Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview of Prokaryotic and Eukaryotic Cells

Cells are the fundamental units of life, and in microbiology, they are classified as either prokaryotic or eukaryotic based on structural and functional differences. Understanding these differences is essential for studying microbial physiology, genetics, and taxonomy.

• Prokaryotes: Organisms whose cells lack a true nucleus and membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with cells containing a true nucleus and various membrane-bound organelles. Includes Fungi, Protozoa, Algae, and all multicellular organisms.

Key Differences:

• Prokaryotes: Usually one circular chromosome (not in a membrane), no histones, no membrane-enclosed organelles, cell walls (peptidoglycan in bacteria, pseudomurein in archaea), divide by binary fission.

• Eukaryotes: Paired chromosomes in a nuclear membrane, histones present, organelles present, cell walls (when present) made of polysaccharides, divide by mitosis.

The Size, Shape, and Arrangement of Bacterial Cells

Size and Morphology

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

• Monomorphic: Most bacteria have a single, consistent shape.

• Pleomorphic: Some bacteria can alter their shape or size in response to environmental conditions.

Common Bacterial Shapes

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes vibrio (curved rods), spirillum (rigid spirals), and spirochete (flexible spirals)

• Star-shaped and Rectangular: Less common morphologies

Arrangements of Bacterial Cells

• Pairs: Diplococci, diplobacilli

• Chains: Streptococci, streptobacilli

• Clusters: Staphylococci

• Groups of Four: Tetrads

• Cubelike Groups of Eight: Sarcinae

Prokaryotic Cell Structure

External Structures

• Glycocalyx: A viscous, gelatinous layer external to the cell wall, composed of polysaccharide and/or polypeptide. It can be a capsule (organized, firmly attached) or a slime layer (unorganized, loose). Functions include protection from phagocytosis, adherence to surfaces, and biofilm formation.

• Flagella: Filamentous appendages for motility, composed of flagellin. Structure includes filament, hook, and basal body. Enable movement via "runs" and "tumbles" and are important for chemotaxis.

• Fimbriae: Short, hairlike structures for attachment to surfaces and biofilm formation.

• Pili: Longer than fimbriae; involved in motility (twitching, gliding) and conjugation (DNA transfer).

Cell Wall

The bacterial cell wall provides structural support, prevents osmotic lysis, and contributes to pathogenicity. Its composition is a key factor in bacterial classification.

• Peptidoglycan: Main component in most bacterial cell walls; consists of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides.

• Gram-Positive Bacteria: Thick peptidoglycan layer, teichoic acids, two rings in flagellar basal body, high susceptibility to penicillin.

• Gram-Negative Bacteria: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), four rings in flagellar basal body, low susceptibility to penicillin.

• Atypical Cell Walls: Mycobacterium (acid-fast, waxy mycolic acid), Mycoplasma (no cell wall, sterols in membrane), Archaea (pseudomurein or no wall).

Plasma (Cytoplasmic) Membrane

The plasma membrane is a phospholipid bilayer with embedded proteins, responsible for selective permeability, energy generation, and, in some bacteria, photosynthesis.

• Fluid Mosaic Model: Describes the dynamic nature of the membrane.

• Functions: Selective transport, ATP production, housing enzymes, and, in some, chromatophores for photosynthesis.

Movement of Materials Across Membranes

• Passive Processes: Simple diffusion, facilitated diffusion, osmosis (movement of water), and effects of isotonic, hypotonic, and hypertonic solutions.

• Active Processes: Active transport (requires ATP and transporter proteins), group translocation (substance chemically modified during transport).

Cytoplasm and Internal Structures

• Cytoplasm: Gel-like matrix containing water, proteins, carbohydrates, lipids, ions, DNA, ribosomes, and inclusions.

• Nucleoid: Region containing the bacterial chromosome (circular DNA), not membrane-bound. Plasmids are small, extrachromosomal DNA molecules.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes), target for several antibiotics.

• Inclusions: Reserve deposits (e.g., metachromatic granules, polysaccharide granules, lipid inclusions, sulfur granules, carboxysomes, gas vacuoles, magnetosomes).

• Endospores: Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) for survival under adverse conditions.

Eukaryotic Cell Structure

External Structures

• Flagella and Cilia: Used for locomotion or moving substances along the cell surface. Composed of microtubules in a 9+2 arrangement.

• Cell Wall: Present in plants, algae, fungi (composition varies: cellulose, chitin, glucan, mannan).

• Glycocalyx: Carbohydrate-rich layer for protection, adhesion, and cell recognition (mainly in animal cells).

Plasma Membrane

• Similar to prokaryotes but contains sterols and carbohydrates for cell recognition.

• Capable of endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis).

Cytoplasm and Organelles

• Cytoskeleton: Microfilaments, intermediate filaments, and microtubules provide structure and facilitate movement.

• Ribosomes: 80S in cytoplasm and on rough ER; 70S in mitochondria and chloroplasts.

• Nucleus: Double-membrane structure containing DNA complexed with histones (chromatin); site of transcription and replication.

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis).

• Golgi Complex: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes for intracellular digestion.

• Vacuoles: Storage and structural support.

• Mitochondria: Site of ATP production via cellular respiration; contain their own DNA and 70S ribosomes.

• Chloroplasts: Site of photosynthesis in plants and algae; contain their own DNA and 70S ribosomes.

• Peroxisomes: Oxidize fatty acids and detoxify harmful substances.

• Centrosomes: Organize microtubules and are important in cell division.

Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells from prokaryotic ancestors. It proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Mitochondria and chloroplasts resemble bacteria in size and shape, have circular DNA, reproduce independently, and possess 70S ribosomes.

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells