Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Notes

"Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells: An Overview

Prokaryotic and eukaryotic cells differ fundamentally in their structure and organization. Understanding these differences is essential for studying microbial life.

• Prokaryote: Derived from Greek for 'prenucleus'.

• Eukaryote: Derived from Greek for 'true nucleus'.

Feature	Prokaryote	Eukaryote
Chromosomes	One circular chromosome, not in a membrane	Paired chromosomes, in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall	Bacteria: peptidoglycan; Archaea: pseudomurein	Polysaccharide (when present)
Division	Binary fission	Mitosis

Size, Shape, & Arrangement of Bacterial Cells

Bacteria exhibit a variety of sizes, shapes, and arrangements, which are important for identification and classification.

• Average size: 0.2 to 2.0 μm diameter × 2 to 8 μm length

• Example: E. coli is 1 μm × 2 μm

• Animal cell comparison: 10–100 μm

• Monomorphic: Most bacteria have a single shape

• Pleomorphic: Some bacteria can have multiple shapes

Common shapes:

• Bacillus (rod-shaped)

• Coccus (spherical-shaped)

• Spiral: Vibrio, Spirillum, Spirochete

• Star-shaped

• Rectangular

Arrangements:

• Pairs: diplococci, diplobacilli

• Clusters: staphylococci

• Chains: streptococci, streptobacilli

• Groups of four: tetrads

• Cubelike groups of eight: sarcinae

Structure of a Prokaryotic Cell

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

• Capsule: Protective outer layer

• Cell wall: Provides shape and prevents osmotic lysis

• Plasma membrane: Selective barrier for transport

• Cytoplasm: Internal fluid containing genetic material and ribosomes

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

• Nucleoid: Region containing circular DNA

• Plasmid: Small, extrachromosomal DNA

• Fimbriae and Pili: Surface appendages for attachment and DNA transfer

• Flagella: Motility structures

Glycocalyx (Sugar Coat)

The glycocalyx is a viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide. It exists in two forms:

• Slime layer: Unorganized and loosely attached

• Capsule: Organized and firmly attached

• Contributes to virulence by preventing phagocytosis

• Protects from antibiotics, chemicals, and desiccation

• Aids in attachment to surfaces and biofilm formation

• Examples: Bacillus anthracis, Streptococcus pneumoniae, Klebsiella pneumoniae

Flagella

Flagella are long, filamentous appendages that provide motility to bacteria.

• Enable movement toward/away from stimuli (taxis)

• Movement occurs via runs and tumbles

• Flagella proteins (H antigens) are used for serotyping (e.g., E. coli O157:H7)

• Bacteria without flagella are called atrichous

Arrangements:

• Peritrichous: Flagella all over the cell

• Monotrichous: Single flagellum at one pole

• Lophotrichous: Tuft of flagella at one pole

• Amphitrichous: Flagella at both poles

Axial Filaments

Axial filaments, or endoflagella, are found in spirochetes and are anchored at one end of the cell. Their rotation causes the cell to move in a corkscrew motion, aiding in movement through viscous environments.

Fimbriae and Pili

• Fimbriae: Hairlike appendages for attachment and biofilm formation (e.g., Neisseria gonorrhoeae, E. coli O157)

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

The Cell Wall

The bacterial cell wall is a rigid structure that prevents osmotic lysis and provides shape. It is primarily composed of peptidoglycan.

• Peptidoglycan: Polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides

• Target for antibiotics (e.g., penicillin) and enzymes (e.g., lysozyme)

Cell Wall Composition and Characteristics

Peptidoglycan is a repeating disaccharide polymer cross-linked by peptides, providing strength and rigidity.

• NAG: N-acetylglucosamine

• NAM: N-acetylmuramic acid

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
Porins	Absent	Present
Penicillin susceptibility	High	Low
Lysozyme sensitivity	High	Low

• Gram-Positive: Thick peptidoglycan, teichoic acids, high susceptibility to penicillin, disrupted by lysozyme

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), periplasmic space, porins, low susceptibility to penicillin

Outer Membrane (Gram-Negative Bacteria)

• Protects from phagocytes, complement, and antibiotics

• Contains LPS (lipopolysaccharide):

  • O polysaccharide: Antigenic

  • Lipid A: Endotoxin

• Porins: Protein channels for molecule transport

Cell Walls and the Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Alcohol dehydrates peptidoglycan, retains crystal violet-iodine complex (purple)

• Gram-negative: Alcohol dissolves outer membrane, crystal violet washes out, safranin stains cells red/pink

Atypical Cell Walls

• Acid-fast cell walls: Like gram-positive but with mycolic acid (e.g., Mycobacterium, Nocardia)

• Mycoplasmas: Lack cell walls, have sterols in plasma membrane

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

The Plasma (Cytoplasmic) Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins, following the fluid mosaic model.

• Selective permeability: allows certain molecules to pass

• Contains enzymes for ATP production

• Some have chromatophores for photosynthesis

• Damage by alcohols, detergents, and antibiotics can cause cell lysis

Movement of Materials Across Membranes

• Passive processes: Move substances from high to low concentration, no energy required

• Active processes: Move substances from low to high concentration, require energy (ATP) and transporter proteins

Passive Processes

• Simple diffusion: Movement of solute from high to low concentration until equilibrium is reached

• Facilitated diffusion: Solute combines with transporter protein in the membrane

• Osmosis: Net movement of water across a selectively permeable membrane from high to low water concentration, often via aquaporins

• Osmotic pressure: Pressure needed to stop water movement

Osmosis and Tonicity

• Isotonic solution: Equal solute concentration inside and outside cell

• Hypotonic solution: Lower solute concentration outside; water enters cell

• Hypertonic solution: Higher solute concentration outside; water leaves cell

Active Processes

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

• Group translocation: Substance is chemically altered during transport (requires PEP)