Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Introduction

This chapter explores the structural and functional differences between prokaryotic and eukaryotic cells, focusing on their components, cell walls, membranes, and specialized structures. Understanding these differences is fundamental to microbiology, as it underpins the classification, physiology, and pathogenicity of microorganisms.

Components of All Cells

Universal Cell Structures

• Plasma (Cell) Membrane: Separates the living cell from its environment, controls entry and exit of substances.

• Chromosomes: DNA molecules carrying hereditary information.

• Ribosomes: Sites of protein synthesis.

• Cytosol: Semi-fluid substance inside the cell membrane.

Prokaryotic vs. Eukaryotic Cells

Key Differences

• Prokaryotes: No nucleus, one circular chromosome, no histones, no membrane-bound organelles, cell wall (peptidoglycan in bacteria, pseudomurein in Archaea), divide by binary fission.

• Eukaryotes: True nucleus, paired linear chromosomes, histones, membrane-bound organelles, cell wall (chitin in fungi, cellulose in plants), divide by mitosis.

Prokaryotic Cell Structure

Shapes and Arrangements

Bacteria exhibit various shapes and arrangements, which are important for identification and classification.

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spiral: Includes vibrio, spirillum, and spirochete forms

• Arrangements: Diplococci (pairs), streptococci (chains), staphylococci (clusters), diplobacilli (pairs), streptobacilli (chains)

Generalized Prokaryotic Cell Structure

Prokaryotic cells have several key structures that contribute to their function and survival.

• Capsule

• Cell wall

• Plasma membrane

• Cytoplasm

• Nucleoid (DNA)

• Ribosomes

• Plasmid

• Fimbriae

• Flagella

Glycocalyx: Slime Layer and Capsule

Structure and Function

• Slime Layer: Loosely organized, promotes adherence, protects from drying, traps nutrients, important in biofilm formation.

• Capsule: Highly organized, prevents phagocytosis, thick layer, increases pathogenicity.

Biofilms

Biofilms are microbial communities that form on surfaces, providing protection and enhanced survival for bacteria.

• Form slime or hydrogels

• Quorum sensing enables cell-to-cell communication

• Advantages: nutrient sharing, resistance to antibiotics and immune system

Flagella and Motility

Structure of Prokaryotic Flagellum

• Filament: Composed of flagellin, forms a helix

• Hook: Connects filament to basal body

• Basal Body: Anchors flagellum to cell wall and membrane; structure differs in Gram-positive and Gram-negative bacteria

Flagellar Arrangements and Motility

• Peritrichous: Flagella distributed over entire cell

• Monotrichous: Single flagellum at one pole

• Lophotrichous: Tuft of flagella at one pole

• Amphitrichous: Flagella at both poles

• Motility: Bacteria move by "running" and "tumbling"; direction determined by flagellar rotation

Axial Filaments (Endoflagella)

• Found in spirochetes

• Anchored at one end, rotation causes corkscrew movement

Fimbriae and Pili

Fimbriae

• Hairlike appendages for adhesion to surfaces and tissues

• Important for colonization and infection

Pili

• Rigid tubular structures made of pilin

• Assist in attachment, motility (gliding/twitching), and genetic material transfer (conjugation)

Bacterial Cell Wall

Structure and Function

• Located outside the plasma membrane

• Prevents osmotic lysis, protects cell, contributes to pathogenicity

• Composed of peptidoglycan (in bacteria)

Peptidoglycan Structure

• Polymer of disaccharides: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

• Rows of carbohydrates linked by polypeptides

• Provides strength and rigidity

Gram-Positive Cell Wall

• Thick peptidoglycan layer

• Teichoic acids (wall and lipoteichoic acids) provide antigenic specificity and regulate cation movement

Gram-Negative Cell Wall

• Thin peptidoglycan layer

• Outer membrane contains lipopolysaccharides (LPS), lipoproteins, and phospholipids

• Periplasmic space between outer and plasma membranes

• No teichoic acids

• LPS contains O polysaccharide (antigen), core polysaccharide (stability), and Lipid A (endotoxin)

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Periplasmic space	Absent	Present

Atypical Cell Walls

Special Cases

• Mycobacterium tuberculosis: Acid-fast cell wall, waxy lipid (mycolic acid), resistant to chemicals and dehydration

