Functional Anatomy of Prokaryotic and Eukaryotic Cells: Microbiology Study Notes

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Overview: Similarities and Differences

Both prokaryotic and eukaryotic cells contain nucleic acids, proteins, lipids, and carbohydrates. They utilize similar chemical reactions for metabolism, protein synthesis, and energy storage. However, they differ significantly in structural organization and complexity.

• Prokaryotic cells lack a membrane-bound nucleus and organelles.

• Eukaryotic cells possess a nucleus and various membrane-bound organelles.

Prokaryotic and Eukaryotic Cell Structure

Prokaryotic cells are generally smaller (0.2–2.0 μm diameter, 2–8 μm length) and have shapes determined by genetics. Common shapes include bacillus (rod-shaped), coccus (spherical), and spiral (corkscrew).

• Bacilli: Rod-shaped bacteria

• Cocci: Spherical bacteria

• Spiral: Includes vibrio, spirillum, and spirochete forms

Bacterial Arrangements

Bacteria can be arranged in characteristic patterns:

• Staphylo-: Grapelike clusters (cocci)

• Strepto-: Chains (cocci or bacilli)

Prokaryotic Cell Anatomy

Prokaryotic cells contain several key structures:

• Nucleoid: Region containing DNA

• Plasmids: Small, circular DNA molecules

• Ribosomes: Sites of protein synthesis

• Cell wall: Provides shape and protection

• Capsule: External protective layer

• Flagella: Motility structures

Glycocalyx

Structure and Types

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

• Capsule: Neatly organized and firmly attached

• Slime layer: Unorganized and loosely attached

Functions of Glycocalyx

• Adherence: Helps bacteria attach to surfaces

• Virulence: Capsules prevent phagocytosis, contributing to pathogenicity

• Biofilm formation: EPS (extracellular polymeric substance) aids in cell attachment

• Protection: Shields against dehydration and toxins

• Nutrition: May serve as a nutrient source

Flagella and Other Appendages

Flagella Structure and Function

Flagella are long, whip-like appendages used for motility. Their structure differs between Gram-negative and Gram-positive bacteria.

• Parts: Filament, hook, and basal body

• Function: Movement via rotation

Flagella Arrangements

• Peritrichous: Flagella all over the cell

• Monotrichous: Single flagellum at one end

• Lophotrichous: Multiple flagella at one end

• Amphitrichous: Flagella at both ends

Flagella Movement

Bacteria move via taxis, responding to environmental stimuli (e.g., chemotaxis). Movement involves alternating "runs" and "tumbles" as flagella rotate.

Other Appendages: Axial Filaments, Fimbriae, Pili

• Axial filaments: Found in spirochetes, enable corkscrew movement

• Fimbriae: Hairlike structures for attachment

• Pili: Used for motility and DNA transfer (conjugation)

Bacterial Cell Wall

Structure and Function

The bacterial cell wall prevents cell lysis due to osmotic pressure, maintains shape, and anchors flagella. It is primarily composed of peptidoglycan, which contains both D and L amino acids.

Peptidoglycan Structure

Peptidoglycan consists of a carbohydrate backbone (N-acetylglucosamine and N-acetylmuramic acid) linked by polypeptides.

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified based on cell wall structure, which is revealed by the Gram stain.

• Gram-positive: Thick peptidoglycan layers, teichoic acids

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides, porins, and a gel-like periplasm

Plasma Membrane

Structure

The plasma membrane is a phospholipid bilayer separating the cell wall from the cytoplasm. It includes proteins and lipids, and follows the Fluid Mosaic Model.

• Phosphate heads: Hydrophilic, face outward

• Fatty acid tails: Hydrophobic, face inward

Functions

• Selective permeability: Controls entry and exit of substances

• Energy generation: Site of nutrient breakdown and ATP production

Movement of Substances Across Membranes

Passive Processes

• Simple diffusion: Movement from high to low concentration, no ATP required

• Facilitated diffusion: Movement via membrane proteins, no ATP required

• Osmosis: Movement of water across membrane, no ATP required

Osmotic Pressure and Solutions

• Isotonic: Equal solute concentration inside and outside cell

• Hypotonic: Lower solute concentration outside cell

• Hypertonic: Higher solute concentration outside cell

Active Processes

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

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

Internal Structures of Prokaryotic Cells

Inclusions

• Metachromatic granules: Store inorganic phosphate

• Polysaccharide and lipid inclusions: Energy reserves

• Sulfur granules: Energy source for sulfur bacteria

• Carboxysomes: Contain enzymes for photosynthesis

• Gas vacuoles: Provide buoyancy in aquatic bacteria

• Magnetosomes: Contain iron oxide, act as magnets

Endospores

Endospores are highly durable, dehydrated cells formed by some Gram-positive bacteria. They are resistant to heat, dehydration, toxins, and radiation, and can survive for millions of years.

Eukaryotic Cell Structure

General Structure

Eukaryotic cells are larger and more complex, containing a nucleus and various organelles.

Eukaryotic Flagella and Cilia

Flagella and cilia are projections for motility or moving substances along the cell surface. Both contain microtubules arranged in a 9+2 array and move in a wave-like manner.

• Flagella: Few and long (e.g., Euglena)

• Cilia: Many and short (e.g., Tetrahymena)

Eukaryotic Cell Wall and Glycocalyx

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

• Pellicle: Flexible protein covering in protozoans

• Glycocalyx: Sticky carbohydrate layer covering plasma membrane; strengthens cell surface and aids attachment

Eukaryotic Plasma Membrane

• Similar to prokaryotes, but contains sterols

• Endocytosis: Plasma membrane engulfs extracellular material

Cytoplasm and Cytoskeleton

• Cytoplasm: Substance inside plasma membrane and outside nucleus

• Cytosol: Fluid portion

• Cytoskeleton: Microtubules (tubulin), microfilaments (actin), intermediate filaments; maintain shape, anchor organelles, enable movement

Ribosomes

• Sites of protein synthesis

• Free in cytoplasm or bound to ER

• Consist of large and small subunits

Nucleus

• Contains DNA

• Surrounded by nuclear envelope

Endoplasmic Reticulum (ER)

• Rough ER: Protein processing and sorting; covered with ribosomes

• Smooth ER: Synthesis of phospholipids, fats, steroids

Golgi Complex

• Processes and sorts proteins from ER

• Proteins leave via secretory vesicles, plasma membrane vesicles, or lysosomes

Lysosomes and Vacuoles

• Lysosomes: Contain enzymes for molecule breakdown; digest bacteria

• Vacuoles: Storage of nutrients, water, waste

Mitochondria

• Double membrane structure

• Inner membrane forms cristae

• Matrix contains DNA, ribosomes, enzymes

• Site of cellular respiration (ATP generation)

Chloroplasts (Plants and Algae)

• Contain thylakoids with chlorophyll

• Site of photosynthesis

• Contain DNA, ribosomes, enzymes

Peroxisomes and Centrosomes

• Peroxisomes: Oxidation of fatty acids and amino acids; breakdown of toxic substances (H2O2 by catalase)

• Centrosomes: Protein fibers and centrioles; form mitotic spindle for cell division

Evolution of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory proposes that eukaryotes evolved when larger bacterial cells engulfed smaller ones, leading to the development of organelles such as mitochondria and chloroplasts. Evidence includes similarities in DNA, ribosomes, and double membranes.

Review Questions

• What are three structural differences between gram-positive and gram-negative cell walls?

• What are two functions of a glycocalyx in bacteria?

• How do pili and fimbriae differ from flagella?