Functional Anatomy of Prokaryotic and Eukaryotic Cells: Mini-Textbook Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells

Microbiology distinguishes between prokaryotic and eukaryotic cells based on structural and functional features. Understanding these differences is fundamental to the study of microbial life.

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

• Eukaryotes: Cells with a true, membrane-bound nucleus. Includes Fungi, Protozoa, Algae, and multicellular organisms.

Feature	Prokaryote	Eukaryote
Chromosomes	Usually one circular, not in membrane	Paired, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Bacteria: peptidoglycan; Archaea: pseudomurein	Polysaccharide (when present)
Ribosomes	70S	80S (70S in mitochondria)
Cell Division	Binary fission	Mitosis

The Size, Shape, and Arrangement of Bacterial Cells

Bacteria exhibit a variety of 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

• Basic shapes:

  • Bacillus (rod-shaped)

  • Coccus (spherical-shaped)

  • Spiral (includes Vibrio, Spirillum, Spirochete)

  • Star-shaped

  • Rectangular

• Arrangements:

  • Pairs: diplococci, diplobacilli

  • Clusters: staphylococci

  • Chains: streptococci, streptobacilli

  • Groups of four: tetrads

  • Cubelike groups of eight: sarcinae

Structures External to the Cell Wall

External structures contribute to bacterial survival, pathogenicity, and motility.

Glycocalyx

• External to cell wall; viscous and gelatinous

• Made of polysaccharide and/or polypeptide

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

• Functions:

  • Contributes to virulence (prevents phagocytosis, aids adherence)

  • Forms biofilms (protects cells, aids attachment)

Negative Stain for Capsules

• Capsules are visualized by negative staining (e.g., nigrosine, safranine)

• Capsules appear as clear halos around colored bacteria against a dark background

Biofilms

• Microbial communities encased in a slimy layer

• Quorum sensing enables coordinated activity

• Biofilms provide protection, facilitate nutrient sharing, and genetic exchange

• Significant in human health: increased resistance to antimicrobials, associated with medical device infections

Flagella

• Filamentous appendages for motility

• Composed of flagellin protein

• Three parts: filament, hook, basal body

• Arrangement varies: peritrichous, monotrichous, lophotrichous, amphitrichous

• Flagella rotate for movement (run/tumble), act as H antigens

Archaella

• Motility structures in Archaea

• Made of glycoproteins (archaellins), rotate like flagella

Axial Filaments

• Also called endoflagella; found in spirochetes

• Anchored at one end; rotation causes corkscrew movement

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment, biofilm formation

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

The Cell Wall

The bacterial cell wall is a complex structure essential for cell shape, protection, and pathogenicity. Its composition is used to differentiate major groups of bacteria.

• Prevents osmotic lysis, protects membrane

• Made of peptidoglycan (in bacteria)

• Site of action for antibiotics (e.g., penicillin)

Peptidoglycan Structure

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

• Rows linked by polypeptides, forming a lattice

• Penicillin interferes with peptide cross-bridges

Gram-Positive Cell Walls

• Thick peptidoglycan

• Teichoic acids (lipoteichoic and wall teichoic acids)

• High susceptibility to penicillin, disrupted by lysozyme

• Two rings in basal body of flagella

Gram-Negative Cell Walls

• Thin peptidoglycan

• Outer membrane with lipopolysaccharide (LPS), lipoproteins, phospholipids

• Periplasmic space between membranes

• Four rings in basal body of flagella

• Low susceptibility to penicillin

Cell Walls and Gram Stain Mechanism

• Crystal violet-iodine crystals form inside cell

• Gram-positive: alcohol dehydrates peptidoglycan, CV-I crystals retained

• Gram-negative: alcohol dissolves outer membrane, CV-I washes out, safranin stains cells

Atypical Cell Walls

• Acid-fast cell walls: Thick peptidoglycan, waxy mycolic acid, stain with carbolfuchsin

• Mycoplasmas: Lack cell walls, sterols in membrane

• Archaea: Wall-less or walls of pseudomurein (lack NAM and D-amino acids)

Structures Internal to the Cell Wall

Plasma (Cytoplasmic) Membrane

• Phospholipid bilayer enclosing cytoplasm

• Peripheral, integral, and transmembrane proteins

• Fluid mosaic model: proteins and lipids move freely

• Selective permeability, contains enzymes for ATP production

Destruction of Plasma Membrane

• Damaged by disinfectants (alcohols, detergents) and antibiotics (polymyxin)

• Causes leakage of cell contents

Movement of Materials Across Membranes

• Passive processes: No energy required; substances move from high to low concentration

• Active processes: Energy required; substances move from low to high concentration

Simple Diffusion

• Movement of solute from high to low concentration

• Continues until equilibrium is reached

Facilitated Diffusion

• Integral membrane proteins act as channels or carriers

• Transporter proteins may be nonspecific or specialized

Osmosis

• Net movement of water across selectively permeable membrane

• Osmotic pressure: pressure needed to stop water movement

Effects of Solutions on Cells

• Isotonic: Equal solute concentrations; no net water movement

• Hypotonic: Lower solute outside; water enters cell

• Hypertonic: Higher solute outside; water leaves cell

Active Transport and Group Translocation

• Active transport: requires transporter protein and ATP; moves substances against gradient

• Group translocation: requires transporter protein and PEP; substance is chemically altered during transport

Cytoplasm

• Thick, aqueous, elastic substance inside plasma membrane

• Contains DNA (nucleoid), ribosomes, inclusions

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

Nucleoid

• Bacterial chromosome: circular, double-stranded DNA

• Plasmids: small, extrachromosomal DNA circles; carry noncrucial genes (e.g., antibiotic resistance)

Ribosomes

• Sites of protein synthesis

• Made of protein and ribosomal RNA

• 70S: 50S (large) + 30S (small) subunits

• Target for antibiotics (e.g., streptomycin, gentamicin)

Inclusions

• Reserve deposits of nutrients

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

Endospores

• Resting cells produced when nutrients are depleted

• Resistant to desiccation, heat, chemicals, radiation

• Produced by Bacillus and Clostridium

• Sporulation: endospore formation; germination: return to vegetative state

• Contain dipicolinic acid (DPA) and Ca2+ ions for DNA protection

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

Characteristic	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Flagella basal body	2 rings	4 rings
Susceptibility to penicillin	High	Low
Lysozyme sensitivity	High	Low
Toxins	Exotoxins	Endotoxins & Exotoxins

Key Equations and Concepts

• Peptidoglycan linkage: $\text{NAG} - \beta(1,4) - \text{NAM}$

• Osmosis: $\text{Water moves from low solute concentration to high solute concentration}$

• Ribosome assembly: $30S + 50S \rightarrow 70S$

Additional info:

• Biofilms are a major factor in chronic infections and medical device contamination.

• Endospores are not reproductive structures; they are survival mechanisms.

• Gram stain is a fundamental technique for bacterial classification and diagnosis.