• Mycoplasma pneumoniae: Lacks cell wall, pleomorphic, sterols in membrane for protection

Damage to the Cell Wall

Mechanisms

• Lysozyme: Enzyme that hydrolyzes peptidoglycan bonds

• Penicillin: Inhibits peptide bridge formation in peptidoglycan

Plasma Membrane in Bacteria

Fluid Mosaic Model

• Phospholipid bilayer with hydrophilic heads and hydrophobic tails

• Integral, transmembrane, and peripheral proteins

• Functions: energy reactions (ATP), nutrient processing, transport

• Self-sealing, proteins and lipids move freely

Membrane Transport

• Passive Transport: No energy required; includes simple diffusion and facilitated diffusion

• Active Transport: Requires energy (ATP); moves substances against concentration gradient

• Group Translocation: Substance is chemically modified during transport

Osmosis and Solutions

• Isotonic: Equal solute concentration inside and outside cell

• Hypotonic: Lower solute outside; water enters cell, may cause lysis

• Hypertonic: Higher solute outside; water leaves cell, causes plasmolysis

Internal Structures of Prokaryotes

Cytoplasm

• 80% water, contains proteins, carbohydrates, lipids, ions

• Serves as pool for building blocks and energy

• Cytoskeleton provides structural support

Nucleoid

• Region containing bacterial chromosome (circular, double-stranded DNA)

• Plasmids: extrachromosomal DNA, often carry antibiotic resistance or toxin genes

Ribosomes

• Sites of protein synthesis

• Composed of protein and rRNA

• Prokaryotic ribosomes: 70S (50S + 30S subunits)

Inclusions

Inclusion	Function
Metachromatic granules	Phosphate reserves
Polysaccharide granules	Energy reserves
Lipid inclusions	Energy reserves
Sulfur granules	Energy reserves
Carboxysomes	CO2 fixation during photosynthesis
Gas vacuoles	Buoyancy
Magnetosomes	Iron oxide inclusions

Endospores

• Produced by Gram-positive bacteria (e.g., Bacillus, Clostridium)

• Formed under nutrient depletion

• Highly resistant to adverse conditions

• Germination returns endospore to vegetative state

• Made of keratin, can survive millions of years

Functional Anatomy of Eukaryotic Cells

Overview

Eukaryotic cells are highly compartmentalized, with membrane-bound organelles performing specialized functions.

• Nucleus: Contains genetic material, site of transcription

• Ribosomes: Site of translation; 80S in cytoplasm, 70S in mitochondria/chloroplasts

• Endoplasmic Reticulum (ER): Rough ER (protein processing), Smooth ER (lipid synthesis, detoxification)

• Golgi Complex: Modifies, sorts, and ships proteins

• Lysosomes: Digestion and waste removal

• Vacuoles: Storage and structural support (plants)

• Mitochondria: ATP production via cellular respiration

• Chloroplasts: Photosynthesis in plants and algae

• Cytoskeleton: Structural support, movement, organelle positioning

Endosymbiotic Theory

• Eukaryotes evolved from symbiotic relationships between larger and smaller prokaryotic cells

• Mitochondria and chloroplasts originated from engulfed bacteria

Cytoskeleton Elements

• Microfilaments: Actin, cell shape, muscle contraction

• Intermediate filaments: Cell shape, organelle stabilization

• Microtubules: Tubulin, cell shape, vesicle/chromosome movement, motility

Flagella and Cilia

• Both made of microtubules (9+2 arrangement)

• Flagella: long, few; Cilia: short, numerous

• Provide locomotion or move substances along cell surface

External Structures of Eukaryotic Cells

Glycocalyx

• Outermost layer, composed of polysaccharides

• Functions: protection, adherence, signal reception

Cell Wall

• Found in plants, algae, fungi

• Composed of chitin (fungi) or cellulose (plants)

• Provides structural support and shape

Cell Membrane

• Phospholipid bilayer with embedded proteins

• Contains sterols for rigidity

• Serves as selectively permeable barrier

Additional info: These notes expand on the original content by providing definitions, examples, and comparisons, ensuring a comprehensive and self-contained study guide for college-level microbiology students.