CHAPTER 3: THE CELL

Overview

• Historical discoveries have shaped our current understanding of microbes.

• Distinguishing structures found in prokaryotic and eukaryotic cells.

STEP 1: Disputing Spontaneous Generation

Historical Experiments

• Spontaneous generation: The notion that life can arise from nonliving matter.

• Francesco Redi (1626–1697): Conducted experiments with meat and maggots, showing that maggots only appeared when flies could access the meat, thus discrediting spontaneous generation.

• Louis Pasteur: Used swan-necked flasks to show that boiled broth remained free of microorganisms unless exposed to air, definitively refuting spontaneous generation.

Cell Theory

Key Concepts

• Robert Hooke (1635–1703): First to describe cells based on microscopic observations of cork.

• Cell theory has three key components:

  1. All living beings are composed of one or more cells.

  2. The cell is the basic unit of structure and organization in organisms.

  3. All cells arise from preexisting cells.

Endosymbiosis

Origin of Eukaryotic Organelles

• Lynn Margulis (1938–2011): Proposed the endosymbiotic hypothesis for the origin of mitochondria and chloroplasts.

• Mitochondria and chloroplasts are derived from bacteria that were engulfed by ancestral eukaryotic cells.

• Evidence: Mitochondrial and chloroplast ribosomes are structurally similar to bacterial ribosomes; their DNA is closely related to bacterial DNA.

Germ Theory

Development and Contributors

• Miasma theory: Disease originated from particles from decomposing matter.

• Germ theory: Microbes are responsible for causing infection and food spoilage.

• Ignaz Semmelweis: Promoted handwashing to prevent disease transmission.

• John Snow: Father of epidemiology, traced cholera outbreaks to contaminated water.

• Joseph Lister: Developed procedures for sterilization in surgery.

• Robert Koch: Established protocols to determine the cause of infectious diseases.

Timeline of Key Discoveries

• Ancient Greeks: Proposed miasma theory.

• 1665: Hooke discovers cells.

• 1674: van Leeuwenhoek observes single-celled organisms.

• 1854: Snow demonstrates waterborne transmission of cholera.

• 1861: Pasteur disproves spontaneous generation.

• 1867: Lister uses antiseptics in surgery.

• 1876–1890: Koch and others identify microbes as causes of infectious diseases.

Structure of a Bacterial Cell

Main Components

• Plasma membrane: Selectively permeable barrier.

• Cell wall: Provides structural support.

• Capsule: Protective outer layer.

• Cytoplasm: Internal fluid containing cell components.

• Nucleoid: Region containing the bacterial chromosome.

• Plasmid: Small, circular DNA molecules.

• Ribosome: Site of protein synthesis.

• Inclusion: Storage bodies.

• Fimbriae: Attachment structures.

• Pilus: Used for DNA transfer.

• Flagellum: Motility structure.

Common Bacterial Shapes

• Coccus: Spherical

• Coccobacillus: Short rod

• Vibrio: Curved rod

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Spirochete: Flexible spiral

Bacterial Arrangements

Grouping Patterns

• Diplo-: Pairs

• Tetrad: Groups of four

• Strepto-: Chains

• Staphylo-: Clusters

• Palisades: Accordion fold

• Sarcina: Cubes

Osmotic Pressure

Effects on Cells

• Isotonic solution: No net movement of water; cell remains unchanged.

• Hypertonic solution: Water leaves the cell; cell shrinks (crenation).

• Hypotonic solution: Water enters the cell; cell may burst (lysis).

• Cells lacking a cell wall are more susceptible to osmotic pressure changes.

The Importance of Cell Walls

• Cell walls protect prokaryotic cells from osmotic pressure changes, helping maintain cell shape and integrity.

• In hypertonic environments, the cell membrane may detach from the wall (plasmolysis).

• In hypotonic environments, the cell wall prevents the cell from bursting.

The Nucleoid

Genetic Material in Prokaryotes

• Chromosome: Single, circular, double-stranded DNA molecule containing all genetic information required by the cell.

