Cell Structure and Function: Microbiology Study Notes

Cell Structure and Function

Domains of Life & Biological Classification

The classification of life forms is fundamental to microbiology, as it helps organize organisms based on shared characteristics. The three domains—Bacteria, Archaea, and Eukaryotes—are distinguished by cellular structure and genetic differences. Viruses, while not classified within these domains, are important in microbiology due to their unique properties.

• Bacteria: Prokaryotic, single-celled organisms with diverse metabolic capabilities.

• Archaea: Prokaryotic, often extremophiles, distinct from bacteria in cell wall composition and genetics.

• Eukaryotes: Organisms with membrane-bound organelles, including plants, animals, fungi, and protists.

• Viruses: Acellular entities, require host cells for replication.

Characteristics Common to All Life

All cellular life shares several fundamental characteristics, though viruses differ in key ways.

• Growth: Increase in size; occurs in all cellular life, not in viruses.

• Reproduction: Increase in number; all cells reproduce, viruses rely on host cells.

• Responsiveness: Ability to react to environmental stimuli (e.g., taxis); present in all cells, limited in viruses.

• Metabolism: Use of nutrients for energy; all cells metabolize, viruses use host metabolism.

Types of Microorganisms

Microbiology studies a variety of microorganisms, each with unique structural and functional properties.

• Bacteria

• Archaea

• Fungi

• Protists

• Viruses

• Algae

Cell Structure: Prokaryotes vs. Eukaryotes

Prokaryotic Cell Structure

Prokaryotic cells (Bacteria and Archaea) lack membrane-bound organelles and a nucleus. Their cellular organization is simpler but highly efficient for survival in diverse environments.

• Nucleoid: Region containing circular DNA.

• Ribosomes: Sites of protein synthesis.

• Cell wall: Provides structure and protection.

• Glycocalyx: External layer for protection and adherence.

• Flagella: Motility structures.

Eukaryotic Cell Structure

Eukaryotic cells possess membrane-bound organelles, including a nucleus, and are structurally more complex. This allows compartmentalization of functions and greater cellular specialization.

• Nucleus: Contains linear DNA, control center.

• Mitochondria: Site of aerobic ATP production.

• Endoplasmic reticulum: Protein and lipid synthesis.

• Golgi apparatus: Protein modification and sorting.

• Lysosomes: Breakdown of nutrients and cellular debris.

• Cytoskeleton: Structural support and movement.

Comparison of Domains: Cell Morphology

Microbial domains can be visually distinguished by cell shape and structure.

• Archaea: Often spherical or irregular (e.g., Methanosarcina).

• Bacteria: Rod-shaped, spherical, or spiral (e.g., E. coli).

• Eukarya: Variable shapes, often larger and more complex (e.g., Amoeba).

Organelles: Prokaryotes vs. Eukaryotes

Organelles are specialized structures within cells. Prokaryotes have fewer organelles, while eukaryotes possess both nonmembranous and membranous organelles.

Organelle	Function	Prokaryotes	Eukaryotes
Ribosomes	Protein synthesis	Present in all	Present in all
Cytoskeleton	Shape/support	Present in some	Present in all
Centrosome/centrioles	Mitosis/cytokinesis	Absent	Present in animals
Nucleus	Control center	Absent	Present in all
Endoplasmic reticulum	Transport/lipid synthesis	Absent	Present in all
Golgi apparatus	Secretion	Absent	Present in some
Lysosomes	Breakdown	Absent	Present in some
Peroxisomes	Neutralize toxins	Absent	Present in some
Mitochondria	ATP production	Absent	Present in most
Chloroplasts	Photosynthesis	Absent	Present in plants/algae

External Structures of Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the outside of some cells, providing protection and aiding in adherence. In bacteria, it exists as either a capsule or a slime layer.

• Capsule: Thick, tightly packed; protects from phagocytosis.

• Slime layer: Loose, water-soluble; enhances adherence.

Flagella

Flagella are long, whip-like extensions used for motility. Their arrangement and structure vary among bacteria, archaea, and eukaryotes.

• Structure: Composed of filament, hook, and basal body.

• Protein: Made of flagellin in bacteria.

• Arrangements: Peritrichous (cover cell), polar (at ends), endoflagella (spirochetes).

Fimbriae and Pili

Fimbriae are short, bristle-like projections used for adherence and biofilm formation. Pili are longer and specialized for DNA transfer (conjugation).

