Structure and Function of Prokaryotic Cells

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 differences between Gram-positive and Gram-negative bacteria. Understanding these features 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 a simpler structure compared to eukaryotes.

• They reproduce primarily by binary fission.

Key Point: Prokaryotes are distinguished from eukaryotes by the absence of a true nucleus and membrane-bound organelles.

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.

• Capsule: Organized, firmly attached to the cell surface; protects bacteria from desiccation and host immune responses.

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

• Functions:

  • Protection from phagocytosis

  • Prevents desiccation

  • Facilitates attachment to surfaces (biofilm formation)

Example: Streptococcus pneumoniae uses its capsule to evade the host immune system.

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 include monotrichous (single flagellum), lophotrichous (tuft at one end), amphitrichous (flagella at both ends), and peritrichous (flagella all over the surface).

Function: Movement toward or away from stimuli (taxis), such as chemotaxis (chemical), phototaxis (light), or magnetotaxis (magnetic fields).

Arrangements of Flagella

Arrangement	Description
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 (Nonmotile Extensions)

• Fimbriae: Short, bristle-like appendages; aid in attachment to surfaces and other cells; important in biofilm formation.

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

Comparison Table: Fimbriae vs. Flagella

Feature	Fimbriae	Flagella
Length	Short	Long
Function	Attachment	Motility
Number per cell	Many	Few to many

Prokaryotic Cell Walls

General Features

• Most prokaryotes have a cell wall composed of peptidoglycan.

• Provides structural support and shape.

• Protects against osmotic pressure.

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

Bacterial Cell Wall Structure

• Peptidoglycan: A polymer consisting of sugars (N-acetylglucosamine and N-acetylmuramic acid) and amino acids.

• Forms a mesh-like layer outside the plasma membrane.

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 (fluid mosaic model).

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

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

Transport Across the Cell Membrane

• Passive Transport: Does not require energy (ATP); substances move down their concentration gradient.

• Types of passive transport:

  • Simple diffusion

  • Facilitated diffusion (via channels or carriers)

  • Osmosis (movement of water)

• Active Transport: Requires energy (ATP); substances move against their concentration gradient using carrier proteins.

Equation for Diffusion Rate (Fick's Law):

$J = -D \frac{dC}{dx}$

Where:

• $J$ = flux (amount per unit area per unit time)

• $D$ = diffusion coefficient

• $\frac{dC}{dx}$ = concentration gradient

Summary Table: Key Differences Between Gram-Positive and Gram-Negative Bacteria

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

Conclusion

Understanding the structure and function of prokaryotic cells is fundamental in microbiology. The differences in cell wall composition, external structures, and membrane transport mechanisms are critical for bacterial classification, pathogenesis, and the development of antimicrobial therapies.