Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Prokaryotic vs. Eukaryotic Cells

Microbiology distinguishes between prokaryotic and eukaryotic cells based on structural and functional characteristics. Understanding these differences is fundamental for studying microbial physiology, genetics, and pathogenicity.

• Prokaryotes: Cells lacking a membrane-bound nucleus; include Bacteria and Archaea.

• Eukaryotes: Cells with a true nucleus and membrane-bound organelles; include fungi, protozoa, algae, and plants.

Key Differences:

Feature	Prokaryote	Eukaryote
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Chromosomes	Usually one circular	Paired, linear
Cell Wall	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharides (cellulose, chitin)
Division	Binary fission	Mitosis
Organelles	Absent	Present

Size, Shape, and Arrangement of Bacterial Cells

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

• Average size: 0.2–2.0 μm diameter, 2–8 μm length

• Monomorphic: Single shape

• Pleomorphic: Variable shapes

Common Shapes:

• Bacillus: Rod-shaped

• Coccus: Spherical

• Spiral: Includes vibrio, spirillum, spirochete

• Star-shaped and Rectangular: Rare forms

Arrangements:

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of four: Tetrads

• Cubelike groups of eight: Sarcinae

Structure of a Prokaryotic Cell

Prokaryotic cells have a simple structure but contain specialized components for survival and adaptation.

• Cell wall: Provides shape and protection

• Plasma membrane: Controls transport

• Cytoplasm: Contains DNA, ribosomes, inclusions

• Capsule: Protective layer

• Flagella: Motility

• Fimbriae and pili: Attachment and DNA transfer

Glycocalyx

The glycocalyx is an external layer that enhances bacterial survival and pathogenicity.

• Composition: Polysaccharide and/or polypeptide

• Types: Capsule (organized, attached), Slime layer (unorganized, loose)

• Functions: Prevents phagocytosis, aids in adherence, forms biofilms

Flagella

Flagella are appendages that provide motility to bacteria, enabling movement toward or away from stimuli.

• Structure: Filament, hook, basal body

• Composition: Protein flagellin

• Arrangement: Monotrichous, lophotrichous, amphitrichous, peritrichous

• Function: Motility (run and tumble), taxis, antigenic properties (H antigens)

Axial Filaments

Axial filaments, or endoflagella, are unique motility structures found in spirochetes, enabling corkscrew movement.

• Location: Anchored at one end, wrap around cell

• Function: Rotation causes movement

Fimbriae and Pili

Fimbriae and pili are surface structures involved in attachment, motility, and genetic exchange.

• Fimbriae: Hairlike, enable attachment and biofilm formation

• Pili: Involved in motility and conjugation (DNA transfer)

Cell Wall

The cell wall is a critical structure for bacterial survival, providing shape, protection, and contributing to pathogenicity.

• Composition: Peptidoglycan (Bacteria), Pseudomurein (Archaea)

• Function: Prevents osmotic lysis, site of antibiotic action

• Types: Gram-positive, Gram-negative, acid-fast, atypical (mycoplasmas, archaea)

Peptidoglycan Structure

Peptidoglycan is a polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked by polypeptides to form a lattice.

• Penicillin: Inhibits peptide cross-bridges, weakening cell wall

Gram-Positive vs. Gram-Negative Cell Walls

Gram staining differentiates bacteria based on cell wall structure, which affects susceptibility to antibiotics and pathogenicity.

• Gram-Positive: Thick peptidoglycan, teichoic acids, two rings in flagella basal body, high susceptibility to penicillin

• Gram-Negative: Thin peptidoglycan, outer membrane with LPS, four rings in flagella basal body, low susceptibility to penicillin

Gram Stain Mechanism

• Crystal violet-iodine: Forms crystals inside cell

• Alcohol: Dehydrates peptidoglycan (Gram-positive), dissolves outer membrane (Gram-negative)

• Safranin: Stains Gram-negative cells

Comparative Table: Gram-Positive vs. Gram-Negative

Characteristic	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Flagella basal body	2 rings	4 rings
Penicillin susceptibility	High	Low

Atypical Cell Walls

Some bacteria have atypical cell walls, such as acid-fast bacteria (waxy mycolic acid) and mycoplasmas (lack cell wall).

• Acid-fast: Mycobacterium, Nocardia

• Mycoplasmas: Sterols in membrane

• Archaea: Pseudomurein or wall-less

Damage to the Cell Wall

Antibiotics and enzymes can damage bacterial cell walls, leading to cell lysis.

• Lysozyme: Hydrolyzes peptidoglycan

• Penicillin: Inhibits peptide bridge formation

• Protoplast: Wall-less Gram-positive cell

• Spheroplast: Wall-less Gram-negative cell

• L forms: Irregular, wall-less cells

Plasma (Cytoplasmic) Membrane Structure

The plasma membrane is a phospholipid bilayer with embedded proteins, responsible for selective permeability and metabolic functions.