• Fimbriae: Important in biofilms, adherence to surfaces.

• Pili: Transfer DNA between cells; not true sexual reproduction.

Archaeal External Structures

Archaea possess similar external structures to bacteria, including glycocalyces, flagella, and fimbriae. Unique to some archaea are hami, which are filamentous structures with hooks for attachment.

Eukaryotic External Structures

Eukaryotic cells may have glycocalyx, flagella (whip-like motion), and cilia (short, numerous, coordinated movement). Cilia are used for motility and moving substances across cell surfaces.

Comparison Summary Across Domains

Characteristic	Archaea	Bacteria	Eukaryotes
Nucleus	Absent	Absent	Present
Membrane-bound organelles	Absent	Present in few	Various types present
Glycocalyx	Present	Capsule/slime layer	Present in some
Flagella	Some, rotate	Some, rotate	Some, undulate
Cilia	Absent	Absent	Present in some
Fimbriae/Pili	Present in some	Present in some	Absent
Hami	Present in some	Absent	Absent
Cell wall	Most, no peptidoglycan	Most, peptidoglycan	Plants, algae, fungi
Cytoplasmic membrane	All	All	All
Cytosol	All	All	All
Endospores	Absent	Some	Absent
Chromosomes	Single, circular	Single, circular	Linear, multiple

Cell Walls & Cell/Plasma Membranes

Bacterial Cell Walls

Bacterial cell walls provide structure, shape, and protection. The main shapes are cocci (spherical) and bacilli (rod-shaped). Cell walls are composed of peptidoglycan and are classified as Gram-positive or Gram-negative based on staining properties.

• Gram-positive: Thick peptidoglycan, teichoic acids, purple stain.

• Gram-negative: Thin peptidoglycan, outer membrane with LPS, pink stain.

Gram-Positive Bacteria Cell Wall

Gram-positive bacteria have a thick peptidoglycan layer, making them more susceptible to antibiotics. Some possess mycolic acid, requiring acid-fast staining.

• Teichoic and lipoteichoic acids: Anchor cell wall to membrane.

• Acid-fast bacteria: Retain red/pink stain due to mycolic acid (e.g., Mycobacterium).

Gram-Positive Bacteria: Endospores

Many Gram-positive bacteria can form endospores, which protect DNA and allow survival in harsh conditions. Endospores are mainly found in Bacillus and Clostridium genera.

Gram-Negative Bacteria Cell Wall

Gram-negative bacteria have a thin peptidoglycan layer and two plasma membranes. The outer membrane contains lipopolysaccharide (LPS), with Lipid A contributing to pathogenic effects.

• Lipid A: Can cause fever, shock, and blood clotting when released.

• Gram stain: Pink due to thin peptidoglycan.

Gram + vs. Gram -: Side by Side Comparison

Gram-positive and Gram-negative bacteria differ in cell wall structure, staining, and susceptibility to antibiotics.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Outer membrane	Absent	Present
Teichoic acids	Present	Absent
LPS	Absent	Present
Stain	Purple	Pink
Antibiotic susceptibility	Higher	Lower

Membrane Structure & Function

Membrane Structure and Functions

The cell membrane controls passage of substances, maintains gradients, and is selectively permeable. Proteins facilitate transport across the membrane.

• Selective permeability: Only certain substances can cross.

• Transport proteins: Allow movement of ions and molecules.

• Concentration/electrical gradients: Essential for cell function.

Membrane Permeability and Transport

Transport across membranes occurs via passive (no energy) or active (requires energy) mechanisms.

• Passive transport: Simple diffusion, facilitated diffusion, osmosis.

• Active transport: Requires ATP to move substances against gradients.

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane. The effects of isotonic, hypertonic, and hypotonic solutions on cells are important in microbiology.

• Isotonic: No net water movement.

• Hypertonic: Water leaves cell, causing shrinkage.

• Hypotonic: Water enters cell, causing swelling.

Equations

Diffusion and osmosis can be described mathematically:

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Osmotic Pressure:

$\Pi = iMRT$

Where: J = flux, D = diffusion coefficient, dC/dx = concentration gradient, \Pi = osmotic pressure, i = van't Hoff factor, M = molarity, R = gas constant, T = temperature.

*Additional info: Equations and mathematical context added for completeness.*