• Plasmids: Free, small, circular DNA molecules; not essential for growth but useful in genetic engineering.

Bacterial Ribosome

Protein Synthesis Machinery

• Composed of 60% ribosomal RNA and 40% protein.

• Consist of two subunits: large (50S) and small (30S) in prokaryotes; together form the 70S ribosome.

• Site of protein synthesis; found in all cells.

Bacterial Internal Structures

Inclusions

• Intracellular storage bodies.

• Vary in size, number, and content.

• Used when environmental sources are depleted.

Endospores

• Produced by some Gram-positive genera: Clostridium, Bacillus, Sporosarcina.

• Two-phase life cycle: vegetative cell (active) and endospore (dormant, highly resistant).

• Sporulation: Formation of endospores.

• Germination: Return to vegetative growth.

Endospores

• Hardiest of all life forms; withstands extremes in heat, drying, freezing, radiation, and chemicals.

• Not a means of reproduction; metabolically inactive and dehydrated.

• Thick coat; longevity can reach 250 million years.

• Destroyed by pressurized steam at 120°C for 20–30 minutes.

Tetanus & Botulism

Pathogenic Endospores

• Endospores of Clostridium spp. are widespread in nature.

• Tetanus neurotoxin (TeNT) by C. tetani: Causes spastic paralysis (lockjaw, opisthotonos).

• Botulinum neurotoxin (BoNT) by C. botulinum: Causes flaccid paralysis.

Botulinum & Tetanus Neurotoxins

• Botulinum toxin: Blocks acetylcholine release, stopping muscle contraction (flaccid paralysis).

• Tetanus toxin: Blocks inhibitory neurotransmitters, causing continuous muscle contraction (spastic paralysis).

Pathology of Tetanus

• Spores enter through wounds, burns, umbilical stumps, frostbite, or crushed body parts.

• Types: Localized (near injury), Cephalic (head/face), Generalized (systemic, including neonatal).

• Generalized tetanus: TeNT travels to CNS, causing muscle spasms and arching posture (opisthotonos); death often due to respiratory muscle paralysis.

Treatment and Prevention (Tetanus)

• Antitoxin therapy with human tetanus immune globulin (TIG).

• Control infection with penicillin or tetracycline; muscle relaxants may be used.

• Vaccine available; booster needed every 10 years (DTaP for children, Tdap for adults).

C. botulinum Food Poisoning

• Rare but often fatal illness caused by BoNT intoxication.

• Can result from infection (wound, infant, adult intestinal botulism) or direct toxin introduction (foodborne, inhalation, iatrogenic).

• Symptoms: Double/blurred vision, difficulty swallowing, neuromuscular symptoms, respiratory failure.

• Children

Treatment and Prevention (Botulism)

• Detect toxin in food, intestinal contents, or feces.

• Administer antitoxin; provide cardiac and respiratory support.

• Treat infectious botulism with penicillin.

• Practice proper food preservation and handling; add preservatives.

Plasma Membrane

Structure and Function

• Phospholipid bilayer with embedded proteins (fluid mosaic model).

• Functions:

  1. Site for energy reactions, nutrient processing, and synthesis.

  2. Passage of nutrients into the cell and discharge of wastes.

  3. Selective permeability.

Transport: Movement of Chemicals Across the Cell Membrane

Types of Transport

• Passive transport: No energy required; substances move down a concentration gradient.

  • Diffusion

  • Osmosis (diffusion of water)

  • Facilitated diffusion (requires a carrier protein)

• Active transport: Requires energy and carrier proteins; can move substances against a gradient.

  • Group translocation (chemical modification of transported molecule)

  • Bulk transport (endocytosis, exocytosis, pinocytosis)

Diffusion

• Simple diffusion: Movement of molecules down a concentration gradient directly across the phospholipid bilayer.

Facilitated Diffusion

• Movement of molecules down a concentration gradient through a membrane protein.