Functional Anatomy of Prokaryotic and Eukaryotic Cells: Structure, Function, and Comparison

Comparing Prokaryotic and Eukaryotic Cells

Overview of Cell Types

Prokaryotic and eukaryotic cells represent two fundamental organizational types in microbiology. Understanding their structural and functional differences is essential for grasping microbial physiology and taxonomy.

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

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

• Key Differences: Chromosome structure, presence of histones, organelles, cell wall composition, and division mechanisms.

Example: Bacteria (prokaryotes) divide by binary fission, while eukaryotic cells divide by mitosis.

The Size, Shape, and Arrangement of Bacterial Cells

Bacterial Morphology

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: Most bacteria have a single, consistent shape.

• Pleomorphic: Some bacteria can vary in shape.

Main Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes Vibrio (curved rod), Spirillum (rigid spiral), Spirochete (flexible spiral)

• Star-shaped and Rectangular forms (rare)

Arrangements of Cocci and Bacilli

Bacterial cells can be arranged in characteristic patterns based on their division planes.

• Cocci: Diplococci (pairs), Streptococci (chains), Tetrads (groups of four), Sarcinae (cubelike groups of eight), Staphylococci (clusters)

• Bacilli: Single, Diplobacilli (pairs), Streptobacilli (chains), Coccobacilli (short rods)

Structure of a Prokaryotic Cell

Cell Components

Prokaryotic cells contain essential structures for survival and reproduction. Not all bacteria possess every structure.

• Capsule: Protective outer layer

• Cell wall: Provides shape and protection

• Plasma membrane: Controls entry and exit of substances

• Cytoplasm: Site of metabolic activity

• 70S ribosomes: Protein synthesis

• Nucleoid: Contains DNA

• Plasmid: Extra-chromosomal DNA

• Fimbriae: Attachment structures

• Pilus: DNA transfer and motility

• Flagella: Motility

• Inclusions: Storage granules

Glycocalyx

Structure and Function

The glycocalyx is an external, viscous, gelatinous layer composed of polysaccharides and/or polypeptides. It exists in two forms:

• Capsule: Neatly organized, firmly attached

• Slime layer: Unorganized, loose

Functions:

• Contributes to virulence by preventing phagocytosis and aiding adherence

• Helps form biofilms, protecting cells and aiding attachment

Flagella, Axial Filaments, Fimbriae, and Pili

Flagella

Flagella are filamentous appendages that propel bacteria. They consist of three parts: filament, hook, and basal body. Bacterial flagella are powered by a proton gradient.

• Filament: Outermost region

• Hook: Attaches filament to basal body

• Basal body: Anchors flagellum to cell wall and membrane

Flagellar Arrangements

Bacteria exhibit various flagellar arrangements, which aid in motility and identification.

• Peritrichous: Flagella all over the cell

• Monotrichous: Single flagellum at one end

• Lophotrichous: Multiple flagella at one end

• Amphitrichous: Flagella at both ends

Flagellar Movement

Flagella rotate to produce "runs" (straight movement) and "tumbles" (change in direction). Flagella proteins serve as H antigens for serovar identification.

Axial Filaments (Endoflagella)

Axial filaments are found in spirochetes and are anchored at one end of the cell. Their rotation causes the cell to move in a corkscrew fashion.

Fimbriae and Pili

Fimbriae are hairlike appendages that allow for attachment and biofilm formation. Pili are involved in motility and DNA transfer (conjugation).

The Cell Wall

Structure and Function

The bacterial cell wall prevents osmotic lysis, protects the cell membrane, and contributes to pathogenicity. It is a site of action for antibiotics and is composed of peptidoglycan.

• Peptidoglycan: Polymer of repeating disaccharides (N-acetylglucosamine and N-acetylmuramic acid) linked by polypeptides, forming a lattice structure.

• Penicillin: Interferes with peptide cross-bridges, weakening the cell wall.

Gram-Positive Cell Walls

Gram-positive bacteria have thick peptidoglycan layers and teichoic acids, which regulate cation movement and provide antigenic specificity.

Gram-Negative Cell Walls

Gram-negative bacteria have thin peptidoglycan, an outer membrane with lipopolysaccharide (LPS), lipoproteins, and phospholipids. The outer membrane protects from phagocytes and antibiotics.