• Fluid mosaic model: Proteins and lipids move freely

• Glycoproteins and glycolipids: Attached carbohydrates

Plasma Membrane Function

The plasma membrane regulates transport, houses enzymes for ATP production, and may contain photosynthetic pigments.

• Selective permeability: Allows passage of specific molecules

• Chromatophores: Pigment-containing foldings in photosynthetic bacteria

Destruction of Plasma Membrane

Antimicrobial agents such as alcohols, detergents, and antibiotics (e.g., polymyxin) can damage the plasma membrane, causing leakage of cell contents.

Movement of Materials Across Membranes

Cells transport materials across membranes via passive and active processes.

• Passive: No energy required; includes simple diffusion, facilitated diffusion, osmosis

• Active: Requires energy (ATP); includes active transport, group translocation

Cytoplasm

The cytoplasm is a gel-like substance containing water, proteins, carbohydrates, lipids, ions, DNA, ribosomes, and inclusions.

• Cytoskeleton: Fibers for cell division, shape, growth, DNA movement

Nucleoid

The nucleoid contains the bacterial chromosome (circular, double-stranded DNA) and plasmids (extrachromosomal DNA).

• Plasmids: Carry nonessential genes, replicate independently, may transfer between cells

Ribosomes

Ribosomes are the sites of protein synthesis, composed of protein and rRNA. Prokaryotic ribosomes are 70S (50S + 30S subunits).

• Antibiotics: Streptomycin, gentamicin, erythromycin, chloramphenicol target prokaryotic ribosomes

Inclusions

Inclusions are reserve deposits of nutrients and other substances within the cytoplasm.

• Types: Metachromatic granules (phosphate), polysaccharide granules, lipid inclusions, sulfur granules, carboxysomes, gas vacuoles, magnetosomes

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) under nutrient depletion.

• Resistant to: Desiccation, heat, chemicals, radiation

• Sporulation: Formation of endospore

• Germination: Return to vegetative state

Eukaryotic Cell Structures & Functions

Flagella and Cilia

Eukaryotic flagella and cilia are projections used for locomotion or moving substances along the cell surface. Both consist of microtubules arranged in a 9+2 array.

• Flagella: Long, few in number

• Cilia: Short, numerous

• Movement: Wavelike

Cell Wall and Glycocalyx

Eukaryotic cell walls are found in plants, algae, and fungi, composed of carbohydrates. The glycocalyx is a carbohydrate-rich layer found in animal cells, strengthening the cell surface and aiding in cell-cell recognition.

• Cell wall: Cellulose (plants), chitin (fungi), glucan/mannan (yeasts)

• Glycocalyx: Carbohydrates bonded to proteins/lipids

Plasma (Cytoplasmic) Membrane

Eukaryotic plasma membranes are similar to prokaryotic membranes but contain sterols and carbohydrates for attachment and recognition. Endocytosis is a unique function in eukaryotes.

• Endocytosis types: Phagocytosis, pinocytosis, receptor-mediated

Cytoplasm

Eukaryotic cytoplasm contains cytosol, cytoskeleton (microfilaments, intermediate filaments, microtubules), and exhibits cytoplasmic streaming.

Ribosomes

Eukaryotic ribosomes are 80S (60S + 40S subunits), found free in cytoplasm or bound to endoplasmic reticulum. 70S ribosomes are present in mitochondria and chloroplasts.

Nucleus

The nucleus is a double-membrane structure containing DNA complexed with histones, forming chromatin. Chromatin condenses into chromosomes during cell division.

Endoplasmic Reticulum

The ER is a folded transport network. Rough ER is studded with ribosomes for protein synthesis; smooth ER synthesizes membranes, fats, and hormones.

Golgi Complex

The Golgi complex modifies proteins from the ER and transports them via secretory vesicles.

Lysosomes and Vacuoles

• Lysosomes: Contain digestive enzymes

• Vacuoles: Storage, shape, formed by endocytosis

Mitochondria

Mitochondria are double-membrane organelles involved in cellular respiration (ATP production), containing 70S ribosomes and circular DNA.

Chloroplasts

Chloroplasts are the site of photosynthesis, containing thylakoids with chlorophyll, 70S ribosomes, and circular DNA.

Peroxisomes and Centrosomes

• Peroxisomes: Oxidize fatty acids, destroy H2O2

• Centrosomes: Organize mitotic spindle, critical for cell division

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotes: larger bacterial cells engulfed smaller ones, leading to mitochondria and chloroplasts.

• Evidence: Mitochondria and chloroplasts resemble bacteria, have circular DNA, reproduce independently, have 70S ribosomes, and double membranes