long
Number	Many	Few
Function	Attachment	Motility
Structure	Proteinaceous	Filament, hook, basal body

Prokaryotic Cell Walls

General Features

• Most prokaryotes have a cell wall composed of peptidoglycan.

• Functions:

  • Provides structural support and shape.

  • Protects against osmotic pressure.

  • Target for many antibiotics (e.g., penicillins).

Bacterial Cell Wall Structure

• Peptidoglycan: A mesh-like polymer of sugars and amino acids.

• Consists of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues, cross-linked by short peptide chains.

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, LPS)
Staining	Retains crystal violet (purple)	Does not retain crystal violet (pink/red)
Sensitivity to Antibiotics	Generally more sensitive	Generally less sensitive

Example: Staphylococcus aureus is Gram-positive; Escherichia coli is Gram-negative.

Prokaryotic Cell Membrane

Structure and Function

• Also called the plasma membrane or cytoplasmic membrane.

• Composed of a phospholipid bilayer with embedded proteins (integral and peripheral).

• Described by the fluid mosaic model.

• Functions:

  • Controls passage of substances into and out of the cell.

  • Site of energy generation (electron transport chain in bacteria).

  • Contains enzymes for metabolic processes.

Transport Across the Cell Membrane

• Transport can be passive (no energy required) or active (requires energy, usually ATP).

Passive Transport

• No energy (ATP) required.

• Molecules move from high to low concentration (down the concentration gradient).

• Types:

  • Simple diffusion: Movement of small or lipid-soluble molecules.

  • Facilitated diffusion: Movement via specific membrane proteins (channels or carriers).

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

Active Transport

• Requires energy (usually ATP).

• Moves substances against their concentration gradient (from low to high concentration).

• Involves carrier proteins (pumps) in the membrane.

• Types of active transporters include uniporters, symporters, and antiporters.

Equation for Active Transport:

$\text{Active Transport:} \\ \text{Carrier Protein} + \text{ATP} + \text{Substrate}_{\text{low}} \rightarrow \text{Carrier Protein} + \text{ADP} + \text{P}_i + \text{Substrate}_{\text{high}}$

Summary Table: Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide (LPS)	Absent	Present
Stain Color (Gram Stain)	Purple	Pink/Red
Antibiotic Sensitivity	More sensitive	Less sensitive

Additional info: The notes also mention that many important antibiotics target peptidoglycan synthesis, making the cell wall a critical target for antimicrobial therapy.