• LPS: O polysaccharide (antigen), Lipid A (endotoxin)

• Porins: Channels for molecule passage

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Alcohol dehydrates peptidoglycan; crystal violet-iodine complex remains.

• Gram-negative: Alcohol dissolves outer membrane; complex washes out; safranin stains cells.

Atypical Cell Walls

Some bacteria have atypical cell walls:

• Acid-fast: 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).

The Plasma (Cytoplasmic) Membrane

Structure

The plasma membrane is a phospholipid bilayer enclosing the cytoplasm, with peripheral, integral, and transmembrane proteins. Some proteins form channels; glycoproteins and glycolipids are present.

Function

• Selective permeability: Allows passage of some molecules

• ATP production: Contains enzymes for energy generation

• Photosynthetic pigments: On foldings called chromatophores

Movement of Materials Across Membranes

Passive Processes

• Simple diffusion: Movement from high to low concentration

• Facilitated diffusion: Uses transporter proteins for ions and larger molecules

• Osmosis: Net movement of water across a selectively permeable membrane

Osmotic pressure: Pressure needed to stop water movement

Active Processes

• 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, Nucleoid, Ribosomes, and Inclusions

Cytoplasm

The cytoplasm is a thick, aqueous, elastic substance inside the plasma membrane, containing DNA, ribosomes, and inclusions. The cytoskeleton aids in cell division, shape, growth, and DNA movement.

Nucleoid

• Bacterial chromosome: Circular, double-stranded DNA

• Plasmids: Small, extrachromosomal DNA circles; carry non-crucial genes

Ribosomes

• Sites of protein synthesis

• 70S: 50S large + 30S small subunits

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

Inclusions

• Metachromatic granules: Phosphate reserves

• Polysaccharide granules: Energy reserves

• Lipid inclusions: Energy reserves

• Sulfur granules: Energy reserves

• Carboxysomes: RuBisCO enzyme for CO2 fixation

• Gas vacuoles: Buoyancy

• Magnetosomes: Iron oxide inclusions

Endospores

Formation and Function

Endospores are resting cells produced by certain bacteria when nutrients are depleted. They are highly resistant to desiccation, heat, chemicals, and radiation, and can survive for thousands of years. Endospore formation (sporulation) is a survival mechanism, not a reproductive process.

• Produced by: Bacillus and Clostridium

• Germination: Endospore returns to vegetative state

Comparing Eukaryotic and Prokaryotic Structures

Flagella and Cilia

Eukaryotic flagella and cilia are projections used for locomotion or moving substances. Flagella are long and few; cilia are short and numerous. Both consist of microtubules arranged in a 9+2 array and move in a wavelike manner.

Cell Wall and Glycocalyx

• Cell wall: Found in plants, algae, fungi; made of carbohydrates (cellulose, chitin, glucan, mannan)

• Glycocalyx: Carbohydrates bonded to proteins and lipids; found in animal cells; strengthens cell surface, aids attachment, and cell–cell recognition

Plasma Membrane

• Structure: Similar to prokaryotes; phospholipid bilayer, integral and peripheral proteins

• Differences: Sterols (complex lipids), carbohydrates for attachment and recognition

• Function: Selective permeability, transport processes, endocytosis (phagocytosis, pinocytosis, receptor-mediated)

Cytoplasm

• Cytosol: Fluid portion

• Cytoskeleton: Microfilaments, intermediate filaments, microtubules

• Cytoplasmic streaming: Movement throughout cell

Ribosomes

• 80S: Large 60S + small 40S subunits; membrane-bound or free

• 70S: In chloroplasts and mitochondria

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotes. Larger bacterial cells engulfed smaller ones, leading to the development of organelles such as mitochondria and chloroplasts. Evidence includes similarities in size, shape, DNA, reproduction, ribosomes, and double membranes between these organelles and bacteria.

• Mitochondria and chloroplasts: Resemble bacteria, have circular DNA, reproduce independently, have 70S ribosomes, double membranes, and genomes similar to bacteria.

Additional info: Eukaryotes branch off from Archaea, supporting the evolutionary relationship.