Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure, Function, and Clinical Relevance

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Introduction

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, focusing on their cellular components, mechanisms of material transport, and clinical significance. Understanding these differences is fundamental in microbiology, especially for the development of antimicrobial therapies and the study of microbial physiology.

Types of Prokaryotes and Eukaryotes

Three-Domain System

• Bacteria: Unicellular prokaryotes with peptidoglycan cell walls.

• Archaea: Unicellular prokaryotes lacking peptidoglycan; often extremophiles.

• Eukarya: Organisms with membrane-bound nuclei and organelles (includes animals, plants, fungi, and protists).

Example: Cyanobacteria (Bacteria), Halophiles (Archaea), Animals (Eukarya).

Distinguishing Characteristics of Prokaryotes and Eukaryotes

Comparative Table

Characteristic	Prokaryotes	Eukaryotes
DNA Location	Not enclosed in a membrane; single circular chromosome	Enclosed in a nuclear membrane; multiple chromosomes
Histones	Absent	Present
Membrane-bound Organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), absent in animals
Cell Division	Binary fission	Mitosis/meiosis
Size	0.2–2.0 μm	10–100 μm

Key Point: These differences are clinically important for targeting pathogens without harming host cells.

Small Unicellular Organisms

• Bacteria and Archaea are the main prokaryotic groups.

• Bacteria are differentiated by morphology, chemical composition, nutritional requirements, biochemical activities, and energy sources.

• Most bacteria in nature exist as biofilms.

Bacteria vs. Archaea

• Bacterial cell walls contain peptidoglycan; archaeal cell walls do not.

• Bacterial membranes are lipid bilayers; some archaea have monolayer membranes.

• Archaea often inhabit extreme environments (halophiles, thermophiles, methanogens).

Cell Size, Shape, and Arrangement

Discussed in detail in the Microscopy chapter. Key points include:

• Bacterial cells: typically 0.2–2.0 μm, various shapes (coccus, bacillus, spiral), arrangements (chains, clusters).

• Eukaryotic cells: larger (10–100 μm), more complex internal structure.

Prokaryotic Cell Structure and Function

Internal Structures

• Plasma membrane: Selective barrier, composed of phospholipids and proteins.

• Cytoplasm: 80% water, contains enzymes, nutrients, ribosomes, and inclusions.

• Nucleoid: Region containing single, circular DNA molecule (no nuclear envelope).

• Ribosomes: Sites of protein synthesis (70S type in prokaryotes).

• Inclusions: Storage granules for nutrients and reserves.

Bacterial Cell Wall

• Peptidoglycan: Repeating disaccharide units (NAG and NAM) cross-linked by peptides.

• Provides structural support and shape; protects against osmotic lysis.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick, multilayered	Thin, single layer
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains lipopolysaccharide)
Sensitivity to Lysozyme	High	Low
Gram Stain	Purple	Pink/Red

Antibacterial Properties of Lysozyme

• Lysozyme is an enzyme found in tears, saliva, and mucus.

• It hydrolyzes the bonds between NAG and NAM in peptidoglycan, especially effective against gram-positive bacteria.

Penicillin Action

• Penicillin inhibits the formation of peptide cross-bridges in peptidoglycan, weakening the cell wall and causing lysis.

• Gram-negative bacteria are less susceptible due to their outer membrane.

Atypical and Acid-Fast Cell Walls

• Atypical cell walls: Mycoplasma (no cell wall), Archaea (varied composition).

• Acid-fast cell walls: Mycobacterium (contain mycolic acid, resist decolorization by acid-alcohol).

Structures External to the Cell Wall

• Glycocalyx: Sticky, gelatinous layer for protection and adherence; forms capsules or slime layers.

• Flagella: Motility structures; arrangement varies (monotrichous, lophotrichous, amphitrichous, peritrichous).

• Fimbriae: Short, hair-like appendages for attachment.

• Pili: Longer, involved in DNA transfer (conjugation).

Biofilms

• Bacteria often exist in biofilm communities, which are more resistant to antibiotics and immune responses.

• Biofilms are clinically significant in medical device infections and chronic diseases.

Eukaryotic Cell Structure and Function

• Nucleus: Membrane-bound, contains DNA organized into chromosomes with histones.

• Membrane-bound organelles: Mitochondria (ATP production), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (protein modification and sorting), lysosomes (digestion), peroxisomes (oxidation of molecules).

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for support and transport.

• Cell wall: Present in plants (cellulose), fungi (chitin), absent in animals.

• Plasma membrane: Phospholipid bilayer with proteins, sterols (for stability), and carbohydrates (for cell recognition).

• Flagella and cilia: Motility structures with a core of microtubules (9+2 arrangement).

Movement of Materials Across Cell Membranes

• Passive Transport: Movement down a concentration gradient; includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Movement against a concentration gradient; requires energy (ATP).

• Group Translocation: Unique to prokaryotes; substance is chemically modified during transport.

• Endocytosis (Eukaryotes only): Includes phagocytosis (engulfing particles), pinocytosis (engulfing fluids), and receptor-mediated endocytosis (specific uptake via receptors).

Example: White blood cells use phagocytosis to engulf bacteria.

Clinical Relevance

• Differences in cell wall and ribosome structure are exploited by antibiotics (e.g., penicillin targets peptidoglycan, streptomycin targets 70S ribosomes).

• Biofilm formation complicates treatment of infections, especially on medical devices.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Organelles	Absent	Present
Cell Wall	Peptidoglycan (bacteria)	Cellulose, chitin, or absent
Ribosomes	70S	80S (cytoplasm), 70S (mitochondria/chloroplasts)
Cell Division	Binary fission	Mitosis/meiosis