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

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Comparing Prokaryotic and Eukaryotic Cells

Overview of Cell Types

Prokaryotic and eukaryotic cells are the two fundamental cell types in microbiology. Their structural and functional differences are essential for understanding microbial physiology and classification.

Prokaryotes: Cells lacking a membrane-bound nucleus and organelles. Includes Bacteria and Archaea.
Eukaryotes: Cells with a true nucleus and membrane-bound organelles. Includes Fungi, Algae, Protozoa, and Helminths.

Key Differences:

Prokaryotes: Usually one circular chromosome, no histones, no organelles, peptidoglycan (bacteria) or pseudomurein (archaea) cell walls, divide by binary fission.
Eukaryotes: Paired chromosomes in nuclear membrane, histones, organelles, polysaccharide cell walls (when present), 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.

Common Shapes:

Bacillus: Rod-shaped
Coccus: Spherical-shaped
Spiral: Includes vibrio (curved rod), spirillum (rigid spiral), and spirochete (flexible spiral)
Star-shaped and rectangular forms also exist

Arrangements:

Pairs: Diplococci, diplobacilli
Clusters: Staphylococci
Chains: Streptococci, streptobacilli
Groups of four: Tetrads
Cubelike groups of eight: Sarcinae

Spiral and other shapes of bacteria: Vibrio, Spirillum, Spirochete Star-shaped bacteria Rectangular bacteria Arrangements of cocci: diplococci, streptococci, tetrad, sarcinae, staphylococci Arrangements of bacilli: single, diplobacilli, streptobacilli, coccobacillus

Structure of a Prokaryotic Cell

Cell Components

Prokaryotic cells contain several key structures, each with specific functions.

Capsule: Protective layer outside the cell wall
Cell wall: Provides structural support and shape
Plasma membrane: Controls entry and exit of substances
Cytoplasm: Site of metabolic activity
Nucleoid: Region containing DNA
Plasmid: Small, circular DNA molecules
Fimbriae: Attachment structures
Pilus: Used for DNA transfer
Flagella: Motility structures
Inclusions: Storage granules

Structure of a prokaryotic cell

Glycocalyx

Structure and Function

The glycocalyx is an external, viscous, gelatinous layer made of polysaccharide and/or polypeptide. It exists in two forms:

Capsule: Neatly organized and firmly attached
Slime layer: Unorganized and loose

Functions:

Contributes to virulence by preventing phagocytosis and aiding adherence
Helps form biofilms, protecting cells and aiding attachment

Capsules in bacteria

Flagella, Axial Filaments, Fimbriae, and Pili

Flagella

Flagella are filamentous appendages that propel bacteria. They consist of three parts:

Filament: Outermost region
Hook: Attaches filament to basal body
Basal body: Anchors flagellum to cell wall and membrane

Structure and attachment of a bacterial flagellum

Flagella arrangements vary:

Peritrichous: Flagella all over the cell
Monotrichous: Single flagellum at one end
Lophotrichous: Multiple flagella at one end
Amphitrichous: Flagella at both ends

Arrangements of bacterial flagella

Flagella allow movement toward or away from stimuli (taxis) and rotate to produce "run" or "tumble" movements.

Flagella and bacterial motility: run and tumble

Axial Filaments

Axial filaments, also called endoflagella, are found in spirochetes. They are anchored at one end and cause the cell to move in a corkscrew manner.

Axial filament in spirochete Leptospira Diagram of axial filaments wrapping around a spirochete

Fimbriae and Pili

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

Fimbriae on a bacterial cell

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 used to differentiate major groups of bacteria.

Structure of a prokaryotic cell with cell wall and capsule

Peptidoglycan Structure

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

Structure of peptidoglycan in Gram-positive bacteria

Gram-Positive Cell Walls

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

Gram-positive cell wall structure

Gram-Negative Cell Walls

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

Gram-negative cell wall structure Cell envelope: Gram-positive and Gram-negative

Gram Stain Mechanism

The Gram stain differentiates bacteria based on cell wall structure:

Gram-positive: Alcohol dehydrates peptidoglycan; crystal violet-iodine complexes remain.
Gram-negative: Alcohol dissolves outer membrane; complexes wash out; safranin stains cells.

Gram-positive and Gram-negative bacteria under light microscope

Atypical Cell Walls

Some bacteria have atypical cell walls:

Acid-fast: Thick peptidoglycan with waxy mycolic acid; stains with carbolfuchsin.
Mycoplasmas: Lack cell walls; sterols in plasma membrane.
Archaea: Wall-less or walls of pseudomurein.

Acid-fast cell wall: Mycobacterium tuberculosis

The Plasma (Cytoplasmic) Membrane

Structure

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

Plasma membrane structure in Vibrio cholerae Lipid bilayer of plasma membrane Phospholipid molecules in lipid bilayer

Function

The plasma membrane is selectively permeable, contains enzymes for ATP production, and may have photosynthetic pigments on foldings called chromatophores.

Chromatophores in bacterial cell

Movement of Materials Across Membranes

Passive Processes

Substances move from high to low concentration without energy expenditure.

Simple diffusion: Movement of solute to equilibrium.
Facilitated diffusion: Transport via membrane proteins.
Osmosis: Net movement of water across membrane.

Principle of simple diffusion Passive diffusion across membrane Facilitated diffusion through transporter proteins Osmosis through lipid bilayer and aquaporin Principle of osmosis Isotonic, hypotonic, and hypertonic solutions

Active Processes

Substances move from low to high concentration with energy expenditure.

Active transport: Requires transporter protein and ATP.
Group translocation: 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

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

Ribosomes

Sites of protein synthesis, composed of protein and ribosomal RNA. Prokaryotic ribosomes are 70S (50S + 30S subunits).

Prokaryotic ribosome structure

Inclusions

Reserve deposits in the cytoplasm, including:

Metachromatic granules (phosphate reserves)
Polysaccharide granules (energy reserves)
Lipid inclusions (energy reserves)
Sulfur granules (energy reserves)
Carboxysomes (CO2 fixation)
Gas vacuoles (buoyancy)
Magnetosomes (iron oxide inclusions)

Endospores

Formation and Function

Endospores are resting cells produced when nutrients are depleted. They are resistant to desiccation, heat, chemicals, and radiation, and can survive for thousands of years. Sporulation is endospore formation; germination is return to vegetative state.

Formation of endospores by sporulation

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 in a 9+2 array and move in a wavelike manner.

Eukaryotic flagella and cilia Microtubule structure of eukaryotic flagella and cilia

Cell Wall and Glycocalyx

Eukaryotic cell walls are found in plants, algae, and fungi, made of carbohydrates (cellulose, chitin, glucan, mannan). Glycocalyx is found in animal cells, strengthens the cell surface, aids attachment, and is involved in cell–cell recognition.

Plasma Membrane

Eukaryotic plasma membranes are similar to prokaryotic ones but contain sterols and carbohydrates for attachment and recognition. Functions include selective permeability, diffusion, osmosis, active transport, and endocytosis (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 found in mitochondria and chloroplasts.

The Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotes: larger bacterial cells engulfed smaller ones, forming organelles like mitochondria and chloroplasts. Evidence includes similarities in size, shape, DNA, reproduction, ribosomes, and double membranes between these organelles and bacteria.

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